LTGHT. 123 



bodies is very conformable to the course of Nature, which 

 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 regarded 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 I 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 properties 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, 

 near to or on the surface of the earth, that matter attains a 

 density cognisable by our means of experiment, and the dense 

 matter is itself, in great part, if not entirely, 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 as that indicated by Dr. Young, would result ; 

 but even on such a supposition, the denser matter would pro- 

 bably exercise the more important influence on the undula- 

 tions. Returning to the somewhat strained hypothesis, that 

 the particles of dense matter in a so-called solid are as distant 

 as the stars in heaven, still a certain depth or thickness of 

 such solid would present at every point- of space a particle or 

 rock in the successive progress of a wave, which particles, to 

 carry on the movement, must vibrate in unison with it. 



