456 Prof. TyndalPs Contributions to Molecular Physics. 



of gas. The transparency of the liquid to the radiant heat was 

 not sensibly affected by the addition of the iodine. The lumi- 

 nous heat was of course cut off; but this, as compared with the 

 whole radiation, was so small as to be insensible in the expe- 

 riments. 



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 opake to light. I dissolved in the liquid a quantity of 

 the iodine sufficient, when introduced into a cell 0*07 of an inch 

 wide, to cut off wholly the light of the most brilliant gas-flame. 

 Comparing the opake solution with the transparent bisulphide, 

 the following results were obtained : — 



Deflection. Absorption. 

 Bisulphide of carbon (opake) . . . 9*0 12*5 



Bisulphide of carbon (transparent) . 9*0 12*5 



Here the presence of a quantity of iodine, perfectly opake to a 

 brilliant light, was without measurable 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 solution which was perfectly opake 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 (opake) . . . 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 dispersion is sought for in the decomposition 

 of white light. Such prisms, filled with the opake solution, inter- 

 cept 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 obtained entirely separated from 

 the luminous one. By means of a prism of the transparent 

 bisulphide, I determined the position of the spectrum of the 

 electric light upon a screen, and .behind the screen placed a 

 thermo-electric pile so that when the screen was removed the 

 extra-red rays fell upon the pile. I then substituted an opake 

 prism for the transparent one : there was no visible spectrum on 

 the screen ; but the removal of the latter at once demonstrated 

 the existence of an invisible spectrum by the thermo-electric cur- 

 rent which it generated, and which was powerful enough to dash 

 violently aside the needles of a large lecture-room galvanometer. 



