212 THE HUMAN MECHANISM 



Solids and liquids are much better conductors of heat than 

 gases, and air when perfectly still is one of the poorest con- 

 ductors of heat with which we have to deal. It is a familiar 

 fact that the skin is chilled much more rapidly by water than 

 by air of the same temperature (why?); and we shall learn in 

 hygiene that warm fabrics owe their warmth mainly to the 

 amount of poor-conducting air stagnant within their meshes. 



Convection. When a warm body is surrounded by a fluid such 

 as water or air, heat is similarly conducted to the adjacent layer 

 of water or air, which thus becomes warmer ; but, unlike the case 

 of the solid, this heated layer now moves off, carrying its heat 

 with it to other parts of the gas or liquid, and so communicating 

 it to other matter with which it subsequently comes in contact. 

 This method of heat transfer is known as convection, which, it 

 will be seen, depends at bottom upon conduction, but which is at 

 the same time conduction modified by the movement of a heated 

 gas or liquid. So long as the air around us is at rest, it does not 

 remove heat readily from the skin, since air is a poor conductor. 

 Air in motion, on the other hand (as in fanning), cools the skin 

 more rapidly, because as each part of the air is heated, it is moved 

 away and replaced by colder air. In this case the air cools the 

 skin by convection (Latin con, " with" ; veliere, "to carry" ). 



The transfer of heat from the internal heat-producing organs 

 to the skin affords an excellent example of the difference between 

 conduction and convection, for some of this heat passes by direct 

 conduction through the subcutaneous tissue to the overlying skin, 

 while some of it is carried to the surface by convection in the blood 

 stream. When 'the arterioles of the skin are dilated, convection is 

 an important means of heat transfer to the surface ; when, in the 

 reverse case, the cutaneous arterioles are constricted to their 

 utmost, convection becomes relatively unimportant and direct 

 conduction alone remains as the chief means of heat transfer to 

 the skin. Moreover, when the subcutaneous tissue contains large 

 amounts of fat, it is a poor conductor of heat, and for this reason 

 fat people when sitting still on cold days often feel colder than 

 lean people do. 



Radiation. Heat is thus removed from the skin by conduction, 

 and at times to an even greater extent by convection. But there 

 is still a third method of heat loss, known as radiation, by which 



