The later discoverers in chemistry are of the following dates, Black 1728-1799, Cavendish 1731-1810, Priestley 1733-1804, Scheele 1742-1786, Lavoisier 1743, guillotined 1794. These were all born about the time of Hales' zenith, nor did he live[19] to see the great results they accomplished. But it should not be forgotten that Hales' chemical work made more easy the triumphant road they trod.
I have spoken of Hales in relation to chemists and physicists because, though essentially a physiologist, he seems to me to have been a chemist and physicist who turned his knowledge to the study of life, rather than a physiologist who had some chemical knowledge.
Whewell points out in his History of the Inductive Sciences[20] that the Physiologist asks questions of Nature in a sense differing from that of the Physicist. The Why? of the Physicist meant Through what causes? that of the Physiologist—To what end? This distinction no longer holds good, and if it is to be applied to Hales it is a test which shows him to be a physicist. For, as Sachs shows, though Hales was necessarily a teleologist in the theological sense, he always asked for purely mechanical explanations. He was the most unvitalistic of physiologists, and I think his explanations suffered from this cause. For instance, he seems to have held that to compare the effect of heat on a growing root to the action of the same cause on a thermometer[21] was a quite satisfactory proceeding. And there are many other passages in Vegetable Staticks where one feels that his speculations are too heavy for his knowledge.
Something must be said of Hales' relation to his predecessors and successors in Botanical work. The most striking of his immediate predecessors were Malpighi 1628-1694, Grew 1628-1711, Ray 1627-1705, and Mariotte (birth unknown, died 1684); and of these the three first were born one hundred years before the publication of Vegetable Staticks. Malpighi and Grew were essentially plant-anatomists, though both dealt in physiological speculations. Their works were known to Hales, but they do not seem to have influenced him.
We have seen that as a chemist Hales is somewhat of a solitary figure, standing between what may be called the periods of Boyle and of Cavendish. This is even more striking in his Botanical position, for here he stands in the solitude of all great original inquirers. We must go back to Van Helmont, 1577-1644, to find anyone comparable to him as an experimentalist. His successors have discovered much that was hidden from him, but consciously or unconsciously they have all learned from him the true method and spirit of physiological work.
It may be urged that in exalting Hales I am unfair to Malpighi. It may be fairer to follow Sachs in linking these great men together and to insist on the wonderful fact that before Malpighi's book in 1671, vegetable physiology was still where Aristotle left it, whereas 56 years later in 1727 we find in Hales' book an experimental science in the modern sense.
It should not be forgotten that students of animal physiology agree with botanists as to Hales' greatness. A writer in the Encyclopædia Britannica speaks of him as "the true founder of the modern experimental method in physiology."
According to Sachs, Ray made some interesting observations on the transmission of water, but on the whole what he says on this subject is not important. There is no evidence that he influenced Hales.
Mariotte the physicist came to one physiological conclusion of great weight[22]; namely, that the different qualities of plants, e.g. taste, odour, etc., do not depend on the absorption from the soil of differently scented or flavoured principles, as the Aristotelians imagined, but on specific differences in the way in which different plants deal with identical food material—an idea which is at the root of a sane physiological outlook. These views were published in 1679[23], and may have been known to Hales. He certainly was interested in such ideas, as is indicated by his attempts to give flavour to fruit by supplying them with medicated fluids. He probably did not expect success for he remarks, p. 360: "The specifick differences of vegetables, which are all sustained and grow from the same nourishment, is [sic] doubtless owing to the very different formation of their minute vessels, whereby an almost infinite variety of combinations of the common principles of vegetables is made." He continues in the following delightful passage: "And could our eyes attain to a sight of the admirable texture of the parts on which the specific differences in plants depends [sic] what an amazing and beautiful scene of inimitable embroidery should we behold? what a variety of masterly strokes of machinery? what evident marks of consummate wisdom should we be entertained with?" To conclude what has been said on Hales' chronological position—Ingenhousz, the chief founder of the modern point of view on plant nutrition, was born 1730 and published his book On Vegetables, etc. in 1779. So that what was said of Hales' chemical position is again true of him considered in relation to nutrition; he did not live to see the great discoveries made at the close of the 18th century.
There is in his writing a limpid truthfulness and simplicity, unconsciously decorated with pretty 18th century words and half-rusticities which give it a perennial charm. And inasmuch as I desire to represent Hales not merely as a man to be respected but also to be loved, it will be as well to give what is known of the personal side of his character before going on to a detailed account of his work.