26 CROONIAN LECTURES 



be determined, it is evident that ultimately we 

 shall know much more regarding the interstel- 

 lar medium than we do at present. 



However much our knowledge may extend, 

 even if it should be proved that an aether exists 

 for the conveyance of light through interstel- 

 lar space, probably it will be found to be as 

 unessential for the production of light itself, as 

 it is unessential for the production of heat. 



The ideas of the union of matter and force 

 followed the same course with regard to heat 

 as they did with regard to light. 



" The discovery of polarisation of heat by 

 Professor Forbes, and its confirmation and ex- 

 tension to dipolarisation by Melloni, almost at 

 a single blow ruined the emission theory" 

 (Whewell). It made the doctrine of emission 

 as untenable regarding heat as it had before 

 been found regarding light. 



As early as 1798, Count Kumford read a 

 paper before the Koyal Society concerning 

 the heat which is excited by friction. "It 

 appears to me," he says, "to be extremely 

 difficult, if not quite impossible, to form any 

 distinct idea of anything capable of being 



