Prof. Tyndall on Calorescence. 395 



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

 tion of 730 



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 calibra- 

 tion and calculating 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^ 



Hence the introduction of the bisulphide lowers the transmis- 

 sion 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 shown 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 introduction 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 Phi- 

 losophical Magazine for November 1864, the diathermancy of 

 this substance is illustrated by the following Table : — 



Table III. — Radiation 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 50 cells . . 90 

 These experiments were made in the following way : — A rock- 

 salt cell was first filled with the transparent bisulphide, and the 



* The partial destruction of the reflexion from the sides of the cell by 

 the introduction of the bisulphide is not here taken into account. 



