Dr Alison on the Principle of T^ital Affinity. 141 



tions, 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 extensive and important chemical changes, 

 essentially different from those which the same elements pre- 

 sent under other circumstances, uniformly attending the phe- 

 nomena 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 ap- 

 plied, 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 gene- 

 ralizing, 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 ex- 

 ample, vindicated the soundness of the principle which I pro- 

 pose 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, ne- 

 cessary to elucidate most questions in physiology, much less 

 than might be supposed from the multiplicity of details, of 

 which what is called the science of organic chemistry is made 

 up. After what Liebig calls the " peculiar mode of attrac- 

 tion'* which operates in living bodies, has led to the formation 

 of certain organic compounds, these compounds lose their con- 

 nection with living bodies, become liable to an infinite number 

 of changes and decompositions, and thus give origin to an in- 

 finite variety of substances — generally of temporary duration 

 only, because retained in their form by attractions of no great 

 intensity — applicable to many useful purposes, but foreign to 

 the inquiries of the physiologist. He is concerned only with 

 the chemical changes which take place in living bodies them- 

 selves^ and during the state oj life ; and the results of recent 

 inquiries seem to me sufficient to shew, that the fundamental 



