Common observation taught men in very early times that green plants draw nourishment from the soil, and that sunlight is necessary to their health. In the age of Galileo a Belgian physician and chemist, Van Helmont, endeavoured to pursue the subject by experiment. He planted the stem of a live willow in furnace-dried earth, which was enclosed in an earthen vessel. Rain-water or distilled water was supplied when necessary, and dust excluded by a perforated lid. The loss of weight due to the falling-off of leaves was neglected. In the course of five years the tree was found to have increased to more than thirty times its original weight; Van Helmont concluded that this increase was due to water only. Malpighi (1671), being guided mainly by his microscopic studies of the anatomy of the stem and leaf, taught that moisture absorbed by the roots ascends by the wood, becoming (apparently at the same time) aerated by the large, air-conducting vessels; that it enters the leaves, and is there elaborated by evaporation, the action of the sun's rays, and a process of fermentation; lastly, that the elaborated sap passes from the leaves in all directions towards the growing parts. It will be seen that this explanation, though incomplete, makes a fair approximation to the beliefs now held; for more than a hundred years after Malpighi's day less instructed opinions were commonly held. Hales (1727) recognised that green plants are largely nourished at the expense of the atmosphere; he dwelt also on the action of the leaves in drawing water from the soil, and in discharging superfluous moisture by evaporation. Joseph Priestley, who had been proving that air is necessary both to combustion and respiration, made an experiment in 1771 to discover whether plants affected air in the same way that animals do. He put a sprig of mint into a vessel filled with air in which a candle had burned out, and after ten days found that a candle would now burn perfectly well in the same air. Air kept without a plant, in a glass vessel immersed in water, did not regain its power of supporting combustion. Balm, groundsel, and spinach were found to answer just as well as mint. Air vitiated by the respiration of mice was restored by green plants as readily as air which had been vitiated by combustion.