XXYl INTRODUCTION. 



in ■vthich cellular or organized matter is generated 

 under the influence of light and heat, are but very 

 imperfectly explained. A knowledge of the chemical 

 composition of soils, and the various substances em- 

 ployed as manures, enables us to comprehend the 

 mode in which the latter act ; and a knowledge of 

 the nature of those substances which plants require, 

 points out the best and most economical methods of 

 restoring to the soil, by manures, those substances 

 which plants remove from it: but our knowledge of 

 this part of the subject is very far from being com- 

 plete; for although it is certain that, in addition to 

 the great elements of organic matter, which plants 

 derive from both air and soil, they likewise absorb 

 small quantities of inorganic or mineral substances 

 from the soil exclusively, the office performed by the 

 latter in the vegetable economy is not yet well under- 

 stood. Many theories, indeed, have been formed 

 respecting their use, but very little is positively 

 known on the subject. 



Although Agricultural Chemistry is in this imper- 

 fect state, and though much still remains to be done 

 in that branch of science, yet it is so far advanced as 

 to be able to render substantial assistance to the 

 practical agriculturist. It can teach him the princi- 

 ples which govern the growth of plants, and, conse- 



