THERMOTAXIS 691 



tinued, is dangerous to life. During the ' hot waves ' not infre- 

 quently experienced in summer in the United States, hundreds of 

 persons have died within a few days from the excessive heat. It is 

 stated that during the unusually hot summer of 1819 the tempera- 

 ture at Bagdad ranged for a considerable time between 108 and 

 120 F. (42 to 49 C.), and there was great mortality. A much 

 higher temperature may be borne in dry air than in air saturated 

 with watery vapour. A shade temperature of 100 F. (37*7 C.) in 

 the dry air of the South African plateaux is quite tolerable, while a 

 temperature of 85 F. (29-4 C.) in the moisture-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 observations of Boycott and Haldane in a deep mine, in the 

 incubating-room of a laboratory, and in a Turkish bath illustrate the 

 important influence of the humidity of the air. In still air the body- 

 temperature rose above normal when the wet-bulb thermometer rose 

 above 31 C. (88 F.), and it remained normal whatever the external 

 temperature might be so long as the reading of the wet-bulb thermo- 

 meter did not exceed that level. The more the wet-bulb thermometer 

 rose above 31 the more rapid was the increase in the body-temperature. 

 In moving air a greater degree of humidity could be borne without 

 increase in the body-temperature, which did not occur till the tem- 

 perature shown by the wet-bulb thermometer exceeded 35 C. The 

 great increase in the evaporation of sweat when the temperature of the 

 air is high is shown by the observation that on a warm day (dry bulb, 

 79 F. ; wet bulb, 67-5 F.) the average loss of moisture from the body 

 was 1,816 grammes for four soldiers during a march of seven miles, 

 while on a cold day (dry bulb, 45 F. ; wet bulb, 38 F.) it was only 

 419 grammes during the same march by the same men (Pembrey). 



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 experiments of Hosslin, and the experi- 

 ence 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 



