ivhkh proceed from the Sim, - 311 



fever, seems to be merely an identical proposition, or a beg- 

 ging.of the question. For where is the proof of their so keep- 

 ing together? Perhaps it may be said it consists in this, 

 that we alwavs find a body is both illumined and warmed 

 as soon as exposed to sunshine; which shows that, wherever 

 the luminous particles are found, they are accompanied by 

 those of caloric. But this may be very possible without 

 supposing the particles in question have kept contiguous in 

 their journey all the way from the sun, or indeed longer 

 than a very small moment of time. Their velocities may 

 differ exceedingly, and yet, as to sense, particles of each kind 

 may every instant arrive simultaneously, and affect the 

 body. For this, nothing more is required than that the in- 

 terval of time between the arrival of one particle after an- 

 other, belonging to either of the rays, be so Very transitory 

 as wholly to elude our perception : a position which it 13 

 presumed will not be disputed. 



The same author, in the way of further proving the great 

 velocity of the rays of caloric, refers to some experiments 

 made by the ingenious M. Pictet,of Geneva, the result of 

 which shows, that they pass over a space of sixty-nine feet 

 in an interval of time too minute to be measured. If how- 

 ever the velocity in question, even supposed mueh slower 

 than the, astonishing speed of light., approximates to that 

 high scale of rapidity, such experiments would be utterly 

 incapable of assisting us when aiming at any measurements* 

 For, though the motion were a hundred times slower than x 

 that of light, still the calorific rays would be propagated 

 from the sun at the rate nearly of two thousand miles in a 

 second of time; a transit far too rapid to be measured or 

 ascertained by M. Pictet's method. 



If, indeed, the sun had been so constituted as that, now 

 and then, a total failure of his emanations took place for 

 twelve or fifteen minutes, their relative velocities could have 

 been determined, according to Mr. Pictet's method, by 

 watching the order of their arrival after such an inter- 

 mission. According to this, should his light,- for exam- 

 ple, move with double the velocitv of his calorific ravs, 

 a lens or speculum might give a bright image of the sun 

 deprived of all power of affecting the thermometer for near 

 four minutes, till the arrival of the rays of caloric, which, 

 would then give a burning focus. In like manner we 

 might wait shorter or longer, before the effects of the de- 

 oxidizing rays could be perceived beyond the violet colour 

 of the prismatic spectrum, according to their velocity com- 

 pired to that of light. Such means of trial are, how- 

 ever, utterly to be despaired of. 



Y 2 It 



