422 ANIMAL HEAT. [CHAP. xxx. 



have lost 10 F. We may here recall the fact, that the quantity of 

 carbonic acid exhaled is greater when the external temperature is 

 low. Mice and guinea-pigs evolve twice as much carbonic acid at 

 32 as at 60 F., and birds more than double. Also more food, 

 and of the calorifacient kind, is required in winter than in Summer, 

 and in cold climates than in warm. 



The nature of the food must vary with the temperature of the 

 climate ; for at the same time that the respirations are less in num- 

 ber in hot than in cold climates, the air is less dense, and contains 

 in a given volume less oxygen ; hence a light and succulent diet 

 is better adapted to the requirements of the system, and com- 

 paratively little heat is developed ; while, in cold climates, a large 

 quantity of highly carbonaceous food is necessary in order to 

 furnish the requisite supply of heat. 



In animals which are very active, and which are not subjected 

 to the cooling influence of an atmosphere many degrees lower in 

 temperature than their own bodies, it is quite possible that a suffi- 

 cient amount of heat may be generated in the combination of the 

 elements of their tissues without the necessity of fatty or starchy 

 food forming a part of their highly nitrogenous diet. In cold cli- 

 mates, however, a large quantity of readily combustible food is 

 necessary in order to furnish the requisite amount of heat. 



Loss of Heat by Evaporation. The animal body is continually sub- 

 ject to evaporation of its fluid parts, just as any other moist sub- 

 stance, and the amount of evaporation, and the consequent loss of 

 heat, will depend on the same causes as in the case of inorganic 

 substances. The moisture, or dryness, of the surrounding air, and 

 its state of motion or rest will mainly influence the result. All 

 the effects of excessive temperature on the body are much more 

 apparent with a moist than with a dry atmosphere ; because in the 

 case of a dry atmosphere a greater amount of evaporation takes 

 place, and hence a greater quantity of heat is removed from the 

 system. 



In England it would be impossible to sustain a vapour bath at a 

 temperature of 110 or 120 for more than 10 minutes, whereas 

 the body may be without danger exposed for the same time to a 

 dry temperature twice as high or more. In some well-known ex- 

 periments Sir Charles Blagden remained immersed for 8 minutes 

 in dry air heated to the extraordinary pitch of 240 or 260 F. An 

 oppressive day in summer is one where the air is moist at the same 

 time that it is hot and stagnant. 



Again, the refrigeration, arising from simple contact of cold air, 



