PROTECTION AGAINST LOSS OF HEAT 405 



less connected with the fall in temperature met with in passing from the inte- 

 rior toward the surface; for the lower the temperature the lower must be the 

 melting point of the fat in order that it may remain in a fluid state. 



The great importance of the subcutaneous fat is most beautifully seen in 

 the case of the great warm-blooded marine animals of the arctics. They live 

 habitually among the ice blocks in a medium which "conducts heat at least 

 twenty times better than air, and yet they are able to maintain a body tem- 

 perature of 35-40 C. The skin is subjected to conditions which "would 

 abstract an enormous amount of heat, but the extraordinarily thick layer of 

 subcutaneous fat isolates the muscles and the organs in short, the real body 

 from the skin. 



For warm-blooded animals which live in the air, the loss of heat is greatly 

 reduced by the hair and feathers; and clothing serves the same purpose for 

 our own bodies. 



Air is itself a very poor conductor of heat, but when in motion it may 

 carry away great quantities of heat. Let us imagine a naked man in an 

 atmosphere colder than his skin. The layer of air immediately adjacent to 

 his skin is first warmed by his body and as a consequence it becomes lighter. 

 It rises and is replaced by fresh, cold air, which in its turn is warmed, re- 

 placed, and so on incessantly. The body produces therefore by virtue of its 

 own heat an uninterrupted current of air, which abstracts great quantities 

 of heat from it. 



This active exchange of air is considerably restricted by the clothing, what- 

 ever the material of which it is made, inasmuch as it prevents free access of 

 the air to the skin. The air inclosed by the clothing is relatively stationary 

 and thus, because of its poor conducting qualities, it constitutes a thermally 

 insulating layer around the body. Moreover, not only the air between the cloth- 

 ing and the skin, as well as between the different garments, but the air in the 

 meshes of the clothing material itself is to be taken into consideration. For 

 clothing materials, like hair and feathers, are of themselves much better con- 

 ductors than air. The amount of air held in the meshes of the clothing of a 

 man as ordinarily dressed (excluding wraps) is estimated at 20-301. (Rubner). 



Important as this layer of surrounding air is, it must not stand absolutely 

 still, but must be kept in continual motion, even if it be very slow motion; for 

 otherwise the air very quickly becomes saturated with water vapor given off by 

 the skin, and then no further loss of water vapor can take place. The result is 

 great discomfort and, under some circumstances, great disturbances in the regu- 

 lation of heat. 



The loss of heat by radiation is likewise reduced considerably by the cloth- 

 ing. Since the clothing materials consist of substances which do not permit the 

 passage of radiant heat, they absorb the heat radiating from the skin and are 

 themselves warmed by it. Consequently this heat remains longer in the neigh- 

 borhood of the body and thus helps to warm the air immediately surrounding 

 it. When one feels that he is losing heat from the immediate neighborhood of 

 the body too rapidly, he covers the garment from which the heat is escaping 

 with still another, which catches the heat radiating from the first and delays it 

 still longer. A shirt, vest, coat, etc., act in this way. 



The radiation of heat from the skin is still further diminished by water 

 vapor, because it reduces the diathermic capacity of the air. 

 25 



