240 
in relation to the instruments by which they are per- 
formed, and the living power necessary to their exe- 
cution, they do not fall to be considered as objects of 
that science. To conclude, that, because carbon is 
retained in the plant, it is at once assimilated and ap- 
plied as food for its support, is to reduce the series 
of important changes, which terminates in vegetable 
nutrition, to one solitary chemical action, which we 
have seen to go on in circumstances and situations 
totally independent of the living powers and proper- 
ties of the plant. 
525. Neither because vegetables are chiefly compos- 
ed of oxygen, hydrogen and carbon, and because wa- 
ter and carbonic acid are composed of the same ele- 
ments, are we therefore entitled to suppose that these 
substances, by a chemical action upon each other, 
are, or can be, converted into vegetable matter, and 
disposed into that fine and varied structure which we 
call a plant. It is this structure itself which alone is ca- 
pable of executing these changes ; and the chemist can 
do no more than furnish the materials to carry them 
on, or, by varying the circumstances of the action, in 
some degree modify its result. The mere exertion 
of the affinities of bodies, be they what they may, 
can never compound an organic structure, whose 
distinctive character or essence depends, not on the 
nature of its substance or elements, but on the mode 
of their composition and arrangement, and on the 
properties which, as living matter, they acquire and 
possess. In physiology, therefore, it is necessary to 
consider, not merely the chemical nature of the bo- 
dies concerned, and of the combinations into which 
