That the phenomena of vegetation are dependent on certain chemical changes occurring in the plant, by which the various elements of its food are elaborated and converted into vegetable matter, was ver y early recognised by chemists; and long before the correct principles of that science were established, Van Helmont maintained that plants derived their nourishment from water, while Sir Kenelm Digby, Hook, Bradley, and others, attributed an equally exclusive influence to air, and enlarged on the practical importance of the conclusions to be deduced from their views. These opinions, which were little better than hypotheses, and founded on ver y imperfect chemical data, are mentioned by Jethro Tull, the father of modern agriculture, only to deny their accuracy ; and he contended that the plants absorb and digest the finer particles of the earth, and attributed the success of the particular system of husbandr y he advocated to the comminution of the soil, by which a larger number of its particles are rendered sufficiently small to permit their ready absorption by the roots