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XXI- — Observations on the Principle of Vital Affinity, as illustrated hy recent 

 discoveries in Oi^ganic Chemistry. By William Pulteney Alison, M.D., 

 F.R.S.E., Professor of the Practice of Medicine in the University of Edin- 

 burgh. 



(Read Ist and 15th February 1847.) 



Part II. 



It may be remembered that, in the paper formerly laid before this Society on 

 this subject, I endeavom-ed to establish the principle still disputed by some phy- 

 siologists, that the laws which regulate the chemical relations, as Avell as those 

 which regulate the visible movements of the particles of matter, undergo a cer- 

 tain determinate modification or change in living bodies, which is essential to the 

 commencement and to the maintenance of the organization of those bodies ; and 

 farther, that I undertook the task of attempting to deduce, from the numerous 

 but somewhat discordant experiments and observations lately made on the sub- 

 ject, certain inferences which appear to be well ascertained, although not gene- 

 rally admitted, as to the essential nature of this change, i. e., as to laws which 

 regulate those chemical actions which are peculiar to the state of life, and essen- 

 tial to the maintenance of organization, both in vegetables and animals. 



In confirmation of my statement of the general principle of Vital Affinity, as 

 distinguished from simply chemical affinities, I have much satisfaction in quot- 

 ing two sentences from the last edition of Liebig's " Animal Chemistry." Some of 

 the statements of general principles made by this author, seem to me open to ob- 

 jection, and some I do not profess to understand ; but the following is simple 

 and precise ; and, considering the authority of Liebig as a chemist, may, I thinli, 

 be held nearly decisive as to the soundness of the principle. " A fundamental 

 error, committed by some physiologists is, that they suppose the chemical and 

 physical forces alone, or in combination with anatomy, sufficient to explain the 

 phenomena of vitality. It is, indeed, difficult to understand how the chemist, 

 who is intimately acquainted with chemical forces, should recognise in the Uving 

 body the existence of new laws, of new^ causes, while the physiologist, who is 

 little or not at all familiar with the action and nature of chemical and physical 

 forces, should think himself ready to explain the same processes with the aid of 

 the laws of inorganic nature alone." — Animal Chemistry {third edition, p. 252.) 



The first and most fundamental of these general principles (likewise consi- 



" dered in my former paper) is the power of vegetable life, under the influence of 



light, to decompose the carbonic acid existing in the atmosphere, — set the oxygen 



free, fix the carbon, and form with it and the elements of water, starch, sugar, 



gum, and the analogous compounds. Our knowledge of this power, of the effects 



VOL. XVI. PAET III. 4 H 



