342 REGULATION OF TEMPERATURE 



(d) The loss of heat varies directly with the time of exposure 

 of the surface. In actual practice, this can be determined for non- 

 living systems only, because, as we shall see, in the living body 

 secondary reactions take place ; alterations in the state of the 

 surface and also of the (Jeep-lying tissues are produced, rendering 

 difficult the application of this law to living beings. If the 

 student uses his common- sense, however, he will readily see that, 

 if all conditions are kept constant except time, the amount of 

 heat lost by conduction must vary directly with the time of 

 exposure. 



(ii) Temperature gradient. Newton's law states that the amount 

 of heat lost by a body in unit time is proportional to the difference 

 in its temperature from that of the surrounding medium. The 

 warm body loses heat and becomes colder, while the environment 

 becomes warmer. This loss and gain goes on till the body and 

 its environment are at the same temperature. Inspired air or 

 cold food is warmed to body temperature at the expense of body 

 heat. The drinking of cold water may cause a temporary 

 reduction of rectal temperature. Conversely, hot food and drink 

 tend to increase the temperature of the body. 



The heat lost by excretion depends not so much on this factor 

 as on the amount and nature of the excreta. Compared with 

 the total heat lost, the amount lost by the heating of inspired air, 

 cold ingesta, and by the excreta is trivial. 



(iii) Force of wind. In still air, the loss by conduction is very 

 slight. Air is not a very good thermal conductor. A layer of 

 warmed air soon forms an envelope round the cooling body and 

 prevents rapid conduction. A very slight movement of the air 

 may produce a very appreciable effect by driving off the warm 

 particles and bringing cool ones into contact with the body. 

 This may arise naturally by the formation of convection currents. 

 The heated air expands and becomes lighter and so rises and 

 allows the colder air to flow in. 



An air current may be felt when it moves at the rate of about 

 0-4 to 0-5 metre per second. A non-perceptible draught with a 

 velocity of 0-2 metre per second playing on the naked arm may 

 increase the heat loss over that experienced in still air by 19 to 

 75 per cent, depending on the temperature of the air. A moderate 

 breeze at 8 metres per second (15 miles per hour) with a tempera- 

 ture of 20 C. causes more rapid chilling of a naked man than 

 exposure to still air at 2 C. (see also p. 346). 



(iv) Humidity of atmosphere. Water is a better conductor 



