CONDUCTION AND RADIATION. 527 



change of heat is going on among neighbouring 

 bodies ; and a body grows hotter or colder, accord- 

 ingly as it receives more caloric than it emits, or 

 the contrary. And thus a body is cooled by recti- 

 linear rays from a cold body, because along these 

 paths it sends rays of heat in greater abundance 

 than those which return the same way. This 

 theory of exchanges is simple and satisfactory, and 

 was soon generally adopted ; but we must consider 

 it rather as the simplest mode of expressing the 

 dependence of the communication of heat on the 

 excess of temperature, than as a proposition of 

 which the physical truth is clearly established. 



A number of curious researches on the effect of 

 the different kinds of surface of the heating and 

 of the heated body, were made by Leslie and 

 others. On these I shall not dwell ; only observ- 

 ing, that the relative amount of this radiative and 

 receptive energy may be expressed by numbers, 

 for each kind of surface ; and that we shall have 

 occasion to speak of it under the term exterior 

 conductivity ; it is thus distinguished from interior 

 conductivity, which is the relative rate at which 

 heat is conducted in the interior of bodies 6 . 



8 The term employed by Fourier, conductibility or conduci- 

 bility^ suggests expressions altogether absurd, as if the bodies 

 could be called conductible, or conducible, with respect to heat : 

 I have therefore ventured upon a slight alteration of the word, 

 and have used the abstract term which analogy would suggest, if 

 we suppose bodies to be denominated conductive in this respect. 



