Chap. 2.] Objections to the Theory ofDefcartes, &c. 173 



ibme diamonds, on being rubbed or chafed, are lumi- 

 nous in the dark. 2. That an electric fpark, not 

 larger, but much brighter, than the flame of a candle, 

 may be produced, and yet that no part of the electric 

 fluid is known to efcape, in fuch a cafe, to diftant 

 places, but the whole proceeds in the direction to which 

 it is deftined by the hand of the operator. Weaker 

 or ftronger fparks of this fluid are alfo known to differ 

 in colour ; the flrongeft are white and the weakeft red, 

 &c. 



To this opinion, however, there are many prefllng 

 and, indeed, infurmountable objections, id, The ve- 

 locity of found bears a very fmall proportion to that of 

 light. Light travels, in the fpace of eight minutes, a 

 diftance in which found could not be communicated in 

 feventeen years; and even our fenfes may convince 

 us, if we attend to the explofion of gunpowder, &c. 

 of the alrnoft infinite velocity of the one compared 

 with that of the other, adly, If lightdepended alto- 

 gether on the vibrations jof a fluid, no folid reafon can 

 be affigned why this fluid fhould ceafe to vibrate in 

 the night, fmce the fun mud always affect forne pare 

 of the circumambient fluid, and produce a perpetual 

 day. 3dly, The artifice of candles, lamps, &c. would 

 be wholly unnecefiary upon this hypothefis, fmce, by 

 a quick motion of the hand, or of a machine contrived 

 for this purpofe, light might on all occafions be eafily 

 produced. 4thly, Would not a ray of light, admitted 

 through a fmall aperture, put in motion, according to 

 this theory, the whole fluid contained in a chamber ? 

 In fact, we know that light is propagated only in right 

 lines, whereas found, which depends upon vibration, 

 is propagated in every direction. 5thly, The fepara- 

 tion or extenfion of the rays, by means of the prifm, 

 can never be accounted for by the theory of a vibrat- 

 ing 



