4 HISTORICAL INTRODUCTION. 



ere we shall be in possession of anything like a full 

 knowledge of it, it will be at once obvious that the 

 very fundamental conditions for a solution of the 

 question were awanting. The beginning, then, of a 

 true scientific agricultural chemistry may be said to 

 date from the brilliant discoveries associated with 

 the names of Priestley, Scheele, Lavoisier, Cavendish, 



and Black that is, towards the close of last century. 



* 



Early Theories on Source of Plant-food. 



While this is so, and while we must regard the 

 early attempts made towards solving this question as 

 being, for the most part, of little scientific value, it 

 is not without interest, from the historical point of 

 view, to glance briefly at some of these old interesting 

 speculations. 



The Aristotelian doctrine, regarding the possibility 

 of dividing matter into the so-called four primary 

 elements, /ire, air, earth, and water, which obtained 

 in one form or another till the birth of modern 

 chemistry, had naturally an important influence on 

 these early theories. 



Van Helmont s Theory. 



Among the earliest and most important attempts 

 made to solve the problem of plant-growth was that 

 by Jean Baptiste Van Helmont, one of the best known 

 of the alchemists, who flourished about the beginning 

 of the seventeenth century. Van Helmont believed 



