PHILOSOPHICAL SOCIETY OF WASHINGTON. 53 



as well as those which produce all the various subsequent trans- 

 formations, whether in plants or animals, of the organic matter 

 thus prepared. This general conception includes of course, in the 

 case of the higher animals, all the chemical phases of the processes 

 of digestion, assimilation and tissue-metamorphosis or metabolism, 

 including secretion and excretion ; in the case of the lower animals 

 and plants, so much of these several functions as belongs to each 

 species. 



Now please to understand that when I say I recognize all the 

 chemical phases of these processes to be the results of the ordinary- 

 chemical laws, I do not entertain any mental reservation with regard 

 to the unrestricted application of these laws. I cannot for a mo- 

 ment agree with those physiologists who have imagined the vital 

 principle to thwart, or interfere with, or counteract these laws in 

 any way. I know, indeed, that we are far from being as thoroughly 

 acquainted, as we may by and by hope to be, with the chemical 

 phenomena of living beings ; that many of the questions are very 

 difficult, so that as yet, with all our labor, we have obtained but 

 partial or even contradictory results ; but I find in this only a reason 

 for further investigation — no logical difficulty of a radical kind. 

 In a general way I recognize that the matter of which living beings 

 are composed is built up of elementary substances belonging to the 

 inorganic world, and that it consists of atoms possessed of the very 

 same properties, and obedient to the very same laws as like atoms 

 in inorganic bodies. Yet I confess I find in all this no reason for 

 denying the existence of a vital principle ; only I do not figure this 

 principle in my mind as a hostile power interfering in any way with 

 the chemical tendencies of the atoms present ; I liken its operations 

 rather to those of the chemist in his laboratory who obtains the 

 results he needs only on the condition of most rigid obedience to 

 chemical laws. 



Intimately associated with some of the chemical processes just 

 enumerated are those chemical processes of respii*atiou, in which 

 the chemical affinities of the oxygen of the atmosphere are directly 

 or indirectly the means of promoting tissue metamorphosis, as well 

 as of reducing at once to simpler forms some portion of the various 

 complex substances derived from the food. These chemical pro- 

 cesses are undoubtedly the chief original sources of the heat and 

 mechanical power manifested by animals. Of course they receive 

 heat also from without bv conduction and radiation : but this is a 



