164 DARK TITHONIC RAYS. 



glass, and by the side of it a piece of writing-paper; the arrangement was next pla- 

 ced in the dark for four hours; it was then mercurialized at 160 Fah. 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 por- 

 tion 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. 



699. Writing-paper is, therefore, far more permeable to the dark tithonic rays than 

 the purest plate glass. 



700. This property it will be hereafter convenient to speak of under the designation 

 of diatithonescence or transtithonescence. 



701. 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. 



702. Transparent rock-salt appears to hold very nearly the same relation of diatitho- 

 nicity as plate glass. 



703. In like manner, the following substances in thin plates obstruct the radiation of 

 tithonicity : Sulphate of lime, beryl, agate, rock-crystal, calc-spar, mica, wafers, metal- 

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



704. The remarkable results described in the Philosophical Transactions by Sir 

 JOHN HERSCHEL (1840, p. 44), but left by him without any explanation, are of the 

 kind now under discussion. He found that paper washed with nitrate of silver, if ex- 

 posed 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- 

 servable, also, with many other tithonographic compounds. Transparent minerals, such 

 as topaz, selenite, Iceland spar, quartz, produced the same results as glass. But on 

 gloomy days the phenomena did not appear, a bright sunshine being apparently requi- 

 site for their production. " When a piece of nitrated paper, for instance, was rolled 

 round a cylindrical surface of moderate convexity, covered with black velvet, and the 

 piece of glass laid gently in contact with it, the effect of sunshine was exalted at the 

 line of contact; but on either side of that line, as the interval increased, the influence of 

 the glass diminished, and at less than half an inch distance no difference could be per- 

 ceived between the impressions under the glass and in the free air." 



705. Now all this is precisely what should happen if the tithonographic compound 

 radiates while it is undergoing decomposition. The rays which come from the sun pass 

 through the glass with but little loss from absorption ; falling upon the nitrate, they de- 

 compose it, and now it commences radiating, but the physical character of these rays 

 is very different from the character they possessed before impinging on the nitrate 

 Now they cannot get through the glass, before they passed without difficulty. So it is, 



