THE SUMMER HEAT OF CITIES. 437 



external conditions not incompatible with health. In man the range 

 of temperature in health is fixed at 97.25° F. to 99.5° F. Any tem- 

 perature above or below these extremes, unless explained by special 

 circumstances not affecting the normal condition of the person, is 

 an indication of disease. This comparatively fixed temperature in 

 health is a remarkable feature of the living animal. When subjected 

 to a temperature above or below the extremes here given it will 

 still maintain its equilibrium. This fixed temperature under vary- 

 ing conditions of heat and cold is due to a " heat-regulating power," 

 inherent in the constitution of every animal, by which it imparts 

 heat when the temperature of the air is high and conserves heat when 

 the latter is low. The heat escapes from the body — 1, by radiation 

 from the surface; 2, by transmission to other bodies; 3, by evapora- 

 tion; and 4, by the conversion of heat into motion. The surface of 

 the body furnishes the principal medium for the loss of heat by the 

 first three methods — viz., radiation, transmission, and evaporation. 

 It is estimated that 93.07 per cent of the heat produced escapes by 

 the processes of radiation, evaporation, conduction, and mechanical 

 work. The remaining heat units are lost by warming inspired air 

 and the foods and drinks taken. There are apparently other subtile 

 influences, so-called " regulators of heat," at work to preserve an 

 equilibrium of temperature in the animal body, but they are not 

 well known. The result of the operation of these forces is this — 

 viz., if, by any means, the heat of the body is increased, compensa- 

 tive losses of heat quickly occur, and the normal temperature is soon 

 restored; and if, on the contrary, the loss of heat is unusually in- 

 creased, the compensative production of heat of the body at once 

 follows, and the equilibrium is at once restored. The important fact 

 to remember is this — viz., the production and loss of heat in the 

 human organism when in health and not subjected to too violent dis- 

 turbing causes are so nicely balanced that the temperature is always 

 maintained at an average of 98.6° F., the extremes being 97.25° F. 

 and 99.5° F. "So beautifully is this balance preserved," Parkes 

 remarks, " that the stability of the animal temperature in all coun- 

 tries has always been a subject of marvel." If, however, anything 

 prevents the operation of the processes of cooling — viz., radiation, 

 evaporation, and conduction — the bodily temperature rises by the 

 accumulation of heat, and death is the result from combustion. In 

 experiments in ovens a man has been able to bear a temperature of 

 260° F. for a short period, provided the air was dry so that evapora- 

 tion could be carried on rapidly. But if the air is very moist, and 

 perspiration is impeded, the temperature of the body rises rapidly, 

 and the person soon succumbs to the excessive heat. Another im- 

 portant fact is this, viz., the normal temperature of the young and 



