328 Mr. G. H. Knibbs : Mathematical Analysis of 



water into vapour. The abstraction in this way of the 

 necessary quantity of heat from the body prevents its accu- 

 mulation therein and a consequent undue rise of temperature. 



The phenomena of diaphoresis show that the quantity of 

 water eliminated by the skin may vary between somewhat 

 wide limits. If the body be in a truly thermostatic condition 

 the quantity eliminated by evaporation from the skin-surface 

 will not necessarily be constant in rate for like external con- 

 ditions. If the rate of thermogenesis, however, does not vary 

 it will be constant, but where this rate changes, the evapora- 

 tion rate will so respond within the stable limits as to tend 

 to maintain the thermostatic equilibrium, always, of course,, 

 approaching this equilibrium state asymptotically. 



As the temperature of the air approaches that of the body,, 

 the radiational and convectional losses become small, then 

 zero, and finally negative. Heat tends to pass from the air 

 to the body. In ordinary cases it actually does not so pass,, 

 for the air is not ordinarily saturated with vapour, and when 

 this is the case the external conditions are such that heat may 

 still be abstracted from the body by the conversion of water 

 into aqueous vapour. 



The increase of temperature operates to make the molecular 

 action concerned in converting liquid H 2 into gaseous 

 H 2 0, more active, but in all ordinary conditions the absolute 

 capacity of the air for water vapour is greatly augmented as the 

 temperature rises. Thus in millimetres pressure the increase 

 is as follows, viz. : — 



Degrees centigrade 0-1 10-11 20-21 30-31 40-41 50-51 



Difference Pressure— Aqueous 



vapour mm. for 1° C 0"34 063 111 1-86 3"00 4"68 



The difficulties of maintaining thermostatic equilibrium 

 thus tend to increase as the condition changes from the 

 transfer of heat by means of evaporation, together with 

 radiation and convection, to the condition when the whole 

 elimination of heat is by evaporation. For in the latter case 

 the rate at which heat is abstracted from the body has to 

 compensate for the heat tending to flow into it from the 

 hotter air surrounding it. This explains the enormous 

 influence of dryness (or defect from complete saturation) as 

 the temperature of the body is approached or exceeded by 

 that of the air. 



The rapidity with which the state of the human body 

 changes indicates that the analogue of a porous vessel is 

 inadequate, and in even the most carefully conducted experi- 

 ments, considerable discrepancy from any simple progression 



