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 imder 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 Liebig, 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 ;" i. e., that the vital principle regulates the changes of chemical 

 composition, as weU 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 ordinai-y 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 fieri 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 the}'' 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 



