426 ANIMAL HEAT. [CHAP. XXX. 



organized bodies, it is necessary to remember, that the heat dis- 

 engaged during the oxidation of carbon or of a metal (i.e. during 

 the combustion of these bodies) is directly proportioned to the 

 amount and not to the intensity of the chemical action. In the 

 words of a distinguished chemist, " The rod of iron that is burnt 

 in oxygen gas, produces a heat which no one will deny ; but the 

 iron which rusts slowly in the air, disengages just as much heat, 

 although its temperature never rises sensibly above that of the 

 surrounding atmosphere. Phosphorus alight burns brilliantly, 

 and produces abundances of heat ; phosphorus in the cold, still 

 burns, but with little lustre, and the heat which it evolves was for 

 a long time denied." * 



It has been already stated (p. 152-4) that a large portion of the 

 food being destitute of nitrogen, is not the best adapted to form part 

 of the tissues of the body into the composition of which that element 

 enters. This calorifacient food (consisting of various quantities of 

 carbon in combination with oxygen and hydrogen, in the proportion 

 in which these last form water, as starch, sugar, cellulose, or gum, 

 or of carbon and hydrogen in combination with a less proportion of 

 oxygen in fatty matters) seems to be devoted in the main to the pro- 

 duction of heat by the combination of its carbon and hydrogen with 

 the oxygen furnished by respiration. It is a question through what 

 changes it passes ere thus consumed ; but of its eventual destination 

 to the production of heat there would appear to be no doubt. 



As was shown in the chapter on Respiration, more oxygen is 

 taken into the blood by the lungs than is required to form the 

 carbonic acid exhaled. This superfluous oxygen disappears, the 

 greater part appearing to enter into combination with hydrogen, 

 while a small quantity goes to oxidize the sulphur and phos- 

 phorus. The air breathed is likewise found to have lost bulk. 

 Now gaseous carbonic acid contains its own bulk of oxygen 

 but oxygen uniting with hydrogen to form water is condensed. 

 Such a condensation of the oxygen would accord with the 

 observed diminution of bulk in air by being respired. It has 

 been remarked that herbivorous animals return to the air, as 

 carbonic acid, only nine out of ten of the volumes of oxyj 

 absorbed in respiration, and that carnivorous animals return in th< 

 same form, only five or six out of ten volumes. The carnivora, 

 then, absorb nearly twice as much oxygen as they employ for 

 oxidizing carbon, and a very large proportion of the remainder, 

 doubt, combines with hydrogen to form water. 



* Dumas, " Balance of Organic Nature," p. 37. 



