Hales, Stephen

(b. Bekesbourne, Kent, England, 17 September 1677; d. Teddington, Middlesex, England, 4 January 1761)

physiology, public health.

Stephen Hales, a clergyman without formal medical training, published his first discoveries in his fiftieth year, yet was soon recognized as the leading English scientist during the second third of the eighteenth century. As the acknowledged founder of plant physiology, he had no worthy successor until Julius von Sachs, a century later. In animal physiology he took “the most important step after Harvey and Malpighi in elucidating the physiology of the circulation.”¹ His experiments concerning “fixed air"—and the apparatus he devised—laid the foundations of British pneumatic chemistry and stimulated the discoveries of Joseph Black, Henry Cavendish, and Joseph Priestley; Hales was a primary influence on the early researches of Lavoisier.

He was born of an old and distinguished Kentish family, but there is no record of his boyhood until, having been “properly instructed in grammar learning,” he was sent to Cambridge, where he entered Benet College (now Corpus Christi) in 1696.² On receiving the B.A. degree, Hales became a fellow of his college in 1703 and was awarded the M.A. that same year. He was ordained deacon in 1709 and left Cambridge to become “perpetual curate,” or minister, of Teddington, a village on the Thames between Twickenham and Hampton Court. He held this position for the rest of his life, and it was at Teddington that most of his scientific work was carried on.

An interest in science was awakened during his years at Cambridge, the university that boasted the great Isaac Newton (who had left for London the year that Hales entered the university) and the naturalist John Ray, whose earliest book was a catalog of the plants of Cambridgeshire. Something of a scientific renaissance took place during Hale’s last years at the university. William Whiston, Newton’s successor as Lucasian professor, was encouraged by Newton’s old friend Richard Bentley, who became master of Trinity, the college of Newton and Ray, in 1700. Bentley helped secure the appointment of a gifted young fellow of the college, Roger Cotes, to the newly established Plumian professorship of astronomy and built for him an observatory over the Great Gate of Trinity. When John Francis Vigani became the first professor of chemistry at Cambridge, Bentley provided him with a laboratory “in the mediaeval chambers that look out on the Bowling Green.”³

In 1703 William Stukeley, the future physician and antiquary, entered Benet College; intent on a medical career, he “began to make a diligent & near inquisition into Anatomy and Botany,”⁴ He became a close friend of Hales; with Stukeley and other students Hales went “simpling” in the surrounding countryside, Ray’s catalog in hand. In a room that Stukeley’s tutor had given him as a sort of laboratory, they performed, chemical experiments and dissected frogs and other small, animals. Together they devised a method of obtaining a lead cast of the lungs of a dog. It was at this time (about 1706) that Hales carried out his first blood-pressure experiments on dogs. He and Stukeley attended Vigani’s chemical lectures and saw his demonstrations in the laboratory at Trinity. Hales, like Stukeley, must have seen the “many Philosophical Experiments in Pneumatic Hydrostatic Engines & Instruments performed at that time” by John Waller, rector of St. Benedict’s Church, who later succeeded Vigani as professor of chemistry. Hales knew Waller, for about 1705 the two men “gathered subscriptions to make the cold bath about a mile & a half out of Town.”⁵ This introduction to pneumatic experiments was probably supplemented by the lectures in experimental physics given by Whiston and Cotes at the observatory in Trinity College. Cotes, in his share of the lectures, demonstrated the experiments of Torricelli, Pascal, Boyle, and Hooke.⁶

Newton’s influence was strongly felt at Cambridge. In 1704 appeared his long-delayed Opticks, a work that, in its later editions, profoundly influenced Hales. We learn from Stukeley that students at Benet College read the Cartesian Physics of Jacques Rohault, but in the edition of Samuel Clarke,⁷ who appended Newtonian footnotes to correct the text.

Hales, like Stukeley, doubtless witnessed Newton’s arrival in Cambridge in April 1705, when he came to offer himself as the university’s candidate for Parliament. On the sixteenth of that month Queen Anne visited Cambridge as the guest of the master of Trinity. As Stukeley recalled it: “The whole University lined both sides of the way from Emanuel college, where the Queen enter’d the Town, to the public Schools. Her Majesty dined at Trinity college where she knighted Sir Isaac, and afterward, went to Evening Service at King’s college chapel.”⁸

Although doubtless incapable of following the mathematical intricacies of Newton’s Principia, Hales mastered the main features of the new system, of the world. He showed both his mechanical ingenuity and some knowledge of celestial physics by devising a machine to show the motions of the planets. A drawing by Stukeley of Hales’s orrery is preserved, along with Stukeley’s diary, in the Bodleian Library, Oxford.⁹

At Teddington, Hales was first preoccupied with his parish duties; only several years later did he resume his scientific work; and it was later still before the scientific world heard from him. About 1712–1713 he took up again his experiments on animals, this time using as his victims two horses and a fallow doe. But he did not “pursue the Matter any further, being discouraged by the disagreeableness of anatomical Dissections.” For several years his scientific endeavors lapsed, yet on 13 March 1717/18 (O.S.) he was elected a fellow of the Royal Society along with his old friend William Stukeley, who was now practicing medicine in London.¹⁰ It was Stukeley, indeed, who brought Hale’s name to the attention of the Royal Society.¹¹ Hales was soon to justify his election.

While conducting his experiments on animal blood pressure. Hales records that “I wished I could have made the like Experiments, to discover the force of the Sap in Vegetables” but “despaired of ever effecting it,” Yet early in 1719 “by mere accident I hit upon it, while I was endeavouring by several ways to stop the bleeding of an old stem of a Vine, which was cut too near the bleeding season, which I feared might kill it.” His account continues:

Having, after other means proved ineffectual, tyed a piece of bladder over the transverse cut of the Stem, I found the force of the Sap did greatly extend the bladder; whence I concluded, that if a long glass Tube were fixed there in the same manner, as I had before done to the Arteries of several living Animals, I should thereby obtain the real ascending force of the Sap in that Stem.¹²

Hales was in no hurry to make an appearance at the Royal Society or to contribute to its proceedings; several months elapsed before he appeared in person to sign the required bond.¹³ On 5 March 1718/19, perhaps at Stukeley’s urging, Hales informed the president, Sir Isaac Newton, “that he had lately made a new Experiment upon the Effect w^ch y^e Suns warmth has in raising y^e sap in trees.”¹⁴

Seven years of silence followed, during which, in the free time his parish duties allowed him, Hales followed out this original clue. The experiments on plants were virtually completed, and the Vegetable Staticks written out, by the middle of January 1724/25. He submitted his manuscript to the Royal Society, where it was read at successive meetings from January to March.¹⁵ In this form it consisted of six chapters, not the seven of the published book; missing was the long chemical chapter “The Analysis of Air.”

Although Hales was urged to publish, two years elapsed before the Royal Society heard from him again. During this time he performed the seventy chemical experiments of chapter 6 of the Vegetable Staticks. In three meetings during February 1726/27 this chemical chapter was read, and at the last of these meetings the book received the imprimatur of the Royal Society, signed by Sir Isaac Newton “Pr. Reg. Soc.”¹⁶ The book was already in press, for some of the early sections were read again to the Society in March, probably from advance sheets. On 13 April 1727 (O.S.) a copy of the Vegetable Staticks, dedicated to George, prince of Wales (the future George II), was presented to the Society; and the curator of experiments, J. T. Desaguliers, was asked to prepare an abridgment of it.¹⁷

Hales next turned to completing and publishing his experiments on animal circulation (mentioned only briefly in the vegetable Staticks) under the title Haemastaticks, putting the two works together as the Statical Essays. Imbued with the empiricism of John Locke and the principles of Newton’s “experimental philosophy,” Hales was also influenced by the doctrines of the iatrophysicists—those physicians including Borelli, Baglivi, and the Scots doctors Archibald Pitcairne and James Keill who insisted, as a certain John Quincy put it, that the application of mechanical principles “to account for all that concerns the Animal Oeconomy” is the best means to “get clear of all suppositions and delusory Hypotheses” and “has appeared to be the only way by which we are fitted to arrive at any satisfactory Knowledge in the Works of Nature.”¹⁸

An immediate stimulus came from James Keill, who, while Hales was still at Cambridge, published a book giving quantitative estimates of the amount of blood in the human body, the velocity of the blood as it left the heart, the amounts of various animal secretions, and so on.¹⁹ Hales may have had Keill’s book in mind when he wrote that

… if we reflect upon the discoveries that have been made in the animal oeconomy, we shall find that the most considerable and rational accounts of it have been chiefly owing to the statical examination of their fluids, viz. by enquiring what quantity of fluids, and solids dissolved into fluids, the animal daily takes in … And with what force and different rapidities those fluids are carried about in their proper channels …²⁰

What Hales called the “statical way of inquiry” he deemed the proper way to study living things. For his now obsolete use of the word “staticks” he had ample authority. The usage originated with Nicholas of Cusa; in his De staticis experimentis — a work several times reprinted in the fifteenth and sixteenth centuries and translated into English in 1650—Cusa outlined a series of “thought experiments” involving the use of the balance.²¹ Statics, for Cusa, meant weighing. Santorio Santorio made the term familiar in medicine with his De medicina statica aphorismi (Venice, 1614), a work on “insensible perspiration” often reprinted and published in English translation in 1712 by John Quincy.²² In 1718 James Keill published his Tentamina medico-physica (a Latin version of his earlier work) and appended to it some studies on perspiration called Medicina statica britannica.²³ Hales was familiar with this book, to which he referred several times.

The Haemastaticks describes the experiments on blood pressure begun at Cambridge, taken up again at Teddington, laid aside because of the “disagreeableness of the Work,” but resumed again after the publication of the Vegetable Staticks. At first intended only as an addition to the earlier book, it grew “into the Size of another Volume, so fruitful are the Works of the great Author of Nature in rewarding, by farther Discoveries, the Researches of those who have Pleasure therein.”²⁴

The first Cambridge experiments, Hales tells us, were stimulated by the confusion that existed as to the magnitude of the arterial blood pressure; some maintained that the pressure was enormous, and even that it might be the cause of muscle motion. The results of his series of investigations were of such importance that they have been described as “the most important step in knowledge of the circulation between Malpighi and Poiseuille.”²⁵

The Haemastaticks opens with an account of Hales’s most dramatic experiment, a bold and bloody one. He tied a live mare on her back and, ligating one of her femoral arteries, inserted a brass cannula; to this he fixed a glass tube nine feet high; when he untied the ligature, the blood rose to the height of more than eight feet. Detaching the tube at intervals, he allowed a measured quantity of blood to flow out, noting how the pressure changed during exsanguination. He succeeded in inserting cannulas into the veins to record the venous pressure of a number of animals, including an ox, a sheep, a fallow doe, three horses, and several dogs.

His interest in the mechanics of the circulation now enhanced, Hales turned his attention to the chief factors that must maintain the blood pressure; the output of the heart per minute and the peripheral resistance in the small vessels. He made a rough estimate of cardiac output by multiplying the pulse rate of an animal by the internal volume of its left ventricle, of which he made a cast in wax after the animal had been killed. He noted that the pulse was faster in small animals than in large ones, and that the blood pressure was proportional to the size of the animal.

Hales next studied peripheral resistance with perfusion experiments. Injecting various chemical substances (brandy, decoction of Peruvian bark, various saline solutions), he compared the rate of flow of the perfusate and showed that certain substances had a pronounced effect on the rate at which the blood could flow through an isolated organ. He attributed this to changes in the diameter of the capillaries and so—although he did not observe the phenomenon directly—discovered vasodilatation and vasoconstriction.

Hales’s experiments convinced him that the force of arterial blood in the capillaries “can be but very little” and wholly inadequate for “producing so great an Effect, as that of muscular Motion.” This “hitherto inexplicable Mystery of Nature must therefore be owing to some more vigorous and active Energy, whose force is regulated by the Nerves.” The recent experiments of Stephen Gray suggested to Hales that this energy, these “animal spirits,” might be electrical, for Gray had shown that the electrical virtue from rubbed glass.

…will not only be conveyed along the Surface of Lines to very great Lengths, but will also be freely conveyed from the Foot to the extended Hand of a human Body suspended by Ropes in…the Air; and also from that Hand to a long Fishing Rod held in it, and thence to a String and a Ball suspended by it.²⁶

Hales was therefore the first physiologist to suggest, with some evidence to support it, the role of electricity in neuromuscular phenomena.²⁷

Despite these achievements, Hales’s most original contribution was to apply to the study of plants the “statical method” which had brought such good results with animals. Like his contemporaries he was impressed by the analogies that he perceived between the animal and the vegetable worlds. Perhaps the most obvious was the fundamental similarity of the role of the sap in plants and of the blood in animals. Since the growth of plants “and the preservation of their vegetable life is promoted and maintained, as in animals, by the plentiful and regular motion of their fluids, which are the vehicles ordained by nature, to carry proper nutriment to every part,” the same methods ought to be used which had illuminated the animal economy: “the statical examination of their fluids.”²⁸ By an accident, as we saw, he was led to his first attempts to measure the force of the sap in vines and to determine the conditions under which it varied. Although he outstripped his predecessors, Hales was not the first to investigate the flow of sap.

The problem of sap flow had long interested the virtuosos of the Royal Society. In 1668 the Philosophical transations proposed to its readers a long series of “queries” concerning plants, “especially the Motion of the Juyces of Vegetables,” asking, for example, whether the “Juyce ascends or descends” by the bark or the pith.²⁹ Among the responses was a letter from Francis Willughby describing some bleeding experiments on trees performed with John Ray which showed that the sap not only ascended but also seemed to descend and move laterally, and that the rise could not be attributed to capillarity, a common explanation.³⁰ When the letter was read to the Society, the two naturalists were requested to “try some experiments, to find, whether there be any circulation of the juice of vegetables as there is of the blood of animals.”³¹ That there might be such a circulation of sap, moving upward through the vessels of woody plants and downward by those between the wood and the bark, was a commonly held view in Hales’s day; as late as 1720 Patrick Blair in his Botanick Essays tried to prove that this was the case.³²

Nor was Hales the earliest to apply the “statical way of inquiry” to plants. J. B. van Helmont’s famous willow tree experiment, which persuaded him that water was the sole principle or nutrient of plants, is a well-known example.³³ Closer to Hales’s time were the quantitative experiments of John Woodward to determine whether water itself, or substances dissolved in it, accounted for the growth of plants. In these experiments Woodward discovered that the phenomenon of transpiration was of considerable magnitude. Growing mint in water, Woodward observed that the plant took up large quantities of water but gave off far more than it retained. He noted that solar heat played a part in the process, but he did not specify or prove that transpiration occurred through the leaves. Much the greatest part of the water imbibed by his plants, he wrote, “does not settle or abide there: but passes through the Pores of them, and exhales up into the Atmosphere.”³⁴

Hales’s experiments on plants, begun in March 1719, were pursued with vigor during the years 1723–1725, using the resources of his own garden and plants and trees provided him from the nearby royal garden of Hampton Court.³⁵ In his early experiments, conducted during the bleeding season, he observed the rise of sap through long glass tubes fastened to the cut end of a branch of a grapevine. In one such experiment Hales joined glass tubes together to a height of thirty-eight feet. The sap was observed to rise in these tubes “according to the different vigor of the bleeding state of the Vine” from one foot up to twenty-five feet. He carefully observed how the sap flow varied with the weather and the time of day.

To measure the sap pressure Hales employed a “mercurial gage,” a bent tube filled with mercury which he fixed to the cut branches of the vine, observing again the variations of the pressure at different times of day.

To determine the force with which trees imbibe moisture from the earth, Hales devised what he called “aqueo-mercurial” gauges. He laid bare the root of a small pear tree, cut it, and inserted it into a large glass tube, which in turn was fixed to a narrow tube eight inches long. When the tubes were filled with water and immersed in a vessel of mercury, the root, he reported, “imbibed the water with so much vigor” that in six minutes the mercury rose eight inches. These experiments were carried out in the summer months, when the trees and vines were in leaf. Hales noted that the more the sun shone on the plants, “the faster and higher the mercury rose”; it would subside toward evening and rise the next day. He observed that sometimes the mercury “rose most in the evening about 6 a clock, as the sun came on the Vine-branch.” Such results may have suggested to him the role that transpiration—or, as he called it, “perspiration”—might play in causing the sap to rise.

Hales’s experiments on transpiration—perhaps the most famous and brilliant of those he performed with plants—were carried out in the summer months of 1724. He grew a large sunflower in a garden pot covered tightly with a thin lead plate pierced by the plant, by a small glass tube to allow some communication with the air, and by another short, stoppered tube through which the plant could be watered. He weighed the pot and plant twice a day for fifteen days, then cut off the plant close to the lead plate, cemented the stump, and by weighing determined that the pot with its earth “perspired” two ounces every twelve hours. Subtracting this from his earlier weighings, he found that the plant perspired in that period an average of one pound, four ounces of water.

Hales then stripped off the leaves of the plant and divided them in groups according to their several sizes. Taking a sample leaf from each group, he measured their surface areas by placing over them a grid made of threads, composing quarter-inch squares. By multiplying the area of each sample leaf by the number of leaves in the group and adding his measurements together, he obtained the total surface area of the leaves. His figures for the loss of water from the leaves compare favorably with those obtained lung after by Sachs. Hales also attempted to estimate the surface area of the roots to determine the rate of absorption per given area, but these figures are of no value because Hales “did not know how small a part of the roots is absorbent, nor how enormously the surface of that part is increased by the presence of root-hairs.”³⁶ He was somewhat more successful in determining the rate of flow of the stem. Always hoping to find analogies between animals and plants, he estimated the total surface area of his sunflower and its weight so as to compare the quantity of water “perspired” by the plant in twenty-four hours with that of an average “well-sized man” over the same period, taking the latter figure from James Keill’s Medicina statica britannica.³⁷

Transpiration could not account for the powerful rise of sap in vines during the bleeding season. Hales devoted a chapter of his book to the experiments which led him to discover root pressure. He cut off a vine, leaving only a short stump with no lateral branches. To it, by means of a brass collar, he fixed a series of glass tubes reaching as high as twenty-five feet. The sap rose gradually nearly to the top of these tubes, both day and night, although much higher in daytime. From this experiment, Hales remarks, “we find a considerable energy in the root to push up sap in the bleeding season.”³⁸

Similar experiments using his mercurial gauge confirmed that the force of the rising sap was “owing to the energy of the root and stem,” Comparing his results with his blood pressure experiments, Hales concluded that this force was “near five times greater” than that of the blood in the femoral artery of a horse and “seven times greater than the force of the blood in the like artery of a Dog.”

Curious whether this force could be detected in vines when the bleeding season was over, Hales performed the same experiment in the month of July and found that the flow of sap ceased when the vine was cut from the stem, thus proving to his satisfaction that after the bleeding season the principal cause of the rise of sap was not root pressure but that which was “taken away, viz. the great perspiration of the leaves,” This was evident, too, from a number of experiments which showed that branches stripped of their leaves did not imbibe water “for want of the plentiful perspiration of the leaves.”³⁹

A series of experiments to discover the direction of the flow of sap, and the portion of the stem through which it moved, were performed by cutting away the bark or slicing off a small section of it. These showed that while there must be some lateral communication, the sap moved upward between the bark and the wood, not downward “as many have thought,” and that there is no circulation of the sap. Plants, Hales suggested, make up for the lack of a circulation by the much greater quantity of fluid that passes through them. Nature’s “great aim in vegetables being only that the vegetable life be carried on and maintained, there was no occasion to give its sap the rapid motion, which was necessary for the blood of animals.”⁴⁰

Hales’s explanation of the sap’s motion invokes the Newtonian principle of attraction. The chief cause is “the strong attraction of the capillary sap vessels,” greatly assisted “by the plentiful perspiration of the leaves, thereby making room for the fine capillary vessels to exert their vastly attracting power.” This “perspiration” results from the sun’s warmth acting on the leaves, which are fittingly broad and flat to serve this purpose of absorbing the sun’s rays.

An experiment to show the “great force, with which vegetables imbibe moisture” was performed by filling an iron pot nearly to the top with peas and water. Over the peas Hales placed a cover of lead, and on the cover he placed a weight of 180 pounds, which— as the peas swelled with the imbibed water—was lifted up.

The role of attraction, “that universal principle which is so operative in all the very different works of nature, and is most eminently so in vegetables,”⁴¹ was illustrated by Hales’s modification of an experiment of Francis Hauksbee, described in query 31 of Newton’s Opticks, showing the rise of water through a glass tube firmly packed with sifted wood ashes. Hales measured the imbibing force with his “aqueomercurial gage,” He quotes Newton’s words that “by the same principle, a sponge sucks in water, and the glands in the bodies of animals, according to their several natures and dispositions suck in various juices from the blood.” Hales adds:

And by the same principle it is, that…plants imbibe moisture so vigorously up their fine capillary vessels; which moisture, as it is carryed off in perspiration, (by the action of warmth,) thereby gives the sap vessels liberty to be almost continually attracting of fresh supplies.⁴²

An influential experiment—it paved the way for some important researches by Sachs—demonstrated the unequal extent of growth in developing shoots and leaves. In the spring, using a comb-like device. Hales pricked, with homemade red paint, dots a quarter of an inch apart along a young vein shoot. Several months later, when the shoot was full-grown, he measured the distances between the dots. The shoots, he discovered, had grown chiefly by a longitudinal extension between the nodes; the oldest (basal) internode had grown the least and the youngest (apical) one, the most.⁴³

Again, concerned with analogies between plants and animals, this experiment led Hales to see if a similar effect could be observed in the growth of the long bones in animals, with their tubelike cavities. He took a half-grown chick and pierced the thigh and shin bones with a sharp pointed iron, making small holes half an inch apart. After two months he killed the bird and found that although the bones had grown an inch in length, the marks remained the same distance apart. In contrast with what he had observed in his vineshoots, the growth had occurred not in the shaft but entirely at the junction of the shaft and its two ends, that is, at the symphyses.

In his experiments on plants Hales frequently noticed bubbles of air emerging from the cut stems of vines or rising through the sap, often in such quantity as to produce a froth. This, he remarked, “shews the great quantity of air which is drawn in thro’ the roots and stem.” The air, he thought for a time, was “perspired off” through the leaves; but an inconclusive experiment led him to suspect that “the leaves of plants do imbibe elastick air.”⁴⁴ By 1725 he had performed a few experiments to prove that a considerable quantity of air is “inspired” by plants. The problem interested him so much that he deferred publication until he could make “a more particular enquiry into the nature of a Fluid,” the air, “which is so absolutely necessary for the support of the life and growth of Animals and Vegetables,”⁴⁵ These investigations, carried out between 1725 and 1727, were embodied in the long chapter, nearly half the final work, called “Analysis of Air.” This chapter was to have momentous consequences for the later development of chemistry.

Since the investigations of Torricelli, Pascal, Ottovon Gueneke, and, of course, Robert Boyle, the physical properties of air had been pretty well understood: the law of its expansibility, its ability to refract light, its approximate density under standard conditions. But it was no longer thought by chemists to be an element.⁴⁶ Any apparent chemical activity, and its ability to sustain life and support combustion, could be explained by the properties of special substances dispersed through it, such as the nitro-aerial particles imagined by Hooke, John Mayow, and others.⁴⁷ Boyle’s description of the atmosphere was widely accepted; it was composed, he wrote, of three kinds of particles: the permanently elastic particles making up the air properly speaking, a “thin, diaphanous, compressible and dilatable Body”; vapors and dry exhalations from the earth, water, vegetables, and animals; and, third, “magnetical steams of our terrestrial globe” and particles of light from the sun and stars.⁴⁸

Yet Boyle, Hooke, and other fellows of the Royal Society had shown that “air” (“factitious air”) could be produced from solid and liquid bodies in certain chemical reactions: the action of acids on oyster shells or coral, the reaction of dilute acids with iron nails, the explosion of gunpowder.⁴⁹ A particularly striking experiment was performed by Frederick Slare in 1694. He poured spirit of niter (nitric acid) over oil of caraway seeds, and the result was a violent explosion that blew up the glass container. Slare expressed amazement that so much air was produced from small amounts of these liquids.⁵⁰ This experiment made a profound impression; Slare’s account was read, and the experiment perhaps repeated, by Roger Cotes in his lectures. It was described, too, by Newton in his Opticks, although without mentioning Slare by name. Hales was doubtless familiar with this experiment, although when he mentioned Slare in the Vegetable Staticks it was for a different experiment.⁵¹ Newton’s Opticks, to which Hales referred so often in his book, would have been sufficient authority for the existence of “factitious airs.” At one point he quotes Newton’s words: “Dense Bodies by Fermentation rarify into several sorts of Air, and this Air by Fermentation, and sometimes without it, returns into dense bodies.”⁵²

Of particular concern for Hales was evidence that air was thought to be of special importance to the plant economy. In France, Guy de La Brosse early in the seventeenth century had argued that plants cannot grow without the air from which they draw “la rosée & la manne.”⁵³ Similar views were advanced by Robert Sharroek, a friend and collaborator of Robert Boyle⁵⁴. This question was taken up in the early meetings of the Royal Society; John Beal suggested in 1663 that it should be determined “what effects would be produced on plants put into the pneumatic engine with the earth about their roots, and flourishing; whether they would not suddenly wither, if the air were totally taken from them.⁵⁵ Not long after, Robert Hooke showed thai lettuce seed would not sprout and grow, and a thriving plant would wither and die, if kept in a vacuum.⁵⁶ In 1669 Beal felt able to conclude that a plant “feeds as well on the Air, as [on] the juice furnish’d through the root⁵⁷ After the discoveries in plant anatomy by Malpighi and Nehemiah Grew, and their description of vessels in plants that appeared, like the trachea of insects, to be tubes for transmitting air, it was suggested that air contributed to the nutrition of plants, or—as John Ray put it—that plants have a kind of respiration.⁵⁸

Except for Malpighi and Grew, whom he cites, Hales may have been unaware of these antecedents. But he was familiar with certain of Boyle’s experiments admired by Roger Cotes. By these experiments, published in 1680–1682, Boyle showed, as Hales put it in the beginning of his “Analysis of Air,” that

... a good quantity of Air was producible from Vegetables, by putting Grapes, Plums, Gooseberries, Cheries, Pease, and several other sorts of fruits and grains into exhausted and unexhausted receivers, where they continued for several days emitting great quantities of Air.⁵⁹

In this famous long chapter Hales describes a large number of experiments—some trivial, some confused, but some extremely interesting—performed to discover the amount of air “fixed” in different substances or given off or absorbed under various circumstances, Strictly speaking, Hales was not a chemist, although he had performed some chemical experiments during his Cambridge days, when he had read or consulted George Wilson’s practical compendium, A Compleat Course of Chemistry (1699).⁶⁰ He knew Boyle’s work and John Mayow’s, and was familiar with Nicolas Lemery’s popular textbook.⁶¹ But his approach was more physical than chemical; and it is not surprising—since he thought of air as a unitary substance characterized by its physical property of elasticity—that he failed to note the different chemical properties of the airs he produced.⁶²

Hales’s true mentor was Newton, whose last query of the Opticks (1718) was in fact a monograph on the role of attractive and repulsive forces in chemical processes, and whose short “Thoughts About the Nature of Acids” Hales had also read.⁶³ He was familiar too with the Chymical Lectures in which John Freind attempted to explain chemical reactions in Newtonian terms.⁶⁴

From Newton, Hales derived the fundamental principles by which he explained the effects he observed. Matter is particulate, and the particles are subject to very special laws of attraction and repulsion. In their free state the particles of air exert upon each other strong repulsive forces, which accounts for the air’s “elasticity.” Yet this elasticity is no immutable property, for Newton had remarked that “true permanent Air arises by fermentation or heat, from those bodies which the chymists call fixed, whose particles adhere by a strong attraction.”⁶⁵ When air enters into “dense bodies” and becomes “fixed,” its elasticity is lost because strong attractive forces overcome the forces of repulsion between its particles.

Hales’s first experiments were distillations in which different substances were strongly heated in a glass or iron retort. The retort was cemented and luted to a globular vessel with a long neck, called a bolthead.⁶⁶ This vessel, with a hole cut in the bottom, was immersed in a basin of water; and by means of a siphon the water level was raised in the neck to a point he carefully marked. The amount of air given off or absorbed was determined by allowing the vessel to cool and noting the change in the water level. With this apparatus Hales measured the air produced by weighed amounts of hog’s blood, tallow, powdered oyster shell, amber, honey, and a variety of vegetable materials. Whereas he obtained little air from ordinary well water, a considerable quantity was yielded by Pyrmont water, leading Hales to comment that this air “contributes to the briskness of that and many other mineral waters.” He distilled iron pyrites, known to be rich in sulfur, and from a cubic inch of this mineral obtained eighty-three cubic inches of air. When he heated minium or red lead (Pb₃ O₄), he obtained a large quantity of air, remarking that this air might account for the increase in weight of lead when it is strongly heated to form minium. This air was doubtless what had “burst the hermetically sealed glasses of the excellent Mr. Boyle, when he heated the Minium contained in them by a burning glass.”⁶⁷

Two other contrivances were used by Hales to measure the air produced or absorbed in chemical reactions, or, as he put it, in “fermentations.” One apparatus consisted of a bolthead placed in a basin of water; over its long neck he inverted a cylindrical vessel, using a siphon to draw up the water a given distance. As in the first apparatus, the amount of air given off or absorbed was determined by the change in the water level.⁶⁸ With this apparatus Hales measured the air produced by decomposing sheep’s blood, by ale drawn from a fermenting vat, by the fermentation of raisins and apples, and by the action of vinegar on powdered oyster shells. Other experiments showed that salt of tartar (potassium carbonate) treated with acids yielded much air, a discovery that later put Joseph Black on the road to his major chemical discovery.⁶⁹ Hales also measured the large amount of air (hydrogen gas) produced from iron filings treated with dilute sulfuric acid. When the iron filings were dissolved in dilute nitric acid, he also obtained much air (in this case, nitric oxide). of particular interest is Hales’s measurement of air produced by the action of oil of vitriol on chalk and his further observation that lime (made from the same chalk) absorbed much air.⁷⁰

His second contrivance has been called his pedestal apparatus.⁷¹ A wooden pedestal is placed upright in a basin of water, and on its expanded top can be placed a candle, a weighed amount of some chemical substance to be ignited, or—in the larger form of this apparatus—a small animal. A glass cylinder is suspended over the pedestal so that its mouth is a few inches under water. As in the other devices, air is withdrawn with a siphon or bellows to raise the water to a convenient level, and a change in the water level indicates the change in the volume of air in the cylinder. With his pedestal apparatus Hales discovered that when phosphorus and sulfur are burned, they absorb air. When he detonated niter (potassium nitrate) by means of a burning glass, he noted the large amount of air produced but observed that the volume steadily decreased, or as he put it, “the elasticity of this new air daily decreased.”⁷²

Repeating an experiment of John Mayow’s, Hales placed a candle on the pedestal, ignited it with a burning glass, and noted the shrinkage in volume. When he used candles of equal size but in vessels of different capacities, he found that they burned longer in the larger ones and that “there is always more elastic air destroyed in the largest vessel.” His burning glass, he found, could not light an extinguished candle “in this infected air.”⁷³ Repeating another of Mayow’s experiments, Hales placed a small animal on the pedestal and measured the air absorbed. Here, to be sure, two effects—both unknown to Hales—contributed to the rise of the water level: the intake of oxygen by the animal and its exhalation of carbon dioxide, much of which dissolved in the water. His results led to a series of rebreathing experiments carried out on himself which convinced him that animal respiration “vitiated” the air. His device, a bladder equipped with valves and breathing tube, enabled him to breathe repeatedly his own expired air. He found that he could continue in this fashion only about a minute. In a modification of his device, a series of diaphragms (flannel stretched over thin hoops) was placed in the bladder. When these were soaked with salt of tartar, especially when the salt was calcined (that is, causticized), he found that he could rebreathe for as long as eight and a half minutes. The salt, “a strong imbiber of sulphureous steams,” in fact absorbs much carbon dioxide.

In his experiments, when he noticed a decrease in volume of air during certain reactions, Hales always spoke of a loss of elasticity and attributed this to the acid sulfurous fumes which “resorb and fix” the elastic particles of ordinary air.⁷⁴ Such fumes, he noted, were produced by burning sulfur, by a lighted candle—indeed, by all “flaming bodies"—and by the expired air of animals and man.

To obviate this effect, Hales devised his most famous apparatus; the first pneumatic trough. Substances were heated in an iron retort; to the long neck of the retort he fixed a bent lead tube which was immersed in a basin of water and projected upward into the open end of an “inverted chymical receiver” filled with water. The released air passing through the bent tube bubbled up through the water and was collected in the top of the glass vessel. Hales’s purpose was not to measure the amount of air, as in his other experiments, but to wash the air by passing it through water, to intercept “a good part of the acid spirit and sulphureous fumes.” By this means he could collect and store air and ascertain whether its elasticity could be preserved. By separating the generator from the collector. Hales invented the pneumatic trough, later used in modified form by Brownrigg, Cavendish, and Priestley.

With his trough Hales collected air from a variety of substances—horn, human bladder stones, pyrite, saltpeter, minium, salt of tartar, and various vegetable materials—and claimed that the grealer part of the air remained for the most part “in a permanently elastick state” and so was true air, not a mere flatulent vapor. He did not explore the different chemical properties of the air produced from different substances—indeed, he had no great reason to believe they could be found. Yet he suspected that there were at least some physical differences. Newton had written of bodies rarefying into “several sorts of air,” an opinion that Hales seems to have shared, for he suggested that since air arises from a great variety of “dense” bodies, it is probable that airs from different sources may differ in the size and density of their constituent particles and may have “very different degrees of elasticity.” But his crude attempts to see if common air and the air produced by salt of tartar (carbon dioxide) differed in density and compressibility disclosed no difference.

Hales’s explanation of combustion was a physical one.⁷⁵ He rejected the notion that fire is “a particular distinct kind of body inherent in sulphur,” as the chemists Willem Homberg and Louis Lemery believed. Instead, he followed Newton in distinguishing between heat and fire: heat is the rapid intestine motion of particles; fire is merely “a Body heated so hot as to emit Light copiously,” and flame is only a “Vapour, Fume or Exhalation heated red hot.” Hales owed much also to the speculations of John Mayow, but he did not believe that combustion results from the activity of some nitro-aerial spirit. Candles and matches cease to burn not because they have rendered the air “effete, by having consumed its vivifying spirit,” but because of “acid fuliginous vapours” that destroy the air’s elasticity. A continual supply of fresh elastic air is necessary to produce the rapid intestine motion of the fuel; this motion is the result of the “action and reaction” of acid sulfurous particles and the elastic particles of air. “Air cannot burn without sulphur, so neither can sulphur burn without air.”

Despite the limitations of his achievement—he had prepared a number of gases without recognizing their differences—Hales passed on to the eighteenth century the conviction that there was such a thing as “fixed air” and that it abounds in all sorts of animal, vegetable, and mineral substances. Air is “very instrumental in the production and growth of animals and vegetables,” serving in its fixed state as the bond of union “and firm connection of the several constituent parts” of bodies, that is, the chief elements or principles of which things are made: “their water, salt, sulphur and earth,” He concluded that air should take the place of “mercury” or “spirit” as a fifth element:

Since then air is found so manifestly to abound in almost all natural bodies; since we find it so operative and active a principle in every chymical operation.... may we not with good reason adopt this now fixt, now volatile Proteus among the chymical principles, and that a very active one, as well as acid sulphur; notwithstanding it has hitherto been overlooked and rejected by Chymists, as no way intitled to that denomination?⁷⁶

For Hales, science was more than the avocation of a country minister: it was a natural extension of his religious life. If he was a devotee of the mechanistic world view and held that the living organism was a self-regulating machine, this was in no way incompatible with his faith. For him, as for many other “physical theologians,” nature testified to the wisdom, power, and goodness of the all-wise Creator “in framing for us so beautiful and well regulated a world.”⁷⁷

But Hales never doubted what Robert Boyle called “the usefulness of experimental philosophy.” Hales’s study of plants would, he was confident, improve man’s skill in “those innocent, delightful and beneficial arts” of agriculture and gardening. He was well aware, too, that his studies of the animal vascular system and respiration would prove of medical value. Like Benjamin Franklin, one of the many who read the Statical Essays and were influenced by them, he was constantly alert to the practical possibilities of his discoveries. In describing his perfusion experiments on animals, Hales took occasion to warn the heavy imbibers of spirituous liquors of the consequences of their vice. Indeed, he soon directed two pamphlets against this growing evil “the Bane of the Nation,” and, according to Gilbert White, was instrumental, under the patronage of Sir Joseph Jeckyll, in securing the passage of the Gin Act of 1736 “and stopping that profusion of spiritous liquors which threatened to ruin the morals and the constitution of the common people.”

In the Vegetable Staticks he had described an ingenious mercury gauge used to determine the pressure exerted by peas expanding in water, and this led him to imagine its adaptation as a “sea gage” to measure the depths of the ocean. He applied his chemical knowledge to suggesting ways of keeping water sweet during long sea voyages and exploring the obstinate problem of distilling fresh from salt water.

With the publication of his Haemastaticks, Hales’s career in pure science came to a close. From 1733 to the end of his life he devoted himself to applying scientific knowledge, technical skill, and his rich inventiveness to alleviating human problems, both medical and social. But even earlier he had turned his attention to a problem which had long challenged the resources of the medical profession: the painful affliction of kidney and bladder stones.

Early in 1727, while the Vegetable Staticks was in press, he obtained a specimen of such a human calculus from a friend, the famous surgeon John Ranby. On distilling this stone. Hales collected a much greater proportion of air than he had obtained from any other substance. Since various chemical agents were known to release this “strongly attracting, unelastic air,” he thought it at last possible to find a solvent to dissolve the calculi and obviate the painful operation of being “cut for the stone.” He carried out a number of experiments and published the results with his Haemastaticks. His attempts to find a useful solvent failed, and the paper is noteworthy chiefly for his success in perfusing a dog’s bladder with one of his solutions and for his invention of a surgical forceps, which Ranby and other surgeons promptly used with success to remove stones from the human urethra. Ironically, it was for this largely useless work on human calculi—not for the remarkable experiments on plants and animals and on air published in the Statical Essays—that Hales was awarded the Royal Society’s Copley Medal in 1739.

His newly acquired expertise entangled Hales in a rather notorious episode.⁷⁸ A Mrs. Joanna Stephens had for some years been treating victims of the stone with a secret proprietary remedy, supposedly with some success. Attempts to persuade her to divulge her secret led Parliament to vote a substantial reward and to set up a group of trustees to receive her disclosure and evaluate the effectiveness of her nostrum. Hales was one of the trustees, and he set to work to determine the effective ingredient in the odd mixture. Experiments convinced him that it was the lye used in soapmaking, and lime from eggshells used in her formula, that seemed to have the desired property of dissolving the stone. The result was Hales’s suggestion—destined to be taken up by others—that limewater might prove an effective if somewhat corrosive remedy.

Hales’s experiments on air and respiration were the stimulus for the invention that more than any other contributed to his contemporary fame: the ventilators he contrived to remove fetid air from prisons, hospitals, and slave ships. His experiments— especially the rebreathing experiments—had convinced him that “elastic” air, free from noxious fumes, was necessary for respiration, for there was great danger in respiring “vitiated air.” These theories fitted well with the current belief that many diseases were attributable to bad air and “miasmas.” After a victory over a rival inventor, Hales’s ventilators were installed in His Majesty’s ships, in merchant vessels, in slave ships, and in hospitals and prisons. The ventilators did not, of course, eliminate airborne bacterial or viral diseases, but they seem to have markedly reduced mortality rates. As one of the first to call attention to the importance of fresh air, Hales deserves his reputation as a pioneer in the field of public health.

These varied activities did not interfere with his parish duties. He preached regularly and presided with some severity over the morals of his village; he enlarged the churchyard and virtually rebuilt the old church. In 1754 Hales engineered a new water supply for the village and, as Francis Darwin remarks, “characteristically records, in the parish register, that the outflow was such as to fill a two-quart vessel in 3 swings of a pendulum, beating seconds, which pendulum was 39 + 2/10 inches long from the suspending nail to the middle of the plumbet or bob.”⁷⁹

Hales’s later years were graced with honors. Oxford conferred on him the doctorate of divinity in 1733. He was one of the trustees of the Georgia colony; and John Ellis, the merchant-naturalist who was governor of the colony and a correspondent of Linnaeus, named after him a genus of American flowering shrubs (Halesia). Hales was one of the founders of what is now the Royal Society of Arts and became one of its vice-presidents in 1755. In 1753 he was chosen a foreign associate of the Paris Academy of Sciences, replacing Hans Sloane, who had died earlier that year. Hales’s portrait was painted by Francis Cotes and by his neighbor at Twickenham, the popular Thomas Hudson.

He had many acquaintances in the neighborhood, among them Alexander Pope (he was one of the witnesses to Pope’s will) and Horace Walpole (who called him “a poor, good, primitive creature”). He was patronized by Frederick Louis and Augusta, the prince and princess of Wales, who lived not far distant at Kew. The prince, it is said, enjoyed surprising Hales in his laboratory at Teddington.

Walpole’s unflattering description bears out the opinion of contemporaries, who spoke of Hales’s native innocence and simplicity of manner. Peter Collinson testified to “his constant serenity and cheerfulness of mind.” He died after a brief illness and was buried under the tower of his beloved church. A monument in Westminster Abbey was erected to his memory by the princess of Wales, with a bas-relief of “the old philosopher” in profile. If there is anything in the church at Teddington recalling Hales to memory, the guidebooks make no mention of it. Instead, they single out a monument to Hales’s most famous (and notorious) parishioner, the actress Peg Woffington.

NOTES

1. J. F. Fulton, Selected Readings. 2nd ed., p. 57.

2. Gentleman’s Magazine, 34 (1764), 273. This article by Peter Collinson seems to have been based on information supplied by Stukeley. It is reproduced in Annual Register (1764), “Characters,” pp. 42–49.

3. G. M. Trevelyan Trinity College (Cambridge, 1946), p. 55. Cf. James Henry Monk, Life of Richard Bentley, 2nd ed., 2 vols, (London, 1833), I, 204.

4. Family Memoirs of the Rev. William Stukeley, M.D., 3 vols. (London—Edinburgh, 1882–1887). 1, 2l Henceforth referred to as Family Memoirs.

5. Ibid. pp. 21–22.

6. Robert Smith, ed., Hydrostatical and Pneumatical Lectures by Roger Cotes (Cambridge, 1738; 2nd ed., 1747). These posthumously published lectures, as we have them, were delivered after 1706 (Cotes refers to Newton’s Latin Optice of that year) and perhaps before 1710 (1 Coles died in 1716. Whiston’s lectures on this subject were never published.

7. Family Memoirs. I, 21, where we read: “Mr. Danny read to us... Pardies Geometry, Tacquets Geometry by Whiston. Harris’s use of the Globes, Rohaults Physics by Clark. He read to us Clarks 2 Volumes of Sermons at Boyles Lectures, Varenius Geography put out by Sr, Isaac Newton & many other occasional peices [sic] of Philosophy, & the Sciences subservient thereto.”

8. A. Hastings While, ed., Memoirs of Sir Isaac Newton’s Life by William Stukeley (London, 1936), p.9. See also Family Memoirs, I, 23–24.

9. The sketch is reproduced in A. E Clark-Kennedy, Stephen Hales (p1. IV), and in R. T. Gunther, Early Science in Cambridge (Oxford, 1937), p. 160. Stukeley says Hales “first projected, & gave the idea of horarys.” Family Memoirs. I, 21. The name “orrery” was attached to such devices after the one later built by John Rowley for his patron, the fourth earl of Orrery.

10. After earning the degree of bachelor of medicine from Cambridge, Stukeley studied “the practical part of physick” under Richard Meade at St. Thomas’ Hospital; early in 1717 he opened his own London practice.

11. On 6 Mar. 1717/18; see Royal Society Journal Book, V (1714–1720), 235. stukeley, although formally elected the same day as Hales, had been nominated much earlier by Edmond Halley, and his nomination evidently approved by the Council.

12. Vegetable Staticks (1727), p. iii. Hales, probably writing his preface late in 1726, states that this accidental observation occurred “about seven years since.”

13. “Mr. Hale [sic] having been formerly Elected, and lapsed the time of his admission, the same was dispensed with by the Society, and he Subscribed the Obligation and was admitted accordingly.” Journal Book, V, entry of 20 Nov, 1718 (O.S.), pp. 250–251.

14. Ibid., p. 289. A good summary of the experiment is here transcribed.

15. Ibid., VI (1720–1726), 438–440 et seq.

16. Ibid., VII (1726–1727). 44–45, 48–50.

17. Philosophical Transactions of the Royal Society, 35 , no. 398, 264–291; no. 399, 323–331. In Apr. and May 1727. Desaguliers repeated before the Royal Society certain of Hales’s experiments. Journal Book, VII, 74, 83.

18. “Of Mechanical Knowledge, and the Grounds of Certainty in Physick,” in his trans, of Santorio’s Medicina statica, 2nd ed. (London, 1720), p. 1.

19. An Account of Animal Secretion, the Quantity of Blood in the Human Body, and Muscular Motion (London, 1708). James Keill was strongly influenced by his older brother, the mathematician and Newtonian disciple John Keill.

20. Vegetable Staticks (1727), pp. 2–3.

21. Cusa’s De staticis, one of the Idiota dialogues, appeared in many eds., sometimes appended to eds. of the De architectura of Vitruvius. The English translation is The Idiot in Four Books; the First and Second of Wisdome, the Third of the Minde, the Fourth of Statick Experiments, or Experiments of the Ballance. By the Famous and Learned C. Cusanus (London, 1650).

22. Medicina statica: Being the Aphorisms of Sanctorius, Translated Into English With Large Explanations (London, 1712). The popular work was, of course, known in a number of Latin eds., some with commentaries by Giorgio Baglivi and Martin Lister. In a preface to the vol. of Philosophical Transactions for 1669 we read “The Ingenious Sanctorius hath not exhausted all the results of Statical indications.” Philosophical Transactions of the Royal Society.4 , no. 45, 897. The Oxford English Dictionary gives as the earliest example of the word in English Sir Thomas Browne’s reference in the Pseudodoxia epidemica (1646) to “the statick aphorisms of Sanctorius.” Quincy’s was not the earliest English version: a trans. had been published by J. Davis (London, 1676).

23. James Keill, Tentamina medico-physica quibus accessit medicina statica britannica (London, 1718).

24. Haemastaticks (1733), preface.

25. Arturo Castiglioni. History of Medicine, E. B. Krumbaar, ed. and trans. {New York. 1941) p. 614. Malpighi was the first to observe the capillaries; J. L. M. Poiseuille studied blood viscosity and rate of flow and introduced the mercury manometer for the measurement of blood pressure.

26. Haemastaticks (1733), pp, 58–59.

27. Hales doubtless knew the passage in Francis Hauksbee’s preface to his Physic-Mechanical Experiments (1709), in which Hauksbee wrote that electricity may possibly explain “the Production and Determination even of Involuntary Motion in the Parts of Animals” for he quotes Hauksbee three times, but on other matters, in the “Analysis of Air.” He surely also knew the concluding passage of the General Scholium of Newton’s Principia, 2nd ed, (1713), in which Newton hints that “an electric and elastic spirit” may account for sensation and cause “the members of animal bodies [to] move at the command of the will, namely, by the vibrations of this spirit, mutually propagated along the solid filaments of the nerves, from the outward organs of sense to the brain, and from the brain to the muscles,”

28. Vegetable Staticks (1727), pp. 2–3.

29. Philosophical Transactions of the Royal Society, 3 , no. 40 (1668), 787–801.

30. The letter was communicated 10 June 1669 (O.S.) and published in Philosophical Transactions of the Royal Society, 4 no. 48, 963–965, See also Charles Raven, John Ray (Cambridge, 1950), pp. 187–188.

31. Thomas Birch, History of the Royal Society of London, 4 vols. (London, 1756–1757), II, 382.

32. The theory of sap circulation was advanced by Christopher Merret in 1664 and by Johann Daniel Major a year later. See Julius von Sachs, History of Botany, p. 456; and J. Reynolds Green, History of Botany in the United Kingdom, p. 76. This theory is clearly set forth by John Locke. See J. A. St. John, ed., Philosophical Works of John Locke, II (London, 1706), 487. Even later than Blair were the claims of a Mr. Fairchild in 1724 to have proved by experiments “a constant Circulation of the Sap in Trees and Plants.” Journal Book, VI, 377.

33. For the background of this experiment, suggested by Cusa in his De staticis, see Herbert M. Howe, “A Root of van Helmont’s Tree,” in Isis, 56 (1965), 408–419. See also A. D. Krikorian and F. C. Steward, “Water and Solutes in Plant Nutrition,” in BioScience, 18 (1968), 286–292.

34. “Some Thoughts and Experiments Concerning Vegetation,” in Philosophical Transactions of the Royal Society, 21 , no. 253 (1699), 193–227. The quotation is from p. 208. Hales, when describing an experiment on the imbibition by a spearmint plant growing in water, wrote; “I pursued this Experiment no farther, Dr. Woodward having long since... given an account... of the plentiful perspirations of this plant.” Vegetable Staticks (1727), p. 28.

35. Hales writes “by the favour of the eminent Mr. Wise,” Vegetable Staticks (1727), pp. 17–18. Hales owed something to his relations with “the skilful and ingenious Mr. Philip Miller” of the Chelsea Physic Garden and author of the popular Gardener’s Dictionary (1724). On Miller sec Green, op. cit., pp. 156–157 and passim.

36. Francis Darwin, Rustic Sounds, p. 126.

37. Vegetable Staticks (1727), p. 10.

38. Ibid., p. 103.

39. See, for example, Hales’s experiments VII and XXVIII, ibid., pp. 28–29, 90.

40. 40. Ibid., p. 136. See also pp. 13–14, where he writes that the sap has “probably only a progressive and not a circulating motion as in animals.”

41. Ibid., p. 96.

42. Ibid., p. 100.

43. Ibid., pp. 329–337. He was struck, as Sachs observes, by the fact that the longitudinal growth allows the capillary vessels to retain their hollowness, as when glass tubes are drawn out to fine threads.

44. Ibid., pp. 102–103, 148. For the inconclusive experiment see experiment CXXII in the chapter “Of Vegetation.” After the publication of the Vegetable Staticks Hales repeated the experiment and convinced himself that leaves imbibe air. He informed Desaguliers of these results by June 1727. See Desaguliers’ postscript to his abstract of Hates’s book in Philosophical Transactions of the Royal Society, 35 , no. 399, 331.

45. Vegetable Staticks (1727), pp. 155–156.

46. The prevailing view in the seventeenth century (of men like Jean Beguin, Lemery, and Homberg) was that there are five elements or principles: three active principles (variously described as spirit, oil, and salt or as mercury, sulfur, and salt) and two passive ones, water and earth. This was clearly a compromise between the Aristotelian theory of the four elements and the tria prima of the Paracelsians. For a clear statement of this view in Hales’s day, see John Harris, Lexicon technicum (1704), article “Principle.”

47. Henry Guerlac, “John Mayow and the Aerial Nitre,” in Actes du Septième Congrès d’histoire des sciences (Jerusalem, 1953), pp. 332–349; and “The Poet’s Nitre,” in Isis, 45 (1954), 243–255.

48. Robert Boyle, General History of Air (London, 1692), p. l. See also Harris, Lexicon technicum (1704), article “Air”; and Cotton Mather, The Christian Philosopher (London, 1721), p. 65.

49. When the experiment on powdered oyster shells was shown to the Society on 15 Mar. 1664/65, the air was collected in a deflated bladder. But when it was repeated a short time later, a large glass filled with water was inverted (“whelmed”) over the reactants; and when the reaction was over, it was found that the “whelmed glass” was about a quarter full of an aerial substance. Birch, op. cit., II, 22, 27. This early anticipation of the principle underlying the pneumatic trough seems to have escaped notice. It is not mentioned in John Parascandola and Aaron J. Ihde, “History of the Pneumatic Trough,” in Isis, 60 (1969), 351–361.

50. Philosophical Transactions of the Royal Society, 18 , no. 212, 212–213.

51. Smith, ed., Hydrostatical... Lectures by Roger Cotes, 2nd ed., pp. 220–223; and Isaac Newton, Optice (1706), p. 325, and Opticks (1718), p. 353. Hales quotes the experiment in which Slare distilled or “calcined” an animal calculus and found that the greatest part of this stone “evaporated in the open fire.” Vegetable Staticks (1727), pp. 188–189.

52. Query 30 of Opticks (1718), pp. 349–350; and Vegetable Staticks (1727), p. 312. Hales also quotes (ibid., p. 165) from another long passage of the Opticks in which Newton speaks of airs formed from those bodies “which Chymists call fix’d, and being rarefied by Fermentation, become true permanent Air. “Query 31, Opticks (1718), p. 372. Newton and Hales both use the word “fermentation” to mean chemical reactions that are accompanied by the production of heat and ebullition. The term originated with Thomas Willis in his De fermentatione sive De motu intestino particularum in quovis corpore (London, 1659).

53. De la nature, vertu, et utilité des plantes (Paris, 1628), p. 75; see also pp. 94–95.

54. The History of the Propagation & Improvement of Vegetables (Oxford, 1660), pp. 40–42, 84–85. Robert Sharrock, an Oxford graduate who became archdeacon of Winchester, supplied prefaces to three of Boyle’s works. His book is dedicated to Boyle.

55. Birch, op. cit., I, 304.

56. Ibid., II, 54, 164; III, 418, 420–421.

57. Philosophical Transactions of the Royal Society, 3 no. 42, 854.

58. The Wisdom of God Manifested in the Works of the Creation, 8th ed. (London, 1722), p. 72. Mather, op. cit., p. 69, was clearly paraphrasing Ray when he wrote “Yea, Malpighius has discovered and demonstrated, that the Plants themselves have a kind of Respiration, being furnished with a Plenty of Vessels for the Derivation of Air to all their Parts.”

59. Vegetable Staticks (1727), p. 156. Boyle’s experiments, carried out with Denis Papin, using the latter’s improved air pump, were published in Boyle’s A Continuation of New Experiments..., which appeared in Latin in 1680 and in English in 1682.

60. For one such Cambridge experiment see Vegetable Staticks (1727), p. 195.

61. Hales also cited Hermann Boerhaave’s New Method of Chemistry. An unauthorized version of Boerhaave’s lectures had appeared in Latin in 1724; Hales seems to have used the English translation by Peter Shaw and E. Chambers, dated 1727. His references to it in the Vegetable Staticks were obviously added while his book was in press.

62. Although he records the combustibility of the gases produced by distilling peas, he failed to note the same property in coal gas. In describing the air produced by the action of dilute acid on iron filings (that is, hydrogen), he does not remark that it is inflammable.

63. Vegetable Staticks (1727), p. 291. Newton’s paper was published by John Harris in the introduction to his Lexicon technicum, II (1710), where Hales consulted it.

64. “And Dr. Freind has from the same principles [as Newton] given a very ingenious Rationale of the chief operations in Chymistry,” Vegetable Staticks (1727), preface, p. v.

65. Ibid., p. 165. Cf. Newton, Opticks (1718), p. 372.

66. The bolthead was a chemist’s globular flask with a long cylindrical neck, what Boyle called a “glass egg with a long neck.” It was named for its resemblance to the head of a bolt or arrow.

67. Vegetable Staticks (1727), p. 287.

68. The method of collecting air by the displacement of water, used in all Hales’s devices, was not original with him. It had been used as early as 1665, probably by Robert Hooke, in an experiment performed at the Royal Society. But it was doubtless from John Mayow that Hales learned of this method; Mayow used it extensively in his Tractatus quinque medicophysici (1674) and illustrated several modifications in an accompanying plate.

69. Henry Guerlac, “Joseph Black and Fixed Air,” in Isis, 48 (1957), 435 and n. 141.

70. Vegetable Staticks (1727), p. 223.

71. Henry Guerlac, “Continental Reputation of Stephen Hales,” Archives internationales d’histoire des sciences, 4 , no. 15 (1951), 396–397. See also Parascandula and Ihde, op. cit., p. 355.

72. Hales (Vegetable Staticks [1727], p. 266) compared his results with the observations of Francis Hauksbee, who had noted the same effect. See Hauksbee’s Physico-Mechanical Experiments on Various Subjects (1709), p. 83

73. Vegetable Staticks (1727), p. 231.

74. Ibid., p. 183.

75. Ibid., pp. 272–275, 278–285.

76. Ibid., pp. 315–316.

77. For Hales’s “argument from design” to justify his scientific work, see in particular his eloquent preface to the Haemastaticks (1733).

78. For a detailed account of this episode, and its later influence on the work of Joseph Black, see Henry Guerlac, “Joseph Black and Fixed Air,” 137–151.

79. “Hales, Stephen,” in Dictionary of National Biography.

BIBLIOGRAPHY

I. Original Works. Hales’s major writings in English are Vegetable Staticks: Or, an Account of Some Statical Experiments on the Sap in Vegetables... Also, a Specimen of an Attempt to Analyze the Air... (London, 1727), also repr. with a useful foreword by M. A. Hoskin (London, 1961); Statical Essays: Containing Vegetable Staticks (London, 1731), the 2nd ed., “with amendments”; Statical Essays, 2 vols. (London, 1733): vol. I is the 3rd ed. of Vegetable Staticks, and vol. II is the 1st ed. of Haemastaticks; or an Account of Some Hydraulic and Hydrostatical Experiments Made on the Blood and Blood-Vessels of Animals, with a separate preface, Hales’s “Account of Some Expenments on Stones in the Kidnies and Bladder,” an appendix with nine “Observations” relating to the motion of fluids in plants, seven additional experiments on air, and “Description of a Sea-gage, Wherewith to Measure Unfathomable Depths of the Sea"—vol. II repr. in facsimile as no. 22 in History of Medicine Series of the Library of the New York Academy of Medicine (New York, 1964), with a short introduction by Andre Cournand, M.D.

Statical Essays, 2 vols. (London, 1738–1740), vol. I is 3rd ed. of Vegetable Staticks and vol. II is 2nd ed., “corrected,” of Haemastaticks; and Statical Essays, 2 vols. (London, 1769), vol. I is 4th ed. of Vegetable Staticks, and vol. II is 3rd ed. of Haemastaticks.

Translations of Hales’s major works are La statique des végétaux, et l’Analyse de l’air..., G. L. L. Buffon, trans. (Paris, 1735), an influential French trans. which has the famous “Préface du traductcur,” in which Buffon praises the experimental method, and includes Hales’s appendix of 1733; Haemastatique, ou la statique des animaux (Geneva, 1744), the first French version of the Haemastaticks, translated by the physician and botanist Francois Boissier de Sauvages; Statique des végétaux, et celle des animaux, 2 pts. (Paris, 1779–1780); pt. I is Buffon’s trans. of the Vegetable Staticks “revue par M. Sigaud de la Fond,” and pt. II is Boissier de Sauvages’s trans. of the Haemastaticks; Sratick der Gewächse (Halle, 1747), translated, with a preface, by the philosopher Christian von Wolff; Statick des Geblüts, 2 pts. (Halle, 1748), pt. I is the Haemastaticks, and pt. II is Wolff’s translation of the Vegetable Staticks; Emastatica, ossia statica degli animali, 2 vols. (Naples, 1750–1752), Italian trans. from the French of Boissier de Sauvages, vol. II has a trans. of Hales’s work on bladder and kidney stones and two medical dissertations by Boissier de Sauvages; and Statica de’ vegetabili ed analisi dell’ ari, D. M. A. Ardinghelli, trans. (Naples, 1756, 1776), with commentary.

Hales’s minor works were A Sermon Preached Before the Trustees for Establishing the Colony of Georgia in America (London, 1734); A Friendly Admonition to the Drinkers of Brandy and Other Distilled Spirit (London, 1734), anonymous, but attributed to Hales; Distilled Spiritous Liquors the Bane of the Nation; Being Some Considerations Humbly Offered to the Hon. the House of Commons (London, 1736); Philosophical Experiments: Containing Useful and Necessary Instructions for Such as Undertake Long Voyages at Sea... (London, 1739); An Account of Some Experiments and Observations on Mrs. Stephens’s Medicines for Dissolving the Stone (London, 1740), also translated into French (Paris, 1742); A Description of Ventilators... (London, 1743), French trans. by P. Demours (Paris, 1744); An Account of Some Experiments and Observations on Tar-Water (London, 1745); Some Considerations on the Causes of Earthquakes... (London, 1750), French trans. by G. Mazeas (Paris, 1751), with the letter of the bishop of London. Thomas Sherlock, on the moral causes of the London earthquakes of 1750; A Sermon Before Physicians, on the Wisdom and Goodness of God in the Formation of Man (London, 1751), the annual Croonian sermon of the Royal College of Physicians, not the Croonian lecture of the Royal Society; An Account of a Useful Discovery to Distill Double the Usual Quantity of Sea-water... and an Account of the Great Benefit of Ventilators...(London, 1756); and A Treatise on Ventilators... (London, 1758).

II. Secondary Literature. General and biographical sources include the following (listed chronologically): “Some Account of the Life of the Late Excellent and Eminent Stephen Hales D.D., F.R.S. Chiefly From Materials Communicvated by P. Collinson, F.R.S.,” in Gentleman’s Magazine, 34 (1764), 273–278, see also Annual Register of World Events (1764), pp. 42–49; Jean-paul Grandjean de Fouchy, “Éloge de M. Hales,” in Histoire de l’Académie royale des sciences for 1762 (Paris, 1764), pp. 213–230; Robert Watt, Bibliographia britannica, 4 vols. (London, 1824), I, col. 457; F.D. [Francis Darwin], “Hales, Stephen,” in Dictionary of National Biography, an excellent summary; Francis Darwin, Rustic Sounds (London, 1917), pp. 115–139, a useful essay by a distinguished botanist; G. E. Burget, “Stephen Hales,” in Annals of Medical History, 7 (1925), 109–116; A. E. Clark-Kennedy, “Stephen Hales; Physiologist and Botanist,” in Nature, 120 (1927), 228–231; George Sarton, “Stephen Hales’s Library,” in Isis, 14 (1930), 422–423; A. E. Clark-Kennedy, Stephen Hales, D.D., F.R.S. An Eighteenth Century Biography (Cambridge-New York, 1929; repr. Ridgewood, N.J., 1965), the only full-length biography; Joeelyn Thorpe, “Stephen Hales,” in Notes and Records. Royal Society of London, 3 (1940), 53–63; and Lesley Hanks, Buffon avant l’“Histoire naturelle” (Paris, 1966), 73–101, which discusses Buffon’s translation of the Vegetable Staticks and the Newtonianism of Hales and Buffon.

On his work in public health, see D. Fraser Harris, “Stephen Hales, the Pioneer in the Hygiene of Ventilation,” in Scientific Monthly, 3 (1916), 440–454.

On animal physiology see the following (listed chronologically): John F. Fulton, Selected Readings in the History of Physiology (Springfield, III.-Baltimore, 1930), pp. 57–60, 75–79, 235, see also the greatly enl. ed., with material supplied by Leonard Wilson (Springfield, III., 1966); and Physiology, in the Clio Medica series (New York, 1931), pp. 35–36, 42–43; Thomas S. Hall, A Source Book in Animal Biology (New York, 1951), pp. 164–171, which reprints Hales’s preface to the Haemastaticks, without the concluding acknowledgment, and experiment I; and Diana Long Hall, “From Mayow to Haller: A History of Respiratory Physiology in the Early Eighteenth Century” (Ph.D. thesis, Yale University, 1966), pp. 118–121.

Hales’s work in plant physiology is discussed in the following (listed chronologically): Julius von Sachs, Geschichte der Botanik (Munich, 1875), pp. 514–521, 582–583, English trans. by Henry E. G. Garnsey, revised by I. B. Balfour, History of Botany (1530–1860) (Oxford, 1906), pp. 476–482, 539; J. Reynolds Green, A History of Botany in the United Kingdom (London, 1914), pp. 198–206 and passim; R. J. Harvey-Gibson, Outlines of the History of Botany (London, 1919), pp. 46–50 and passim; and Ellison Hawks and G. S. Boulger, Pioneers of Plant Study (London, 1928), pp. 228–230.

Hales’s chemistry is discussed in the following (listed chronologically): Hermann Kopp, Geschichte der Chemie, 4 vols. (Brunswick, 1843–1847), III, 182–183 and passim; Ferdinand Hoefer, Histoire de la chimie, 2nd ed., 2 vols. (Paris, 1869), II, 338–342; Henry Guerlac, “The Continental Reputation of Stephen Hales,” in Archices internationales d’histoire des sciences, 4 , no. 15 (1951), 393–404; Milton Kerker, “Hermann Boerhaave and the Development of Pneumatic Chemistry,” in Isis, 46 (1955), 36–49; Henry Guerlac, “Joseph Black and Fixed Air, A Bicentenary Retrospective,” in Isis, 48 (1957), 124–151, 433–456; Rhoda Rappaport, “G.-F. Rouelle: An Eighteenth-Century Chemist and Teacher,” in Chymia, 6 (1960), 94; Henry Guerlac, Lavoisier—The Crucial Year (Ithaca, N.Y., 1961), passim, for Hales’s influence upon Lavoisier; J. R. Partington, History of Chemistry, III (London, 1962), 112–123; and John Parascandola and Aaron J. Ihde, “History of the Pneumatic Trough,” in Isis, 60 (1969), 351–361

Henry Guerlac

Stephen Hales

1677-1761

English Physiologist and Clergyman

Although trained as a clergyman with no formal education in physiology, chemistry, or medicine, Stephen Hales became one of the most important British scientists of the eighteenth century. He can be considered the father of plant physiology, and his studies of circulation in animals were the most important after William Harvey's (1578-1657) over a century earlier. Hales conducted experiments on gases with equipment of his own design that influenced the later work of Joseph Black (1728-1799),Henry Cavendish (1731-1810), Joseph Priestley (1733-1804), and Antoine Lavoisier (1743-1794). He also did important research on respiration and in public health involving ventilation and fresh air.

Stephen Hales was born in Bekesbourne, county of Kent, England, on September 17, 1677, into an established, influential family. In 1696 he entered Benet College at Cambridge University, where he received a bachelor's degree and then a master's in 1703. In 1709 he was ordained as a priest in the Anglican Church and left Cambridge to become minister at Teddington, a small town on the Thames River between Twickenham and Hampton Court and about 15 miles (24 km) west of London. Hales kept this religious post until his death in January 1761, and did most of his research work at Teddington.

While at Cambridge Hales gained his first knowledge of scientific experimentation. In 1703 William Stukeley, later to become a well-known London physician, entered Benet College; and he and Hales developed a close friendship. In a room provided by Stukeley's tutor, the young men set up a laboratory where they conducted chemistry experiments and dissected small animals, including frogs. By 1707 Hales was attempting to measure blood flow in dogs. In these years before he left Cambridge, Hales also attended chemistry lectures by John Francis Vigani and his successor, John Waller.

After taking his religious post at Teddington, Hales spent several years attending to his parish duties before renewing his interest in science. By 1714 Hales began a study of the force of circulating blood—what we call blood pressure—in larger animals such as horses and deer. In 1719 he turned his attention to a study of the force of sap in plants. By this time Hales had been nominated to a fellowship in the Royal Society, a prestigious honor that validated his efforts. Membership in this body meant that oral accounts of Hales's research presented at the Society's meetings would be heard by the most important scientists and physicians in Britain at that time. Finally, Hales began to publish his work; in 1727 his book Vegetable Staticks; Or, an Account of Some Statical Experiments on the Sap in Vegetables was published in London. In 1733 Haemastaticks; Or, an Account of Some Hydraulic and Hydrostatical Experiments Made on the Blood and Blood-Vessels of Animals appeared.

These two works described in great detail Hales's meticulous experiments, which were characterized by his close observation and measurement. His most famous experiment is described in the first few pages of Haemastaticks. Hales tied down a horse and cut open an artery about three inches from the stomach. Into this artery he inserted a brass pipe only 0.167 inch (0.424 cm) in diameter. Then he used another brass tube fitted to the first one to attach a glass tube 9 feet (2.74 m) in length. Hales could then watch the mare's blood rise with each heartbeat over 8 feet (2.43 m) into the glass pipe. After these measurements Hales removed the glass tube and measured the blood released until the horse died. Then he studied the heart and surrounding arteries to see how much blood remained in the carcass. Hales repeated these experiments in cows, deer, dogs, and sheep, and carefully recorded the results. He also conducted other experiments to detect the flow of blood through the lungs; in describing the tiny blood vessels in the lungs, Hales was apparently the first person to use the term "capillary." Hales demonstrated the same curiosity and attention to detail in his studies of sap and water flow in plants; he invented the U-tube manometer to aid him in this research.

Between 1725 and 1727 Hales conducted important research on gases. In his plant experiments Hales had noticed bubbles of air that appeared when a plant stem was cut. Hales developed several apparatuses that would allow him to measure the amount of air absorbed or released when various animal, plant, or chemical compounds were heated and cooled. His most famous device was the pneumatic trough, with which he could collect different gases. Although Hales did not differentiate one gas from another, this research and the pneumatic trough became very useful to later chemists who did manage to isolate the various gases.

In 1739 Hales received the Copley Prize from the Royal Society, which recognized the importance of his scientific work. Six years earlier he had been awarded the Doctor of Divinity degree from Cambridge. In the 1750s Hales helped develop a channel and drainage system that supplied Teddington with fresh water. He was also interested in the problem of fresh air in confined spaces such as ships, hospitals, and prisons, and he developed a ventilator and bellows system to supply fresh air in these areas. His ventilation systems were installed in British navy warships and in several prisons, including the notorious Newgate. Hales's ventilators did not remove bacteria, which were unknown at the time, but mortality among the sailors and prisoners was reduced. In addition to his clerical duties and his scientific researches, Hales also served as one of the original trustees of the American colony of Georgia. Although he never visited America, Hales left his entire library to the colonies in his will. Unfortunately, the books disappeared and have never been located.

A. J. WRIGHT

Stephen Hales

The English scientist and clergyman Stephen Hales (1677-1761) pioneered the study of plant physiology, contributed the first major account of blood pressure, and invented a machine for ventilating buildings.

Stephen Hales was born in Bekesbourne, Kent, on Sept. 17, 1677. He entered Corpus Christi College, Cambridge, in 1697, where he studied theology and took a degree in arts in 1703. He received his doctorate from Cambridge in 1733. At Cambridge, Hales was caught up in the backwash of Isaac Newton's great work at the university, and he acquired an interest in astronomy, physics, and chemistry as well as in biology. In 1708 Hales became perpetual curate of Teddington in Middlesex, and here he remained until his death. In 1719 he married Mary Newce, who died 2 years later without issue.

In 1711 Hales began his studies on blood pressure. True to his mechanistic views, he carefully measured the blood pressure of three horses and produced the first recorded estimates of blood pressure. Furthermore, he studied the pulse rates of various-sized animals and measured the heart's capacity to pump blood through the pulmonary veins. Hales also studied the effects of heat, cold, and various drugs on the blood vessels and experimented with animal reflexes.

Even though some research had been carried out by Jan van Helmont and Marcello Malpighi, Hales rightfully merits the title of father of plant physiology. Certainly in a century given over almost exclusively to the taxonomy of Carl Linnaeus, Hales's work was unique. In 1718, the year he became a member of the Royal Society, he read a paper entitled "Upon the Effect of ye Sun's warmth in raising ye Sap in trees." He then carefully measured sap pressure, velocity, and circulation in plants. Until this time sap circulation in plants was believed to parallel blood circulation in animals. Hales, however, clearly demonstrated that the transpiration of leaves draws the sap toward them at the same rate that the roots push sap upward. Furthermore, he found that plants draw some of their food from the gases in the air. He invented the pneumatic trough for collecting gases and developed gages and techniques to measure sap pressure.

Hales published his findings in Vegetable Staticks (1727), reissued in 1733 as volume 1 of his Statical Essays. Volume 2 was Haemastatics, primarily a summary of his earlier work on blood circulation. For his work he received the Copley Prize in 1739.

Hales thought his invention of a ventilator was his greatest contribution to the well-being of mankind. As early as 1741 Hales presented to the Royal Society a description of a ventilator to rid mines, prisons, hospitals, and shops of noxious airs. He published A Description of Ventilators (1743) and A Treatise of Ventilators (1758). Hales also sought ways to distill pure water from seawater, preserve meat and water for long ocean voyages, preserve foods in tropical climates, measure earthquakes, and prevent forest fires.

In 1751 Hales became clerk of the closet of the princess dowager, and subsequently he was made chaplain to her and to her son, the future George III. Though offered the canonry of Windsor by the royal family, Hales maintained an active ministry at Teddington until his death on Jan. 4, 1761.

Further Reading

The best work on Hales is Archibald E. Clark-Kennedy, Stephen Hales:An Eighteenth Century Biography (1929). General background studies include Charles Singer and E. Ashworth Underwood, A Short History of Medicine (1928; 2d ed. 1962), and Abraham Wolf, A History of Science, Technology, and Philosophy in the Eighteenth Century (1938; 2d rev. ed. 1952). □

Hales, Stephen

English Physiologist 1677-1761

Stephen Hales was a preeminent scientist of the late eighteenth century and the founder of plant physiology . Born in Kent, England, in 1677, Hales grew up in an upper-class Kent family and was educated at Cambridge University. Though he received no formal training in botany during college, Hales obtained a solid background in science, including physics and mechanics. Upon graduation from Cambridge, Hales moved to Teddington, a town on the Thames River in England, where he lived the rest of his life.

Hales has been called the first fully deductive and quantitative plant scientist. He made many significant discoveries concerning both animal and plant circulation. Crucially, Hales measured plant growth and devised innovative methods for the analysis and interpretation of these measurements.

Hales's most original contribution was his transfer of application of the so-called statical method he and others had used on animals to plant specimens . The basis behind the statical method was the belief that the comprehension of living organisms was possible only through the precise measurements of their inputs and outputs. Thus the way to understand a human being would be to measure the fluids and other materials that had entered and left it. In the case of a tree, a statistician would measure changes in the amount and quality of the water it consumed and the sap it contained.

In 1706, under the influence of Isaac Newton's new mechanics, Hales tried to figure out the mechanism that controlled animal blood pressure by experimenting on dog specimens. At the same time, Hales had the idea that the circulation of sap in plants might well be similar to the circulation of the blood in humans and other animals. As he was exploring animal circulation, Hales grew increasingly interested in plant circulation. He wrote later in his book Vegetable Staticks of his first circulation experiments: "I wished I could have made the like experiments to discover the force of the sap in vegetables."

After a decade of quiet research and study, Hales did indeed devise such experiments on plants. He attached glass tubes to the cut ends of vine plants. He then watched sap rise through these tubes, and he monitored how the sap flow varied with changing climate and light conditions. In 1724, Hales completed Vegetable Staticks (quoted above), wherein he distinguished three different aspects of water movement in plants. These he called imbibition, root pressure, and leaf suction.

The prevalent notion among Hales's contemporaries was that the movement of plant sap was similar to the circulation of human blood, which was discovered by William Harvey in 1628. Crucially, Hales demonstrated that this theory was false. Instead, he demonstrated the constant uptake (absorption) of water by plants and water's constant loss through transpiration (evaporation into the air). Drawing on this principle, Hales made many exact and careful experiments using weights and measures. All of these he repeated using different types of plants (willows and creepers, for example) in order to verify his conclusions. Thus, from his beginnings as a physiologist, Hales went on to create a mechanics of water movement.

see also de Saussure, Nicholas; Physiologist; Physiology, History of; Water Movement.

Hanna Rose Shell

Bibliography

Isley, Duane. "Hales." One Hundred and One Botanists. Ames, IA: Iowa State University Press, 1994.

Morton, Alan G. "Hales." History of Botanical Science. London: Academic Press, 1981.