50 



On the Absoi'ption-Sjjecira of Iodine. 



[May 18, 



cliloroform, alcohol, and in aqueous solutions of potassium iodide were 

 prepared, and the absorption produced by different thicknesses of these 

 solutions observed. 



Solutions of iodine in alcohol, and of iodine in an aqueous solution of 

 potassium iodide, of different strengths were then prepared, and the ab- 

 sorption produced by layers of these solutions, var3dng in thickness from 

 5 mm. to 25 mm., was observed. 



Solutions of iodine in both liquids were, when of equal strength, of 

 the same colour and the absorption-spectra were similar, and also similar 

 to that of the solid iodine, except that the absorption did not appear to 

 extend quite so far down the spectrum ; but as the transition from strong 

 absorption to none at all is a very gradual one, it is somewhat difficult 

 to determine the exact point at which the absorption ends. Tig. 2 repre- 

 sents the mean results obtained from thirty observations through different 

 thicknesses of the alcohol solutions. 



Iodine dissolved in bisulphide of carbon absorbs most strongly light 

 of mean refrangibility ; and the absorption-spectrum resembles that of 

 the vapour of iodine as represented in the map to M. Thalen's memoir, 

 ' Le spectre d'absorption de la vapeur d'lode' (Upsal, 1869), except that 

 the absorption is continuous, and does not extend quite so far towards 

 the less-refrangible end of the spectrum. 



Eig. 1 represents the mean result of eighty-one measurements of the posi- 

 tion of the absorption produced by different thicknesses of these solutions. 

 The rays whose wave-lengths lie between 450 and 560 (approximately) 

 are first absorbed ; and as the thickness of the layer or the concentration 

 of the solution is increased, the absorption extends towards both ends of 

 the spectrum, though more rapidly towards the less-refrangible one. 



Just as was the case with the solution of iodine in alcohol, and with 

 solid and liquid iodine, light of a wave-length of about 650 passes un- 

 absorbed through a considerable thickness of the solution ; but a thick- 

 ness which is sufficient to stop the whole of the red rays still allows the 

 blue and violet ones to pass ; hence, whilst dilute solutions of iodine in 

 bisulphide of carbon and other liquids of that class appear of a kind of 

 red, when the light passes through a greater thickness, or the solution 

 is more concentrated, they appear blue or violet. 



Iodine is insoluble in cold hydrogen sulphate ; but when some frag- 

 ments of iodine are placed in a test-tube, partially filled with strong 

 hydrogen sulphate, and the tube heated, the iodine first melts, and then 

 gradually colours the liquid, till it becomes about the same tint as a 

 very dilute solution of iodine in bisulphide of carbon. 



•0100 grm. of iodine was placed in a test-tube containing 25 c. c. 

 of strong hydrogen sulphate, and the test-tube carefully heated over 

 a spirit-lamp until the whole of the liquid iodine had disappeared ; 

 the acid appeared of the same colour as a solution of iodine in chloroform, 

 containing about the same amount of iodine. The colour of the acid did not 



