tions and suggestions on matters connected with health, agriculture, &c.
He was equally distinguished as a botanical and as an animal physiologist. His most important book, ‘Statical Essays,’ deals with both subjects. This book, founded chiefly on papers read before the Royal Society, was well received at the time, and was translated into French, German, Dutch, and Italian. It consists of two volumes, of which the first, dealing with plant-physiology, was published under the separate title of ‘Vegetable Staticks,’ in 1727.
The study of the anatomy of plants made, as Sachs points out, small advance during the eighteenth century, but there was a revival of plant-physiology, to which Hales's work was the most original and important contribution. Much of his work was devoted to the study of the loss of water which plants suffer by evaporation, and to the means by which the roots make good this loss. In these subjects many of his experiments remain of fundamental importance. With regard to the passage of water up the stems of trees it is worth notice that he made a suggestion which has quite recently, under different auspices, met with a good deal of approval, namely, that the ‘force is not from the roots only, but must proceed from some power in the stem and branches’ (Veg. Staticks, p. 110). It is especially characteristic of his work that he sought a quantitative knowledge of all the functions which he investigated. Thus he calculated the available amount of water in a given area of soil, and compared it with the loss of water due to the evaporation from the plants growing on that area. He also estimated the rain and dew fall from the same point of view; the variation in root force at different times of day; the force exerted by peas as they imbibe water and expand; the rate of growth of shoots and leaves by using the method still in use, of marking them at equal intervals.
With regard to the nutrition of plants in general he was far in advance of his age in two particulars: (1) He wrote well and clearly against the theory of the circulation of sap, then and long afterwards in vogue, a theory which rendered any advance in knowledge impossible; (2) finding that gas could be obtained from plants by dry distillation, he was led to believe that gas might be condensed or in some way changed into the substances found in plants. In thus recognising the fact that the air may be a source of food to plants, he was a forerunner of Ingen-Housz and De Saussure, the actual founders of the central principle of vegetable nutrition; but his views were not clearly enough elaborated or supported by experiment, and they failed to make much impression. He connected the assimilative function of leaves with the action of light, but, misled by the Newtonian theory as to the nature of light, he supposed that light, the substance, was itself a food.
The latter half of ‘Vegetable Staticks’ contains a mass of experiments on the gases which he distilled from various substances. He began the work in connection with his theory of the gaseous nutrition of plants, and seems to have been led on by its intrinsic interest. It led him to speculate on combustion and on the respiration of animals, and if his work had no direct chemical outcome, it prepared the way for the work of Priestley and others by teaching them how to manipulate gases by collecting them over water. His papers on sea-water and on the water of chalybeate springs also contain interesting chemical speculations.
Hales's contributions to animal physiology have been well summarised by Dr. Michael Foster: ‘He not only exactly measured the amount of blood pressure under varying circumstances, the capacity of the heart, the diameter of the blood-vessels and the like, and from his several data made his calculations and drew his conclusions, but also by an ingenious method he measured the rate of flow of blood in the capillaries in the abdominal muscles and lungs of a frog. He knew how to keep blood fluid with saline solutions, got a clear insight into the nature of secretion, studied the form of muscles at rest and in contraction, and speculated that what we now call a nervous impulse, but which was then spoken of as the animal spirits, might possibly be an electric change. And though he accepted the current view that the heat of the body was produced by the friction of the blood in the capillaries, he was not wholly content with this, but speaks of the mutually vibrating action of fluids and solids in a way that makes us feel that, had the chemistry of the time been as advanced as were the physics, many weary years of error and ignorance might have been saved.’ In first opening the way to a correct appreciation of blood pressure, Hales's work may rank second in importance to Harvey's in founding the modern science of physiology. In his work on animals and plants alike the value of what he did depends not merely on facts and principles established, but on his setting an example of the scientific method and his making widely appreciated a sound conception of the living organism as a self-regulating machine.
Hales's best known invention was that of
