INTRODUCTION.

IX

chemistry of the metals was studied and investigated,long before the nature and properties of vegetable andanimal substances were examined. The labours of thealchemists brought to light many valuable discoveriesrespecting the uses, nature, and properties of earthyand metallic substances ; but it was only in the lastcentury, when the constitution of the air, and othergaseous bodies was discovered, that anything approach-ing to correct ideas respecting the nature of animal andvegetable substances, was entertained.

The important discoveries of Dr. Priestley led the wayto a complete revolution in the science, and may almostbe said to have laid the foundation of agricultural che-mistry. It is true that before his time there had beenmany careful and accurate observers, and multitudes oflaborious and valuable experiments had been made onplants, by such men as Yan Helmont—Evelyn—Boyle—Hales, and others ; but before the time of Priestley, andhis contemporaries, Ingenhousz and Bonnet, little pro-gress had been made in studying the chemical changes onwhich the growth of plants depends. We are indebtedto Hales for much curious information respecting therise and motion of the sap in plants, the perspiration,or evaporation, which is constantly going on to a greateror less extent, from the surface of the leaves; and the