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



that the portion on which the glass has been placed will whiten 

 powerfully, but all the rest will remain unchanged. 



This, therefore, proves that colourless glass is nearly opake 

 to the dark tithonic rays, a result observed also in the case of 

 the dark rays of heat. 



I made a comparative trial of the relative permeability of 

 colourless plate glass and common writing-paper. A sensitive 

 surface was exposed until it had slightly but very plainly com- 

 menced to turn brown. On one portion I now laid a piece 

 of clear glass, and by the side of it a piece of writing-paper ; 

 the arrangement was next placed in the dark for four hours; 

 it was then mercurialized at 160°Fahr. for an hour, and the 

 result was very striking. Notwithstanding the long exposure 

 to the mercury vapour, all those portions that had not been 

 covered were perfectly unaffected, the portion that had been 

 covered by the glass was of an intensely deep brown colour, 

 but the portion covered by the paper was marked by a distinct 

 but very faint white stain. It was therefore plain, that from 

 the uncovered portions all the tithonicity had radiated away, — 

 from the portions covered by the writing-paper the same effect 

 almost to the same extent had occurred, the paper, however, 

 slightly obstructing the passage of the rays, — but radiation had 

 been wholly prevented from those parts covered by the colour- 

 less glass. 



Writing-paper is therefore far more permeable to the dark 

 tithonic rays than the purest plate glass. 



This property it will be hereafter convenient to speak of 

 under the designation of Diatithonescence or Transtithones- 

 cence. 



Blue, red and yellow glass obstruct to a great extent the 

 process of radiation. In several trials it seemed as though 

 the yellow was more transparent than the others, but there 

 was not much difference. 



Transparent rock-salt appears to hold very nearly the same 

 relation of diatithonicity as plate glass. 



In like manner the following substances in thin plates ob- 

 struct the radiation of tithonicity : — Sulphate of lime, beryl, 

 agate, rock-crystal, calc-spar, mica, wafers, metallic bodies, 

 cloth of cotton, wood, ivory, coloured glass, &c, &c. 



The remarkable results described in the Philosophical 

 Transactions by Sir John Herschel (184-0, p. 44), but left 

 by him without any explanation, are of the kind now un- 

 der discussion. He found that paper washed with nitrate of 

 silver, if exposed to the sun under a piece of glass, darkened 

 much more rapidly than if the glass were away. This effect 

 was by no means limited to that variety of paper, but was ob- 



