342 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
§ 6 . 
In Table III. a fact is revealed which is worth a little further attention. The mea- 
surements there recorded show that the absorption of a layer of iodide of methyl, 
strongly coloured with iodine (which, doubtless, had been liberated by the action of 
light), was precisely the same as that of a perfectly transparent layer of the liquid. 
The iodine, which produced so marked an effect on light, did not sensibly affect the 
radiant heat emitted by the platinum spiral. Here are the numbers : — 
Absorption. 
Iodide of Methyl (transparent) : 53-2 
Iodide of Methyl (strongly coloured with iodine) . . . 53*2 
In this case, the incandescent spiral, or a flame, was visible when looked at through 
the liquid; I therefore intentionally deepened the colour (a rich brown), by adding 
iodine, until the layer was of sufficient opacity to cut off wholly the light of a brilliant 
jet of gas. The transparency of the liquid to the radiant heat was not sensibly affected 
by the addition of the iodine. The luminous heat was of course cut off ; but this, as 
compared with the whole radiation, was so small as to be insensible in the experiments. 
It is known that iodine dissolves freely in the bisulphide of carbon, the colour of the 
solution in thin layers being a splendid purple ; but in layers of moderate thickness it 
may be rendered perfectly opaque to light. I dissolved in the liquid a quantity of the 
iodine sufficient, when introduced into a cell O07 of an inch wide, to cut off wholly the 
light of the most brilliant gas-flame. Comparing the opaque solution with the transpa- 
rent bisulphide, the following results were obtained ; — 
Deflection. Absorption. 
Bisulphide of Carbon (opaque) .... 9‘0 12‘5 
Bisulphide of Carbon (transparent) . . . 9’0 12 '5 
Here the presence of a quantity of iodine, perfectly opaque to a brilliant light, was with- 
out measureable effect upon the heat emanating from our platinum spiral. The liquid 
was sensibly thickened by the quantity of iodine dissolved in it. 
The same liquid was placed in a cell 0-27 of an inch in width ; that is to say, a solu- 
tion which was perfectly opaque to light, at a thickness of 0-07 was employed in a layer 
of nearly four times this thickness. Here are the results : — 
Deflection. Absorption. 
Bisulphide of Carbon (transparent) . . . 13-6 18-8 
Bisulphide of Carbon (opaque) .... 13-7 19-0 
The difference between the two measurements lies within the limits of possible error. 
Bisulphide of carbon is commonly used to fill hollow prisms, when considerable dis- 
persion is sought for in the decomposition of white light. Such prisms, filled with the 
opaque solution, intercept entirely the luminous part of the spectrum, but allow the extra- 
red rays free passage. A heat-spectrum of the sun, or of the electric light, may be thus 
