ON THE CULTIVATION OF THE EARTH. 65 



" they assimilate it by means of peculiar organs, 

 " and it is by examining their physical and 

 " chemical constitution, and the substances and 

 " powers which act upon them ; and the modi- 

 " fications which they undergo, that the scientific 

 " principles of agricultural chemistry are ob- 

 " tained. 



" According to those ideas, it is evident 

 * that the study ought to be commenced by 

 " some general inquiries into the composition and 

 " nature of material bodies, and the laws of their 

 " changes. The surface of the earth, the atmo-^ 

 " sphere, and the water deposited from it, must, J 

 " either together or separately, afford all the 

 " principles concerned in vegetation j and it is 

 " only . by examining the chemical nature of 

 " these principles, that we are capable of dis- 

 " covering what is the food of plants ; and the 

 " manner in which this food is supplied and pre- 

 " pared for their nourishment ; the principle of 

 " the constitution of .bodies, consequently, should 

 " fi^Herthe first subject for our consideration. 



" By methods of analysis, dependent upon 

 " chemical and electrical instruments, discovered 

 " in late times, it has been ascertained that all 

 "the varieties of material substances may be 

 " resolved into a comparatively small number of 

 " bodies, which, as they are not capable of being 



