394 Prof. Tyndall on Calorescence. 



companions. We must, in fact, discover a substance which shall 

 filter the composite radiation of a luminous source by stopping 

 the visible rays and allowing the invisible ones free transmission. 



Could we obtain a black elementary body thoroughly homo- 

 geneous, and with all its parts in perfect optical contact, experi- 

 ments 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 jelly prepared by Mr. Faraday, is exceed- 

 ingly 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 can- 

 not 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 experiments 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 suit- 

 able 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 trans- 

 mitted 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 radiation of 

 the electric light. A cylindrical cell, 2 inches in length and 

 2*8 inches in diameter, with its ends stopped by plates of per- 

 fectly transparent rock-salt, was placed empty in front of an 

 electric lamp ; the radiation from the lamp, after having crossed 



* Philosophical Transactions, vol. ciiv. p. 333 ; Philosophical Magazine, 

 S.4. vol. xxviii. p. 446. 



