THE EXTERNAL ENVIRONMENT 229 



The calorific loss per square centimetre per second is, therefore, 

 in all : 



?x + ft = (K, + K 2 ) (T-0- 



Let it be noted that the temperature, T, of the external surface 

 of the skin is neither easy to measure nor constant. The tem- 

 perature of the organism, about 37-50C, is maintained by a calorific 

 regimen, a permanent flow of heat. The characteristic of the 

 regimen is that the skin receives on its internal surface as much 

 heat as it loses on its external surface. It is easily proved that 

 it receives : 



?3 =(K 1 + K 2 )(37-60C T'), 

 T' being the internal cutaneous temperature (see fig. 144). 



Hence : 



& = ?i + q*' 



But the transmission of the heat takes place through a thick- 

 ness e, of the skin, whose conductivity has to be ascertained. 

 The number of small calories able to traverse a surface of 

 1 square centimetre with a thickness of 1 centimetre in one second, 

 and for a difference of temperature equal to 1C between the 

 internal and external walls, is called the co-efficient of conductivity 

 c. In these conditions the heat transmitted is : 



(T' T) 



** = c r 



in small calories, and 



?4 = 99 = q\ + ft = q (generally). 



The advantage of thus having three expressions for the same 

 calorific loss is that we can eliminate T and T', which are 

 not known, and solve the problem with known temperatures 

 only ; 37-50 C. for the internal, and t for the external (the air 

 temperature). A very simple operation will give : 

 cK^Ki + Kg) (37 -SOT t) 



' 



The average thickness of the human skin e = 0-2 cms. The 

 co-efficients are : 



c (i) = 0-00060 c ; K! = 0-00015 c ; K 2 = 0-000066 c. This gives : 

 q =497 X 10- 7 X (37-50C 1). 



Per twenty-four hours, or in (3,600 X 24) seconds, a square 

 centimetre of skin will lose a quantity of heat : 



Qj == 0-004294 (37-50C 1) in great calories. 



( x ) Jules Lefevre estimates c at O'OOOGOc ; Adamkiewiez found for the 

 muscles 0*00012 c., certainly too small a value, and Bordier's experiments 

 also resulted in 0-00014 C; 



