
THE PRINCIPLE OF VITAL AFFINITY. 171 
be afterwards noticed, where we see chemical compounds uniformly formed in 
living bodies, quite distinct from any that can be formed by the chemist from the 
same elements, and quite distinct from those to which the same elements uni- 
formly revert, after the phenomena of life are over. 
Lastly, we must remember, when we see this apparent inversion or altera- 
tion of the ordinary chemical relations of matter, taking place in the interior of 
_ living bodies, that in that scene, by the admission of all, matter comes under the 
| dominion of mechanical laws, which operate in no other department of nature; 
so that it is quite conformable to analogy to suppose that its chemical relations 
will undergo a similar modification. 
When all these considerations are duly weighed, I cannot perceive what fur- 
ther evidence can be required in order to justify the expression which I have 
quoted from LiepiG, viz., that the “new combinations,’ as well as the forms, 
assumed by that matter which goes to the composition of organized beings, “ indi- 
| cate the existence of a power distinct from all other powers of nature, viz., the 
| vital principle ;” 2. ¢., that the vital principle regulates the changes of chemical 
| composition, as well as the changes of position which the particles of that matter 
' undergo; which is more simply expressed. by saying, that there are vital affinities 
as well as vital contractions and attractions. 
But even if we are to regard it as doubtful whether or not ordinary chemical 
_ affinities can determine, under any conditions, this decomposition of carbonic acid 
_ and evolution of oxygen by its contact with carbon and the elements of water, I 
| maintain that it is sound philosophy, when we see this and other rapid and exten- 
_ sive and important chemical changes, essentially different from those which the 
| same elements present under other circumstances, uniformly attending the pheno- 
mena of life in vegetables,—to investigate and generalize the laws by which these 
| changes are regulated, as laws of living action, leaving it open to future inquirers, 
| if they can, to resolve them into other laws of more general application. For 
| although I acknowledge the force of the aphorism, “ Frustra fit per plura quod 
| potest fieri per pauciora,” still I apprehend, that in every case to which this 
aphorism is applied, the potest jiert must be established, not by conjecture, but 
| by experiment; otherwise we fall into the error, so strongly condemned by Bacon 
| and others, of prematurely generalizing, and supposing the laws of nature to be 
| fewer and more comprehensive than they really are. 
| Having thus, in reference to this first and simplest example, vindicated the 
| soundness of the principle which I propose to illustrate, I think we may next 
| shew, that the main object of inquiry in the chemical department of physiology 
| is more simple and precise, and the extent of that inquiry, necessary to elucidate 
| most questions in physiology, much less than might be supposed from the multi- 
| plicity of details, of which what is called the science of organic chemistry is made 
