THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 47 



The relation between active change of matter and re-active 

 genesis of molecular vibration, is clearly shown by the con- 

 trasts between different organisms, and between different 

 states and parts of the same organism. In plants the genesis 

 of heat is extremely small, in correspondence with their ex- 

 tremely small production of carbonic acid: those portions 

 only, as flowers and germinating seeds, in which considerable 

 oxidation is going on, having decidedly raised temperatures. 

 Among animals we see that the hot-blooded are those which 

 expend much force and respire actively. Though insects are 

 scarcely at all warmer than the surrounding air when they 

 are still, they rise several degrees above it when they exert 

 themselves; and in mammals, which habitually maintain a 

 temperature much higher than that of their medium, exertion 

 is accompanied by an additional production of heat. 



This molecular agitation accompanies the falls from un- 

 stable to stable molecular combinations; whether they be 

 those from the most complex to the less complex compounds, 

 or whether they be those ultimate falls which end in fully 

 oxidized and relatively simple compounds; and whether they 

 be those of the nitrogenous matters composing the tissues or 

 those of the non-nitrogenous matters diffused through them. 

 In the one case as in the other, the heat must be regarded as 

 a concomitant. Whether the distinction, originally 



made by Liebig, between nitrogenous substances as tissue- 

 food and non-nitrogenous substances as heat-food, be true or 

 not in a narrower sense, it cannot be accepted in the sense 

 that tissue-food is not also heat-food. Indeed he does not 

 himself assert it in this sense. The ability of carnivorous 

 animals to live and generate heat while consuming matter 

 that is almost exclusively nitrogenous, suffices to prove that 

 the nitrogenous compounds forming the tissues are heat-pro- 

 ducers, as well as the non-nitrogenous compounds circulating 

 among and through the tissues : a conclusion which is in- 

 deed justified by the fact that nitrogenous substances out of 

 the body yield heat, though not a large amount, during 



