444 THE REGULATION OF TEMPERATURE 



TABLE LXIX 



Time to Kill Spores of Hay Bacilli at Variolas 

 Temperatures 



That the time factor is of immense importance may be adduced 

 from the preceding experiment (Table LXIX.) 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 liave entered ovens heated 

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

 extreme superficial vasodilatation. Young girl labourers are said to experience 

 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 withstood a 

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

 water at 45° C. is unbearable. 



It i« 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 ivhich is vaporised entails the use of 536 calories. This is 

 in accordance with the principle of energetics laid down by Le Chatelier 

 (p. 9 et seq.). 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 expan- 

 sion. 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 eva])oration of moisture from the surface thus causes cooling of 

 the body (see also clothes, p. 452) . 



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

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

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



1. Area and nature of moist surface exposed. 



2. Colour of moist surface exposed. 



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 



