8 PEOFESSOE TTNDALL ON CALOEESCENCE. 



parts in perfect optical contact, experiments already published would lead me to expect 

 that such a body would form an effectual filter for the radiation of the sun or of the 

 electric light. While cutting off the visible radiation, the black element would, I 

 imagine, allow the invisible to pass. Carbon in the state of soot is black, but its 

 parts are not optically continuous. In black glass the continuity is far more perfect, 

 and hence the result established by Melloni, that black glass possesses a considerable 

 power of transmission. Gold in ruby glass, or in the state of jeUy prepared by Mr. 

 Faraday, is exceedingly transparent to the invisible calorific rays, but it is not black 

 enough to quench entirely the visible ones. The densely brown liquid bromine is 

 better suited to our purpose ; for, in thicknesses sufficient to quench the light of our 

 brightest flames, this element displays extraordinary diathermancy. Iodine cannot be 

 applied in the solid condition, but it dissolves freely in various liquids, the solution in 

 some cases being intensely dark. Here, however, the action of the element may be 

 masked by that of its solvent. Iodine, for example, dissolves freely in alcohol ; but 

 alcohol is so destructive of the extra-red rays, that it would be entirely unfit for experi- 

 ments the object of which is to retain these rays while quenching the visible ones. The 

 same remark applies in a greater or less degree to many other solvents of iodine. 



The deportment of bisulphide of carbon, both as a vapour and a liquid, suggests the 

 thought that it would form a most suitable solvent. It is extremely diathermic, and 

 there is hardly another substance able to hold so large a quantity of iodine in solution. 

 Experiments already recorded prove that, of the rays emitted by a red-hot platinum 

 spiral, 94*5 per cent, is transmitted by a layer of the liquid 0*02 of an inch in 

 thickness, the transmission through layers 0*07 and 0*27 of an inch thick being 87*5 

 and 82 "5 respectively*. The following experiment with a layer of far greater thickness 

 exhibits the deportment of the transparent bisulphide towards the more intense radia- 

 tion of the electric light. A cylindrical cell, 2 inches in length and 2*8 inches in dia- 

 meter, with its ends stopped by plates of perfectly transparent rock-salt, was placed 

 empty in front of an electric lamp ; the radiation from the lamp, after having crossed 

 the cell, fell upon a thermo-electric pile, and produced a deflection of 



73°. 



Leaving the cell undisturbed, the transparent bisulphide of carbon was poured into 



it : the deflection fell to _ 



72°. 



A repetition of the experiment gave the following results : — 



Deflection, 

 o 

 Through empty cell .;.... 74 



Through bisulphide 73 



Taking the values of these deflections from a Table of calibration and calculating 

 • Philosophical Transactions, vol. cliv. p. 333 ; Philosophical Magazine, Ser. 4. vol. xxviii. p. 446. 



