FAMILIAR LETTERS ON CHEMISTRY. 21 



the act of combination is accompanied by the disengagement of heat. It is indif- 

 ferent whether this combination takes place rapidly or slowly, at a high or at a low 

 temperature ; the amount of heat liberated is a constant quantity. 



The carbon of the food being converted into carbonic acid within the body, must 

 give out exactly as much heat as if it had been directly burnt in oxygen gas, or in 

 common air; the only difference is, the production of the heat is diffused over unequal 

 times. In oxygen gas the combustion of carbon is rapid, and the heat intense ; in 

 atmospheric air it burns slower, and for a longer time, the temperature being 

 lower ; in the animal body the combination is still more gradual, and the heat is 

 lower in proportion. 



It is obvious that the amount of heat liberated must increase or diminish with 

 the quantity of oxygen introduced in equal times by respiration. Those animals, 

 therefore, which respire frequently, and consequently consume much oxygen, pos- 

 sess a higher temperature than others, which, with a body of equal size to be heated, 

 take into the system less oxygen. The temperature of a child (102) is higher than 

 that of an adult (99.5). That of birds (104 to 105.4), is higher than that of qua- 

 drupeds (98.5 to 100.4), or that of fishes, or amphibia, whose proper temperature, 

 is from 2.7 to 3.6 higher than that of the medium in which they live. All 

 animals, strictly speaking, are warm-blooded ; but in those only which possess 

 lungs is the temperature of the body quite independent of the surrounding medium. 



The most trustworthy observations prove that in all climates, in the temperate 

 zones, as well as at the equator or the poles, the temperature of the body in man, 

 and in what are commonly called warm-blooded animals, is invariably the same ; 

 yet how different are the circumstances under which they live ! 



The animal body is a heated mass, which bears the same relations to surrounding 

 objects as any other heated mass. It receives heat when the surrounding objects 

 are hotter, it loses heat when they are colder than itself. 



We know that the rapidity of cooling increases with the difference between the 

 temperature of the heated body and that of the surrounding medium ; that is, the 

 colder the surrounding medium the shorter the time required for the cooling of the 

 heated body. 



How unequal, then, must be the loss of heat in a man at Palermo, where the 

 external temperature is nearly equal to that of the body, and in the polar regions, 

 where the external temperature is from seventy to ninety degrees lower ! 



Yet, notwithstanding this extremely unequal loss of heat, experience has shown 

 that the blood of the inhabitant of the arctic circle has a temperature as high as 

 that of the native of the south, who lives in so different a medium. 



This fact, when its true significance is perceived, proves that the heat given off 

 to the surrounding medium is restored within the body with great rapidity. This 

 compensation must, consequently, take place more rapidly in winter than in sum- 

 mer, at the pole than at the equator. 



Now, in different climates the quantity of oxygen introduced into the system by 

 respiration, as has been already shown, varies according to the temperature of the 

 external air; the quantity of inspired oxygen increases with the loss of heat by 

 external cooling; and the quantity of carbon or hydrogen necessary to combine 

 with this oxygen must be increased in the same ratio. 



It is evident that the supply of the heat lost by cooling is effected by the mutual 

 action of the elements of the food and the inspired oxygen, which combine together. 

 To make use of a familiar, but not on that account a less just illustration, the ani- 

 mal body acts, in this respect, as a furnace, which we supply with fuel. It signifies 

 nothing what intermediate forms food may assume, what changes it may undergo in 

 the body, the last change is uniformly the conversion of its carbon into carbonic 

 acid, and of its hydrogen into water. The unassimilated nitrogen of the food, 

 along with the unburned or unoxidized carbon, is expelled in the urine or in the 

 solid excrements. In order to keep up in the furnace a constant temperature, we 

 must vary the supply of fuel according to the external temperature, that is, accord- 

 ing to the supply of oxygen. 



In the animal body the food is the fuel ; with a proper supply of oxygen we 

 obtain, the heat given out during its oxidation or combustion. In winter, when we 

 takje exercise in a cold atmosphere, and when consequently the amount of inspired 

 oxygen increases, the necessity for food containing carbon and hydrogen increases 

 in the same ratio ; and by gratifying the appetite thus excited, we obtain the most 

 efficient protection against the most piercing cold. A starving man is soon frozen 

 to death. The animals of prey in the arctic regions, as every one knows, far 

 exceed in voracity those of the torrid zone. 



In cold and temperate climates, the air, which incessantly strives to consume the 

 body, urges man to laborious efforts in order to furnish the means of resistance to 



