PROFESSOR TTKDALL ON CALORESCENCE. 9 



the transmission, that through the empty cell being 100, we obtain the following 



results: — 



Transmission. 



For the first experiment .... 94*9 

 For the second experiment . . . . 94"6 



Mean . . . 94-8 



Hence the introduction of the bisulphide lowers the transmission only from 100 

 to 94-8*. 



A perfect solvent of the iodine would be perfectly neutral to the total radiation ; and 

 the bisulphide of carbon is sho\vn by the foregoing experiment to approach tolerably 

 near perfection. We have in it a body capable of transmitting with little loss the total 

 radiation of the electric light. Our object is now to filter this total, by the introduc- 

 tion into the bisulphide of a substance competent to quench the visible and transmit 

 the invisible rays. Iodine does this with marvellous sharpness. In a short paper " On 

 Luminous and Obscure Radiation," published in the Philosophical Magazine for Novem- 

 ber 1864, the diathermancy of this substance is illustrated by the following Table : — 



Table III. — Eadiation through dissolved Iodine. 



Source. Transmission. 



Dark spiral of platinum wire . . . 100 



Lampblack at 212° Fahr 100 



Red-hot platinum spiral 100 



Hydrogen-flame 100 



Oil-flame 97 



Gas-flame 96 



White-hot spiral 95-4 



Electric light, battery of 60 cells . . 90 



These experiments were made in the following way : — A rock-salt cell was first filled 

 with the transparent bisulphide, and the quantity of heat transmitted by the pure Kquid 

 to the pile was determined. The same cell was afterwards filled with the opaque solu- 

 tion, the transmission through which was also determined. Calling the transmission 

 through the transparent liquid 100, the foregoing Table gives the transmission through 

 the opaque. The results, it is plain, refer solely to the iodine dissolved in the bisul- 

 phide, — the transmission 100, for example, indicating, not that the solution itself, but 

 that the body dissolved is, within the limits of error, perfectly diathermic to the radia- 

 tion from the first four sources. 



The layer of liquid employed in these last experiments was not sufficiently thick 

 to quench utterly the luminous radiation from the electric lamp. A cell was therefore 



* The partial destruction of the reflexion from the sides of the cell by the introduction of the bisulphide is 

 not here taken into account. 



MDCCCLXVI. C 



