1885.] on hoiu Thought presents itself in Nature. 185 



that these last underlying motions exist, though we have reason to 

 suspect them. If they do exist, then the heat motions are secondary ; 

 if they do not exist, then the heat motions are the primary motions. 



A similar doubt exists in regard to the undulations of which light 

 and radiant heat consist. In the present state of our knowledge it 

 continues undecided whether they are primary motions, or whether 

 they are waves transmitted through still more subtile motions under- 

 lying them, and if this be the case then they are secondary motions. 

 But whether primary or secondary these etherial motions are mani- 

 festly of quite a diiferent order from the motions which take place in 

 sound. They are to be classed along with the subtile internal motions 

 that go on icithin the molecules of bodies, and are very different from 

 that general shifting backwards and forwards of vast accumulations 

 of molecules which we hear as sound. This is that fundamental 

 difference between light and sound of which I spoke earlier in this 

 discourse. 



ExTEENAL Source of our other Sensations. 



When we extend this scrutiny to those events in the outer world 

 which act on us through our other senses, we find motions throughout 

 the material universe, motions everywhere, motions underlying every 

 phenomenon; and no phenomenon has yet been met with in the 

 material universe that when adequately examined does not resolve 

 itself entii-ely into motions and the relations between motions. If a 

 chemist comes across an object surrounded by those light waves which 

 would give it to his eyes the appearance of being of the steel grey of 

 sodium ; if on approaching a knife to it he finds he can divide it, and- 

 that the molecular motions of the edge of his knife are made to 

 retreat inwards to the small extent which causes him to feel the 

 amount of resistance which sodium offers ; if he allows it to drop and 

 finds that it moves towards the earth as ponderable matter does ; if 

 he places it in the pan of his balance and finds that the upper mole- 

 cular motions of the pan approach those beneath in the degree which 

 requires a counterpoise corresponding to the weight of sodium to be 

 moved into the other scale to prevent the rest of the pan from 

 descending ; if he sees the motions of combustion arise when he 

 throws it on warm water, and ascertains by his spectroscope that the 

 motion ensues in the surrounding space which occasions the charac- 

 teristic lines of sodium ; if on applying some of its salts to his tongue 

 those motions occur in that organ which result in his perceiving a 

 saline taste ; if on approaching chemical tests to it he finds all the 

 alterations of molecular motion arise which are manifested when a 

 compound of sodium is j^i'ocluced : would any chemist hesitate one 

 moment to pronounce that the object with which he has been dealing 

 is sodium ? Yet at every step he has done nothing but move objects 

 about, and it has been motions and nothing hut motions and their changes 

 which he has really observed. 



