24 CROONIAN LECTURES 



of and against the luminiferous aether thus : 

 (p. 186, ed. 1867) : " At the utmost our as- 

 sumption, on the one hand, is that whenever 

 light, heat, etc., exist, ordinary matter exists, 

 though it may be so attenuated that we cannot 

 recognise it by the test of other forces such as 

 gravitation ; and that to the expansibility of 

 matter no limit can be assigned. On the other 

 hand, a specific matter without weight must be 

 assumed, of the existence of which there is 

 no evidence but in the phenomena, for the 

 explanation of which its existence is supposed. 

 To account for the phenomena the aether is 

 assumed, and to prove the existence of the 

 aether the phenomena are cited. For these 

 reasons, and others above given, I think the 

 assumption of the universality of ordinary 

 matter is the least gratuitous." 



Mr. W. K. Clifford, in a lecture at the Koyal 

 Institution, says : "It has been supposed for a 

 long time that light consists of waves trans- 

 mitted through an extremely thin ethereal 

 jelly that pervades all space ; it is easy to see 

 the very rapid tremor which spreads through 

 a jelly when you shake it at one point. From 

 this hypothesis we can deduce the laws of the 



