C 1 3 



it will afford a general idea of the connexion of the 

 different parts of the subject, and of their relative im- 

 portance ; it will enable me to give some historical 

 details of the progress of this branch of knowledge, 

 and to reason from what has been ascertained, con- 

 cerning what remains to be investigated and disco- 

 vered. 



The phenomena of vegetation must be consider- 

 ed as an important branch of the science of organized 

 nature ; but though exalted above inorganic matter, 

 vegetables are yet in a great measure dependent for 

 their existence upon its laws. They receive their nour- 

 ishment from the external elements ; they assimilate 

 it by means of peculiar organs ; and it is by examin* 

 ing their physical and chemical constitution, and the 

 substances and powers which act upon them, and the 

 modifications which they undergo, that the scientific 

 principles of Agricultural Chemistry are obtained. 



According to these ideas, it is evident that the 

 study ought to be commenced by some general en- 

 quiries into the composition and nature of material 

 bodies, and the laws of their changes. The surface 

 of the earth, the atmosphere, and the water deposited 

 from it, must either together or separately afford all the 

 principles concerned in vegetation ; and it is only by 

 examining the chemical nature of these principles, 

 that we are capable of discovering what is the food of 

 plants, and the manner in which this food is supplied 

 and prepared for their nourishment. The principles 

 of the constitution of bodies, consequently, will form 

 the first subject for our consideration, 



