Assimilation.
Hales' belief that plants draw part of their food from the air, and again that air is the breath of life, of vegetables as well as of animals (p. 148), are based upon a series of chemical experiments performed by himself. Not being satisfied with what he knew of the relation between "air" (by which he meant gas) and the solid bodies in which he supposed gases to be fixed, he delayed the publication of Vegetable Staticks for some two years, and carried out the series of observations which are mentioned in his title-page as "An attempt to analyse the air, by a great variety of chymio-statical experiments" occupying 162 pages of his book[46].
The theme of his inquiry he takes (Vegetable Staticks, p. 165) from "the illustrious Sir Isaac Newton," who believed that "Dense bodies by fermentation rarify into several sorts of Air; and this Air by fermentation, and sometimes without it, returns into dense bodies."
Hales' method consisted in heating a variety of substances, e.g. wheat-grains, pease, wood, hog's blood, fallow-deer's horn, oyster-shells, red-lead, gold, etc., and measuring the "air" given off from them. He also tried the effect of acid on iron filings, oyster-shells, etc. In the true spirit of experiment he began by strongly heating his retorts (one of which was a musket barrel) to make sure that no air arose from them. It is not evident to me why he continued at this subject so long. He had no means of distinguishing one gas from another, and almost the only quality noted is a want of permanence, e.g. when the CO2 produced was dissolved by the water over which he collected it. Sir E. Thorpe[47] points out that Hales must have prepared hydrogen, carbonic acid, carbonic oxide, sulphur dioxide, marsh gas, etc. It may, I think, be said that Hales deserved the title usually given to Priestley, viz. "the father of pneumatic[48] chemistry."
Perhaps the most interesting experiment made by Hales is the heating of minium (red-lead) with the production of oxygen. It proves that he knew, as Boyle, Hooke and Mayow did before him, that a body gains weight in oxidation. Thus Hales remarks: "That the sulphurous and aereal particles of the fire are lodged in many of those bodies which it acts upon, and thereby considerably augments their weight, is very evident in Minium or Red Lead which is observed to increase in weight in undergoing the action of the fire. The acquired redness of the Minium indicating the addition of plenty of sulphur in the operation." He also speaks of the gas distilled from minium, and remarks "It was doubtless this quantity of air in the minium which burst the hermetically sealed glasses of the excellent Mr Boyle, when he heated the Minium contained in them by a burning glass" (p. 287).
This was the method also used by Priestley in his celebrated experiment of heating red-lead in hydrogen; whereby the metallic lead reappears and the hydrogen disappears by combining with the oxygen set free. This was expressed in the language of the day as the reconstruction of metallic lead by the addition of phlogiston (the hydrogen) to the calx of lead (minium). Thorpe points out the magnitude of the discovery that Priestley missed, and it may be said that Hales too was on the track and had he known as much as Priestley it would not have been phlogiston that kept him from becoming a Cavendish or Lavoisier. What chiefly concerns us however is the bearing of Hales' chemical work on his theories of nutrition. He concludes that "air makes a very considerable part of the substance of Vegetables," and goes on to say (p. 211) that "many of these particles of air" are "in a fixt state strongly adhering to and wrought into the substance of" plants[49]. He has some idea of the instability of complex substances and of the importance of the fact, for he says[50] that "if all the parts of matter were only endued with a strongly attracting power, [the] whole [of] nature would then become one unactive cohering lump." This may remind us of Herbert Spencer's words: "Thus the essential characteristic of living organic matter, is that it unites this large quantity of contained motion with a degree of cohesion that permits temporary fixity of arrangement," First Principles, § 103. With regard to the way in which plants absorb and fix the "air" which he finds in their tissues, Hales is not clear; he does not in any way distinguish between respiration and assimilation. But as I have already said he definitely asserts that plants draw "sublimed and exalted food" from the air.
As regards the action of light on plants, he suggests (p. 327) that "by freely entering the expanded surfaces of leaves and flowers" light may "contribute much to the ennobling principles of vegetation." He goes on to quote Newton (Opticks, query 30): "The change of bodies into light, and of light into bodies is very conformable to the course of nature, which seems delighted with transformations." It is a problem for the antiquary to determine whether or no Swift took from Newton the idea of bottling and recapturing sunshine as practised by the philosopher of Lagado. He could hardly have got it from Hales since Gulliver's Travels was published in 1726, a year before Vegetable Staticks. Timiriazeff, in his Croonian Lecture[51], was the first to see the connexion between photosynthesis and the Lagado research.
Nevertheless Hales is not quite consistent about the action of light; thus (p. 351) he speaks of the dull light in a closely planted wood as checking the perspiration of the lower branches so that "drawing little nourishment, they perish." This is doubtless one effect of bad illumination under the above-named conditions, but the check to photosynthesis is a more serious result. In his final remarks on vegetation (p. 375) Hales says in relation to greenhouses, "it is certainly of as great importance to the life of the plants to discharge that infected rancid air by the admission of fresh, as it is to defend them from the extream cold of the outward air." This idea of ventilating greenhouses he carried out in a plant house designed by him for the Dowager Princess of Wales, in which warm fresh air was admitted. The house in question was built in 1761 in the Princess's garden at Kew, which afterwards became what we now know as Kew Gardens. The site of Hales' greenhouse, which was only pulled down in 1861, is marked by a big Wistaria which formerly grew on the greenhouse wall. It should be recorded that Sir W. Thiselton-Dyer[52] planned a similar arrangement independently of Hales, and found it produced a marked improvement of the well-being of the plants.
It is an illuminating fact that though Hales must have known Malpighi's theory of the function of leaves (which was broadly speaking the same as his own), he does not as far as I know refer to it. In his preface, p. ii, he regrets that Malpighi and Grew, whose anatomical knowledge he appreciated, had not "fortuned to have fallen into this statical[53] way of inquiry." I believe he means an inquiry of an experimental nature, and I think it was because Malpighi's theory was dependent on analogy rather than on ascertained facts, that it influenced Hales so little.
There is another part of physiology on which Hales threw light. He was the first I believe to investigate the distribution of growth in developing shoots and growing leaves by marking them and measuring the distance between the marks after an interval of time. He describes (p. 330) and figures (p. 344) with his usual thoroughness the apparatus employed: this was a comb-like object, shown in [Plate IX], made by fixing five pins into a handle, ¼ inch apart from one another: the points being dipped in red-lead and oil, a young vine-shoot was marked with ten dots ¼ inch apart. In the autumn he examined his specimen and finds that the youngest internode or "joynt" had grown most, and the basal part having been "almost hardened" when he marked, had "extended very little." In this—a tentative experiment—he made the mistake of not re-measuring his plants at short intervals of time, but it was an admirable beginning and the direct ancestor of Sachs'[54] great research on the subject.
In his discussion on growth it is interesting to find the idea of turgescence supplying the motive force for extension. This conception he takes from Borelli[55].
Hales sees in the nodes of plants "plinths or abutments for the dilating pith to exert its force on" (p. 335); but he acutely foresees a modern objection[56] to the explanation of growth as regulated solely by the hydrostatic pressure in the cell. Hales says (p. 335): "but a dilating spongy substance, by equally expanding itself every way, would not produce an oblong shoot, but rather a globose one."
It is not my place to speak of Hales' work in animal physiology, nor of those researches bearing on the welfare of the human race which occupied his later years. Thus he wrote against the habit of drinking spirits, and made experiments on ventilation by which he benefited both English and French prisons, and even the House of Commons; then too he was occupied in attempts to improve the method of distilling potable water at sea, and of preserving meat and biscuit on long voyages[57].
Plate IX
Plate 18 from Hales's Vegetable Staticks
Fig. 40. Instrument devised by Hales to make prick-marks on a young shoot of Vine (Fig. 41); the distribution of stretching after growth is shown in Fig. 42. The use of a similar instrument for marking surfaces is shown in Figs. 43 and 44
We are concerned with him simply as a vegetable physiologist and in that character his fame is imperishable. Of the book which I have been using as my text, namely, Vegetable Staticks, Sachs says: "It was the first comprehensive work the world had seen which was devoted to the nutrition of plants and the movement of their sap.... Hales had the art of making plants reveal themselves. By experiments carefully planned and cunningly carried out he forced them to betray the energies hidden in their apparently inactive bodies[58]." These words, spoken by a great physiologist of our day, form a fitting tribute to one who is justly described as the father of physiology.