+ o6 THE POPULAR SCIENCE MONTHLY. 



tive force by any voluntary operation, which would be one of the 

 prerogatives of life ; they draw it from the calorific energy stored up in 

 the organs traversed by the blood. Besides, there is a fixed relation 

 between the quantity of heat that disappears and the mechanical la- 

 bor that appears. Yet, it is to be remarked that, if all motion by liv- 

 ing beings is a transformation of animal heat, that heat is not wholly 

 transformed into motion. It is partly wasted by transpiration through 

 the skin, by touch, and especially by radiation ; it is used in keeping 

 up to a constant point the temperature of the animal, subjected to 

 many causes of refrigeration. 



The mechanical labor performed by an animal is very complex. In- 

 dependently of visible muscular motions, there are all the changes of 

 place in the interior organs, the continual pasisage of the blood, the 

 contractions and dilatations of a great number of parts. Now, these 

 actions are only possible in so far as the phenomena of breathing are 

 taking place in the active region. Prevent arterial blood from coining 

 to the muscle, that is to say, prevent combustion taking place, and 

 consequent heat evolving in it, and, although the structure of the or- 

 gan suffers no harm, it loses its contractile power. Mere compression 

 of the supplying artery of the muscle, so as to check the flow of blood 

 in it, causes the organ to grow cool, and lose its power. The labors 

 of Hirn and Beclard have clearly established the relations between 

 heat and muscular motion. Later experiments by Onimus have fixed, 

 with equal precision, the efficiency of heat through the movements of 

 circulation. 



We have said that the heat-producing power of aliments will be 

 the more considerable in proportion as they contain a greater quantity 

 of elements that need a large supply of oxygen for their combustion. 

 Therefore, meat and fats repair the losses of the system much more 

 speedily than vegetable substances. The latter are suitable for the in- 

 habitants of warm countries who do not require to produce heat, which 

 the atmosphere supplies them with abundantly. The inhabitants of 

 cold regions, on the contrary, whose accessions of heat ought to be as 

 continual as energetic, are urged by instinct to use meats and fats, 

 which throw out great heat in their combustion. For instance, it is a 

 physiological necessity that the Lapps should feed on the oil of cetacca, 

 as it is a necessity for men of the tropics to consume only very light 

 food. The activity of respiratory combustion and the kind of alimen- 

 tation thus vary with climate, so that there is always a certain propor- 

 tion maintained between the thermic state of the surrounding medium 

 and that of the animal furnace. In like manner, in the same climate, 

 persons who perform great mechanical labor must eat more than those 

 who put forth but little movement. This fact, long ago observed, has 

 received of late the clearest and surest demonstration. Yet, perhaps, 

 it is not kept sufficiently in view in the management of public ali- 

 mentation. Many examples prove the benefit that industry would de- 



