EXAMINATION OF THE THEORY OF M. DECQUEREL. 71 



substances in nature, each saving its own manifestations when exposed to the sun's ray, 

 we should make inquiries like the following : How is it that a piece of black cloth ex- 

 posed to the moonbeams does not become warm ? How is it that a cannister of hot 

 water is not luminous to the eye ? In the rays that come from the moon, and those 

 that are emitted by warm water, is there no intrinsic difference, or does the phenom- 

 enon depend on the receiving surface alone ! What becomes of the beautiful exper- 

 iments of MELLOM, on the physical independence of light and heat, since these are 

 uiaiulv founded on the fact, that by the use of absorbent media we can separate one 

 from the other ? How is it that the rays of an electric spark, passing through quartz, 

 cau make the Bolognian stone phosphoresce, but passing through glass, equally trans- 

 parent and equally colourless, can do no such thing ? The receiving surface is the 

 same in both cases, and, as far as human eyesight can discover, the light that comes 

 through the glass is as pure and unaltered as the light that came through the quartz, 

 but the results are diametrically opposed. And is it not more consonant to reason to 

 suppose that the glass was opaque, or impervious to some agent existing in that beam, 

 which freely passed the quartz opaque to it, but transparent both to light and the 

 tithonic rays ! 



261. We might multiply cases like these, and give similar arguments from chemical 

 changes on sensitive surfaces ; but the instances already cited seem abundantly suffi- 

 cient to overturn the hypothesis in question. Before it can be admitted, it must give 

 a reason why the retina is not affected with the sensation of vision when rays from hot 

 water fall on the eye, w 7 hy a thermometer will not rise when placed in the moonshine, 

 why sulphuret of lime or barytes will not phosphoresce when covered with a piece 

 of glass. 



262. We are thus forced to admit that rays of light, rays of heat, tithonic rays, 

 phosphoric rays, and probably many other radiant forms, have an independent exist- 

 ence, and that they can be separated, by proper processes, from each other. 



26-3. It must, however, be understood, that the conclusion here arrived at essentially 

 depends on the following facts : 1st. The constant visibility of light 2d. The uniform- 

 ity of the action which heat exhibits in expanding bodies. If the progress of science 

 should require us to admit that there can exist light which impresses our eyes with a 

 sensation of darkness, or heat which can neither raise a thermometer nor produce the 

 sensation of warmth, then the force of the foregoing arguments will be essentially af- 

 fected. Such extensive changes in the universal acceptation of words will, unques- 

 tionably, be very slowly received. 



