458 PHYSIOLOGICAL PHYSICS. [Chap. xxxv. 



inwards to the ice, and its melting is delayed. Wool- 

 len clothing is not, however, used in hot weather to 

 keep the external heat from the body, for a reason 

 that is very apparent. The body is kept cool in a hot 

 atmosphere by the large amount of heat that is given 

 off from its surface by evaporating the sweat and in 

 other similar ways. It is not by keeping the heat from 

 gaining access to the body that it is kept cool, but 

 by a large amount of heat being given off. But if 

 the body be enveloped in woollen clothing this heat 

 is prevented from passing off, is retained, and the 

 cooling does not occur. In hot climates, therefore, 

 clothing made of good conductors of heat is sought. 

 This is the reason why linen, materials, which conduct 

 heat better than woollen substances, are used. The 

 varying conductivity of different substances also 

 explains why two bodies at the same temperature 

 may seem to the touch to be very different. A piece 

 of iron at the same temperature as a piece of wood, 

 will seem colder, because it conducts the heat from 

 the hand more readily. Again, if a piece of metal 

 and a piece of wood be both at the same temperature, 

 and that a high one, the nietal will seem to be much 

 hotter, and may burn the hand, while the wood may 

 be held without pain. The metal, being a good con- 

 ductor, readily communicates its heat, while the wood 

 does not. 



Convection of leeat is to be distinguished' 

 from conduction. Convection is the propagation of heat 

 by the transference of heated particles from one place 

 to another. Liquids have little conductivity for heat, 

 but a mass of liquid will speedily become heated 

 through its whole substance, if exposed to a source of 

 heat. This is due to convection currents. Thus, if 

 water in a flask be held over a spirit lamp, the layer 

 of water in contact with the bottom of the vessel 

 becomes heated and then rises to the surface, its 



