316 DISCOURSE ON THE STUDY 



radiating out heat in all directions, and receiving it 

 by a similar means of communication from others, 

 and thus tending, in any space filled, wholly or in 

 part, with bodies at various temperatures, to esta- 

 blish an equilibrium or equality of heat in all parts. 

 The application of this idea to the explanation of 

 the phenomenon of dew we have already seen (see 

 167.). The laws of such radiation, under various 

 circumstances, have been lately investigated in a 

 beautiful series of experiments on the cooling of 

 bodies by their own radiation in vacuo, by Messrs. 

 Dulong and Petit, which offer some of the best ex- 

 amples in science of the inductive investigation of 

 quantitative laws. 



(352.) The communication of heat between bodies 

 in contact, or between the different parts of the 

 same body, is performed by a process called con- 

 duction. It is, in fact, only a particular case of 

 radiation, as has been explained above (217.); but 

 a case so particular as to require a separate and 

 independent investigation of its laws. The most 

 important consideration introduced into the enquiry 

 by this peculiarity is that of time. The com- 

 munication of heat by conduction is performed, for 

 the most part, with extreme slowness, while that 

 performed by direct radiation is probably not less 

 rapid than the propagation of light itself. The 

 analysis of the delicate and difficult points which 

 arise in the investigation of this subject in its 

 reduction to direct geometrical treatment has been 

 executed with admirable success by the late Baron 

 Fourrier, whose recent lamented death has deprived 

 science of an ornament it could ill spare, thinned 



