458 Dr. Draper on a new Imponderable Substance, and a 



been carefully prepared so as to be exceedingly sensitive, were 

 unaffected by the radiant heat of copper at any temperature 

 up to a red heat. These dark rays therefore have no kind of 

 effect on such surfaces. A sensitive plate may be made so hot 

 that it cannot be touched, yet its surface remains unchanged, 

 and even the radiant heat emitted by brightly incandescent 

 bodies has no effect, as I also proved. 



Lastly, — Proof of the existence of dark tithonic rays 

 analogous to the rays o/'dark heat. 



The experiments, now to be described, were made with Da- 

 guerreotype plates iodized at first to a pale lemon yellow, then 

 brought to a golden hue by immersion in the vapour of bro- 

 mine, and lastly exposed for a short time to the vapour of 

 iodine again. 



Having exposed such a plate, fig. 2, a b, to the action of 

 weak daylight or lamplight for a period of time which would 

 cause it to whiten powerfully all over if placed in the vapour 

 of mercury, carry it into a room which is totally dark, and 

 suspend at a distance of one-eighth of an inch from its surface 

 a metallic screen c d, the under-surface of which is blackened. 

 Let all remain in the dark four or five hours, and then re- 

 move the sensitive plate a b, and expose it to the vapour of 

 mercury. All that portion of it which was not covered by the 

 screen c d, will undergo no change, but that which was be- 

 neath c d will whiten powerfully. 



From this remarkable result I infer, that the tithonicity 

 that had originally disturbed the surface of the plate equally 

 all over, has escaped away from those portions that were un- 

 covered ; but that its escape has been entirely prevented by 

 the action of the screen ; and this must be through radia- 

 tion, for the screen is at a distance and has never touched 

 the plate. And, further, that the rays that do thus escape 

 away are absolutely invisible to the eye. 



Now, suppose a piece of black cloth, placed in the rays of 

 the sun until it has become warm, were carried into a cold 

 room and half its surface screened by some material, as a piece 

 of glass, at a short distance ; there cannot be a doubt that the 

 uncovered portion would cool fast by radiation, but the screen- 

 ed portion more slowly, for its radiation would be arrested by 

 the glass plate. 



The two cases are absolutely alike. 



Tithonicity therefore radiates exactly after the manner of 

 heat. 



This also furnishes proof, in addition to those I have here- 

 tofore given in this Journal, that not only does tithonicity be- 

 come latent in bodies, but that it becomes latent in two 



