210 
being excited by friction V Thus, then, we see, 
that, although in his inquiries concerning the mo- 
tions of light, Newton makes no mention of attrac- 
tion, yet, in his profound meditations on the general 
phenomena of nature, he not only employs that term, 
but points to the very species of attractive force, 
which the foregoing investigations have suggested ; 
and, with almost prophetic sagacity, has foretold the 
existence of those " virtues or forces" in the elementa- 
ry particles of bodies, on which the researches of Mr 
Davy have rendered it probable that their attractive 
energies depend. 
486. From these researches, it would appear, that 
the elements of matter naturally possess different 
electrical states, which . respectively determine their 
attraction or repulsion by the opposite poles of the 
Voltaic battery ; and these natural electricities Mr 
Davy supposes them to retain in all the combinations 
into which they enter f. If this doctrine be just, 
and if the rays of light do obey laws similar to those 
of the electric fluid, there is no difficulty in concei- 
\ Ing that the particles of matter may exert different 
forces upon the luminous rays ; and hence their re- 
fraction would vary both according to the natural 
electricities of the elements of the body, and to the 
properties of the individual rays ; and the modified 
operation of such fo;ces might be conceived suffi- 
cient to account for the varied refraction and extinc- 
tion of the different rays of light. 
* Optics, Query 31. 
t Phil. Trans. 1807. 
