INTRODUCTION. XV 



of converting lead, iron, &c., or the base metals as 

 they called them, into gold. Hence, it is not to be 

 wondered at, that the chemistry of the metals was 

 studied and investigated, long before the nature and 

 properties of vegetable and animal substances were 

 examined. The labors of the alchemists brought to 

 light many valuable discoveries respecting the uses, 

 nature, and properties, of earthy and metallic sub- 

 stances; but it was only in the last century, when 

 the constitution of the air and other gaseous bodies 

 was discovered, that anything approaching to correct 

 ideas respecting the nature of animal and vegetable 

 substances was entertained. 



The important discoveries of Dr. Priestley led the 

 way to a complete revolution in the science, and may 

 almost be said to have laid the foundation of agricul- 

 tural chemistry. It is true that, before his time, 

 there had been many careful and accurate observers, 

 and multitudes of laborious and valuable experiments 

 have been made on plants, by such men as Van Ilel- 

 mont — Evelyn — Boyle — Hales, and others ; but, be- 

 fore the time of Priestley and his contemporaries, 

 Bonnet, Ingenhousz, Henry, and Percival, little pro- 

 gress had been made in studying the chemical 

 changes on which the growth of plants depends. We 

 are indebted to Hales for much curious information 



