WITH THOSE OF PONDERABLE MATTER. 99 



by Sir David Brewster, that " the particles of light 

 are identical with the ultimate atoms of bodies, and 

 that there is a specific affinity between definite atoms 

 and definite rays, though we do not understand 

 its nature." (Transactions of the Brit. Association, 

 vol. i. p. 231.) He also observes in another 

 work, that " when a portion of light enters a 

 body, and is never again seen, we are entitled to 

 say that it is detained by some power exerted 

 over it by the particles of the body ; that it enters 

 into chemical combination with them, and pro- 

 duces the various chemical effects by which their 

 colours are changed, and the juices of plants 

 elaborated," to which he adds, " It is not easy to 

 allow that such effects are produced by undula- 

 tions of an aethereal medium." (Optics, chap. 

 xxxiv.) 



That every description of ponderable matter is 

 actually convertible into light by a sufficiently 

 intense heat, or by electricity, will appear from 

 the following undeniable facts : 



1. That the quantity of light generated by 

 ordinary combustion, friction, or percussion, is 

 always in proportion to the rapidity with which 

 ponderable matter is ignited and volatilized. 



2. That the colour of light thus produced always 

 depends on the species of matter employed. 



all bodies that burn with a green light must be compounds of 

 two or more primitive elements ; while others, again, which afford 

 different proportions of all the coloured rays of the spectrum are 

 still more complex. 



