ANIMAL HEAT 507 



laden atmosphere of Bombay may be oppressive. The 

 reason is that in dry air the sweat evaporates freely and 

 cools the skin, while in moist air, although according to 

 Rubner the loss of heat by radiation and conduction is 

 increased, the loss of heat by evaporation of sweat is 

 diminished in a still greater degree. In saturated air at the 

 body temperature no loss of heat by perspiration or by 

 evaporation from the pulmonary surface is possible ; the 

 temperature of an animal in a saturated atmosphere at 35 to 

 40 C. soon rises, and the animal dies. In animals like the 

 dog, which sweat little or not at all on the general surface, the 

 regulation of the heat-loss by respiration is relatively more 

 important than in man. 



The winter fur of Arctic animals is a special device of 

 Nature to meet the demands of a rigorous climate, and 

 combat a tendency to excessive loss of heat. The experi- 

 ments of Hosslin and the experience of squatters in Australia 

 go to show that even domesticated animals have a certain 

 power of responding to long-continued changes in external 

 temperature by changes in the radiating surfaces which 

 affect the loss of heat. It is said that in the hot plains of 

 Queensland and New South Wales the fleeces of the sheep 

 show a tendency to a progressive decrease in weight. And 

 Hosslin found that a young dog exposed for eighty-eight days 

 to a temperature of 5 C. developed a thick coat of fine woolly 

 hairs. Another dog of the same litter, exposed for the same 

 length of time to a temperature of 31*5 to 32 C., had a 

 much scantier covering. The increased protection against 

 heat-loss in the case of the ' cooled ' dog was not sufficient 

 fully to compensate for the lowered external temperature. 

 The metabolism that is to- say, the heat-production was 

 also increased. And although the food was exactly the same 

 for both animals in quantity and quality, the dog at 5 C. 

 put on less than half as much fat in the period of the 

 experiment as the * heated ' dog, but the same amount of 

 ' flesh.' 



The voluntary factor in the regulation of the heat-loss is of 



