7 
which ought to be understood and followed, and 
which, in their progression and application, can 
hardly fail to be highly beneficial to the commu- 
nity. 
A view of the objects in this Course of Lec- 
tures, and of the manner in which they are to be 
treated, will not, I hope, be considered as an impro- 
per introduction. It will inform you what you are 
to expect ; it will afford a general idea of the con- 
nection of the different parts of the subject, and of 
their relative importance ; 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, concerning what remains to 
be investigated and discovered. 
The phenomena of vegetation must be con- 
sidered as an important branch of the science of 
organized nature ; but though exalted above in- 
organic matter, vegetables are yet in a great mea- 
sure dependent for their existence upon its laws. 
They receive their nourishment from the external 
elements ; 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 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 mate- 
rial bodies, and the laws of their changes. The 
surface of the earth, the atmosphere, and the water 
deposited from it, must either together or sepa- 
b 4 < 
