CONDUCTIVITY. 



91 



on a summer day, not solely by conduction from the warm ground, but 

 through the joint effect of convection and conduction. 



The tremulous motion of objects at a distance, seen through hot air, 

 is due to this convection, as already explained in Chapter V. Unequally 

 heated masses of air are moving up and down irregularly, and refracting 

 the light, making it appear to the eye to come now from one point now 

 from another. 



When convection is prevented, it is found that air is a bad conductor, 

 and probably, on this bad conduction of air, depends the non-conducting 

 power of woollen clothes, blankets, and other loosely woven textures. 

 The wool is in itself a very bad conductor, while, through its being 

 matted together, it entangles the air, which is a still worse conductor, 

 so as to prevent convection currents, and thus the whole layer of wool 

 and air conducts badly. 

 Even were the wool it- 

 self a good conductor, 

 the same effect might be 

 produced, for the path 

 by which the heat must 

 get from one side to the 

 other, travelling through 

 the wool, is an indirect 

 one, and so the wool 

 slope of temperature is 

 very gradual. * This 

 probably explains the 

 efficacy as a non-con- 

 ductor of slag wool, 

 a material consisting of 

 blast-furnace slag, blown 

 out into fine fibres by 

 steam. The slag is itself 

 not so bad a conductor, 

 but, when loosely packed, 



the path from one side to the other of a layer of the wool is very 

 much longer than the thickness of the layer. For example, the path 

 from A to B (Fig. 62) through the wool is much greater than the direct 

 path. Then, again, the cross-section of the material is in reality 

 only a fraction of the whole cross-section of the layer, both conditions 

 combining to diminish the conduction through the wool, while the 



* From the table given later, it will be seen that wool and cotton are nearly equal 

 in their conductivities, yet undoubtedly woollen clothes are the warmer. This is in 

 great measure due to the fact that woollen cloth is more open in texture, and there- 

 fore holds a thicker layer of air for the same weight. Probably, also, the more open 

 texture of the wool allows a rather freer exchange between the air within the 

 meshes and the air outside, so that the vapour-laden air can pass from the skin to 

 the outside. The more closely woven cotton hinders the passage of the air, and it 

 is more likely, in cooling down, to deposit its moisture in the cloth, this deposition 

 being aided by the hygroscopic property of cotton. If now evaporation takes 

 place freely from the outer surface of the moistened cloth, the temperature is 

 lowered there, and heat is more rapidly conducted from the skin, which may thus 

 be chilled. 



FIG. 61. 



