162 



DAKK TITHONIC RAYS. 



676. One of the most striking proofs of this is the facility with which impressions of 

 the moon's disc may be obtained on Daguerreotype and other sensitive plates. Even 

 with lenses of comparatively small diameter, and in the space of a few minutes, strong 

 impressions of the moon's surface may be taken. There is no more difficulty in ob- 

 taining these sketches than there is in copying a building or a statue, or any other ob- 

 ject on which the sun is shining. But the moonbeams have hitherto given no trace of 

 the presence of heat. 



677. I found, moreover, by direct trial, that plates which had 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. 



678. Lastly, Proof of the existence of DARK. TITHONIC RAYS analogous to the rays of 



DARK HEAT. 



679. The experiments now to be described were made with Daguerreotype plates 

 iodized at first to a pale lemon yellow, then brought to a golden hue by immersion in 

 the vapour of bromine, and lastly exposed for a short time to the vapour of iodine again. 



680. Having exposed such a plate (Jig. 102), ab, 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 dis- 

 tance 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 remove 

 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 

 beneath c d will whiten powerfully. 



681. From this remarkable result I infer that the tithonicity that had originally dis- 

 turbed the surface of the plate equally all over, has escaped away from those portions 

 that were uncovered, but that its escape has been entirely prevented by the action of the 

 screen ; and this must be through RADIATION, for the screen is at a distance, and has 

 never touched the plate. And, farther, that the rays that do thus escape away are abso- 

 lutely invisible to the eye. 



682. 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 un- 

 covered portion would cool fast by radiation, but the screened portion more slowly, for 

 its radiation would be arrested by the glass plate. 



683. The two cases are absolutely alike. 



684. Tithonicity, therefore, radiates exactly after the manner of heat. 



685. This also furnishes proof, in addition to those I have heretofore given, that not 

 only does tithonicity become LATENT in bodies, but that it becomes latent in two ways, 

 transiently and permanently, exactly after the manner of heat. 



686. The same result is obtained when other sensitive surfaces are employed, the 



