Ill 



of animal heat and motion, which ascribes them to the chemical 

 action (principally oxidation, or a combustion at low temperatures), 

 experienced by the food. A principal object of the present com- 

 munication is to point out the relation of this theory to the dynami- 

 cal theory of heat. It is remarked, in the first place, that both 

 animal heat and weights raised or resistance overcome, are we- 

 chanical effects of the chemical forces which act during the combina- 

 tion of food with oxygen. The former is a dynamical mechanical 

 effect, being thermal motions excited ; the latter is a mechanical 

 effect of the statical kind. The whole mechanical value of these 

 effects, which are produced by means of the animal mechanism 

 in any time, must be equal to the mechanical value of the work 

 done by the chemical forces. Hence, when an animal is going up- 

 hill or working against resisting force, there is less heat generated 

 than the amount due to the oxidation of the food, by the thermal 

 equivalent of the mechanical effect produced. From an estimate 

 made by Mr Joule, it appears that from |^ to | of the mechanical 

 equivalent of the complete oxidation of all the food consumed by a 

 horse may be produced, from day to day, as weights raised. The 

 oxidation of the whole food consumed being, in reality, far from 

 complete, it follows that a less proportion than |, perhaps even 

 less than f , of the heat due to the whole chemical action that 

 actually goes on in the body of the animal, is given out as heat. 

 An estimate, according to the same principle, upon very imperfect 

 data, however, is made by the author, regarding the relation between 

 the thermal and the nonthermal mechanical effects produced by a man 

 at work ; by which it appears that probably as much as ^ of the 

 whole work of the chemical forces arising from the oxidation of his 

 food during the 24 hours, may be directed to raising his own weight, 

 by a man walking up-hill for 8 hours a-day ; and perhaps even as 

 much as I of the work of the chemical forces, may be directed to the 

 overcoming of external resistances by a man exerting himself for 

 6 hours a-day in such operations as pumping. In the former case 

 there would be not more than |, and in the latter not more than f 

 of the thermal equivalent of the chemical action emitted as animal 

 heat, on the whole, during the 24 hours, and the quantities of heat 

 emitted during the times of working would bear much smaller pro- 

 portions respectively than these, to the thermal equivalents of the 

 chemical forces actually operating during those times. 



