CONDUCTION OF HEAT 341 



which has a low thermal conductivity will decrease the rate at 

 which heat arrives at the surface and retard heat loss. 



Every one knows that certain substances do not readily conduct heat. 

 In the hot room of a Turkish bath, where all the objects are at the same 

 temperature, metallic objects feel much hotter than those of wood, bone, 

 rubber, etc. A familiar example of the retention of heat by a body sur- 

 rounded by some bad conductor is found in the hay box or Norwegian 

 cooker, which consists of a wooden box having a thick lining of felt. The 

 partially cooked meal while still hot is placed in the box and the inter- 

 vening space firmly packed with hay or paper. The felt-lined lid is closed 

 and the meal left to cook itself. After several hours the temperature will 

 have fallen only a few degrees. The Dewar or thermos flask depends on 

 a vacuum as non-conductor. 



Animals which have a good layer of subcutaneous fat lose 

 heat much more slowly than lean ones. 



Fat acts as a heat insulator and retards loss by conduction. 

 On a moderately warm day, the obese person becomes uncomfort- 

 ably warm. He is unable to eliminate heat with sufficient 

 rapidity, and, as a consequence, his temperature rises, and may 

 cause an increase in general metabolism amounting to 50 per cent, 

 over that of a thin person. Aquatic mammals rely on their 

 adipose tissue to protect them from a too rapid loss of 

 heat. 



Several measurements of the temperature inside the body have 

 been made. The natural cavities, e.g. the rectum, and vagina, 

 are generally used, both being deep cavities into which the ther- 

 mometer or thermal elements can be inserted for a considerable 

 distance. Measurements have been made of the temperature in 

 the inside of the stomach in patients with gastric fistulae. The 

 temperature of freshly voided urine was suggested by Stephen 

 Hales in 1731 as representing that of the interior of the body. 

 It has been found that, while the temperature of the surface of 

 the body is about 32 C., as the depth from the surface increases, 

 the temperature rapidly rises till the depth of about 5 cm. is 

 reached. It continues to rise much more slowly for the next two 

 cm. or so. Apparently the temperature after this is fairly uniform, 

 i.e. 37 C. That is, the subcutaneous tissues cause a temperature 

 lag or thermal gradient of about 5 C. This is due to the large 

 proportion of water which enters into the composition of the 

 tissues, giving the body the high specific heat of about 0-83. 

 A man weighing 60 kg. would consequently have a water equiva- 

 lent of 50-4 kg. of water, so that a rise of 0-1 C. would only be 

 registered after 5-04 cals. of heat had been rendered latent. 



