CONDUCTION OF HEAT. 



179 



COIDUCTIOI OF HEAT. 



IF two solid bodies, having different temperatures, be placed in close con- 

 tact, it will be observed that the hotter body will gradually fall in temperature, 

 and the colder gradually rise, until the temperatures become equal. This pro- 

 cess is not, like radiation, sudden, but very gradual ; the colder body receives 

 increased temperature slowly, and the hotter loses it at the same rate. Differ- 

 ent bodies, however, exhibit a different facility in this gradual transmission of 

 heat by contact. In some it passes more rapidly from the hotter to the colder, 

 and in others the equalization of temperature is not produced until after the 

 lapse of a considerable time. 



This quality in bodies, by which heat passes from one to the other through 

 their dimensions, is called their conducting power, and the heat thus transmitted 

 is said to be conducted by the body. One body is said to be a better conductor 

 than another, when the equalization of temperature is effected more speedily ; 

 and when the equalization is accomplished more slowly, the body is said to be 

 a bad conductor. 



To make this process more intelligible, let us suppose A, fig. 1, a small 

 square block of red-hot iron, and let B C be a bar of iron, the section of which 



Fig. 1. 



is square. Let the extremity, B, be placed close against the block A, and let 

 a screen, S, pierced by A B, be placed so as to intercept the effect of radiation 

 from A. Let thermometers, t l', &c., be inserted at different points of the bar 



