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. [135a] He has some idea of the instability of complex substances, and of the importance of the fact, for he says [135b] 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, before Vegetable Staticks.
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 green-houses, “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 [137a] planned a similar arrangement independently of Hales, and found it produced a marked improvement of the well-being of the plants.
It is worthy of note, 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 [137b] 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 made by fixing into a handle five pins 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 it, 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’ [138a] 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. [138b]
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 [138c] 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