138 CORRELATION OF PHYSICAL FORCES. 



seems delighted with transmutations. Water, which is a 

 very fluid, tasteless salt, she changes by heat into vapour, 

 which is a sort of air, and by cold into ice, which is a hard, 

 pellucid, brittle, fusible stone, and this stone returns into 

 water by heat, and vapour returns into water by cold. * * 

 And, among such various and strange transmutations, why 

 may not nature change bodies into light, and light into 

 bodies ? ' 



Newton has here seemingly in his mind the emissive 

 theory of light ; but the passages might be applied to either 

 theory ; the analogy he saw in the change of state of matter, 

 as in ice, water, and vapour, with the hypothetic change into 

 light, is very striking, and would seem to show that he regard- 

 ed the change or transmutation of which he speaks as one 

 analogous to the known changes of state, or consistence, in 

 ordinary matter. 



The difference between the view which 1 am advocating 

 and that of the ethereal theory as generally enunciated is, 

 that the matter which in the interplanetary spaces serves as 

 the means of transmitting by its undulations light and heat, I 

 should regard as possessing the qualities of ordinary, or as it 

 has sometimes been called gross, matter, and particularly 

 weight ; though, from its extreme rarefaction, it would mani- 

 fest these properties in an indefinitely small degree ; whilst, on 

 the surface of the earth, that matter attains a density cognisa- 

 ble by our means of experiment, and the dense matter is 

 itself, in great part, the conveyer of the undulations in which 

 these agents consist. Doubtless, in very many of the forms 

 which matter assumes it is porous, and pervaded by more 

 volatile essences, which may differ as much in kind as matter 

 does. In these cases a composite medium, such a that indi- 

 cated by Dr. Young, would result ; but even on such a suppo- 

 sition, the denser matter would probably exercise the more 

 important influence on the undulations. Returning to the 

 somewhat strained hypothesis, that the particles of dense 



