Class of Chemical Bays analogous to the Rays of Dark Heat. 455 



but unlike his bride, in the course of events he became feeble 

 and decrepit, and losing all his strength was rocked to sleep 

 in a cradle. The goddess, pitying his condition, metamor- 

 phosed him into a grasshopper. 



The fact and the fable agree pretty well, and indeed the play- 

 ful coincidence might be carried much further. The powers of 

 photography, which bring architectural remains and the forms 

 of statuary so beautifully and impressively before us, might 

 seem to be prefigured by the speaking image of the son of 

 Tithonus and Aurora that was to be seen in the deserts of 

 Egypt. And besides this, such words as Tithonoscope, Ti- 

 thonometer, Tithonography, Tithonic effect, Diatithones- 

 cence, are musical in an English ear. In this paper I shall 

 therefore use the term Tithonicity and its derivatives in the 

 same manner that we use electricity and its derivatives. 



This communication takes up the consideration of three 

 distinct' facts : — 



1st. The proof of the physical independence of Tithoni- 

 city and Light. 



2nd. The proof of the physical independence of Tithoni- 

 city and Heat. 



3rd. The proof of the existence of dark Tithonic rays, 

 analogous to the rays of dark heat. Under this head it 

 will be shown, that tithonicity like heat enters transiently 

 into bodies producing specific changes on them, and then 

 slowly and invisibly radiates away. And the physical consti- 

 tution of the new class of rays thus formed is entirely differ- 

 ent from that of rays that come from incandescent sources ; a 

 distinction having a striking analogy in the case of heat. 

 Tithonicity becomes transiently and permanently latent in 

 bodies. 



The Plate (PI. I.), which accompanies this paper, serves to 

 show that by the agency of absorbent media we may detect the 

 existence of tithonic rays in every part of the spectrum unac- 

 companied by light. The results, there projected, were ob- 

 tained by an arrangement such as that in Plate I. 'fig. 1 . From 

 a heliostat mirror a a, a beam of the sun's light was thrown 

 in a horizontal position, and falling on a screen b b, a portion 

 of it passed through a circular aperture one-fourth of an inch 

 in diameter. At the distance of ten or twelve feet it fell on a 

 glass trough c c, with parallel faces, into which any coloured 

 solution could be placed ; immediately behind the trough 

 there was a double convex lens d d, of three feet focal length, 

 and between them a second screen ff t with an aperture cor- 

 responding to the centre of the lens, half an inch in diameter. 

 Behind the lens was situated a prism of flint glass e, which 



