Prof. Tyndall on Color escence. 437 



the focus and the source of rays a cell containing the solution 

 of iodine. The transmitted obscure rays were found of sufficient 

 power to inflame paper, or to raise platinum-foil to incan- 

 descence. 



These experiments, however, were not unattended with danger. 

 The bisulphide of carbon is an extremely inflammable substance ; 

 and on the 2nd of November, while employing a very powerful 

 battery and intensely heated carbon-points, the solution took 

 fire, and instantly enveloped the electric lamp and all its appur- 

 tenances in flame. The precaution, however, had been taken of 

 placing the entire apparatus in a flat vessel containing water, 

 into which the flaming mass was summarily turned. The bisul- 

 phide of carbon being heavier than the water, sank to the bottom, 

 so that the flames were speedily extinguished. Similar accidents 

 occurred twice subsequently. 



Such occurrences caused me to seek earnestly for a substitute 

 for the bisulphide. Pure chloroform, though not so diathermic, 

 transmits the obscure rays pretty copiously, and it freely dis- 

 solves iodine. In layers of the thickness employed, however, 

 the solution was not sufficiently opake; and in consequence of 

 its absorptive power, but comparatively feeble effects were ob- 

 tained with it. The same remark applies to the iodides of 

 methyle and ethyle, to benzole, acetic ether, and other sub- 

 stances. They all dissolve iodine, but they enfeeble the results 

 by their action on the extra-red rays. 



I had special cells constructed for bromine and chloride of 

 sulphur : neither of these substances is inflammable ; but they 

 are both intensely corrosive, and their action upon the lungs and 

 eyes is so irritating as to render their employment impracticable. 

 With both of these liquids powerful effects were obtained ; still 

 their diathermancy, though very high, did not come up to that 

 of the dissolved iodine. Bichloride of carbon would be invalu- 

 able if its solvent power were equal to that of the bisulphide. It 

 is not at all inflammable, and its own diathermancy appears to 

 excel that of the bisulphide. But in reasonable thicknesses the 

 iodine which it can dissolve is not sufficient to render the solu- 

 tion perfectly opake. The solution forms a purple colour of in- 

 describable beauty. Though unsuited to strict crucial experi- 

 ments on dark rays, this filter may be employed with good effect 

 in class experiments. 



Thus foiled in my attempts to obtain a solvent equally good 

 and less dangerous than the bisulphide of carbon, I sought to 

 reduce to a minimum the danger of employing this substance. 

 At an earlier period of the investigation I had constructed a tin 

 camera, within which were placed both the lamp and its mirror. 

 Through an aperture in front, 2| inches wide, the cone of 



Phil. Mag. S 4. Vol. 31. No. 211. June 1866. 2 G 



