LATENT HEAT OF EVAPORATION 345 



mechanism called into play to prevent untoward results. On one 

 point preliminary emphasis must be laid, viz. the times during 

 which these extreme temperatures were endured were of very 

 short duration. That the time factor is of immense importance 

 may be adduced from the preceding experiment (Table LVIII.) 

 by Christen. 



In warm-blooded animals, 45 C. is generally considered a 

 lethal temperature, but death may occur at a lower temperature 

 (42), provided the time of exposure is sufficiently prolonged. 



Bakers' assistants are known regularly to have entered ovens heated 

 to over 126 C. When the temperature approached 160, they experienced 

 extreme superficial vasodilatation. Young girl labourers are said to ex- 

 perience no inconvenience from a stay of 5-10 minutes in a kiln heated to 

 about 130 C. Chaubert, the " Fire King," is reported as having with- 

 stood a temperature of between 226 and 315-5 C. Yet immersion in a 

 bath of water at 45 C. is unbearable. It is a matter of common knowledge 

 that those who habitually work in abnormally overheated places consume 

 large quantities of fluid, and sweat profusely. The latent heat of water 

 is the greatest of all substances known. Every gram of water which is 

 vaporised entails the use of 536 calories. This is in accordance with the 

 principle of energetics laid down by Le Chatelier (p. 9). In general, if 

 any change is brought about by the incidence of energy, then alteration 

 will take place in the substance acted on to nullify these changes, i.e. a 

 reaction of opposite direction occurs. 



Substances which expand on heating will be cooled by mechanical 

 expansion. Water increases its volume in passing from the liquid to the 

 gaseous state, and therefore, as the result of such an expansion, the parent 

 fluid is cooled. The evaporation of moisture from the surface thus causes 

 cooling of the body (see also clothes, p. 351). 



The quantity of heat lost by the evaporation of moisture (a) in 

 the bronchial passages, etc., (b) in the form of sensible and in- 

 sensible perspiration, depends mainly on five factors, viz. : 



1. Area of moist surface exposed. 



2. Colour ,, ,, ,, 



3. Gradient of temperature between body and environment. 



4. Force of wind (partial pressure equilibrium). 



5. Humidity of the air. 



(1) The effective area of surface exposed to cooling depends 

 in great measure on the state of the superficial blood vessels. 

 Dilatation of these enormously increases the area. 



/ 



(2) The effect of colour is merely a matter of the differential 

 absorption of radiant energy producing local heating in proportion 

 to the amount of energy absorbed and so causing a more or less 

 rapid evaporation. The black moist muzzle of the bull-dog is 

 much cooler than its white, comparatively dry skin. 



