Sir John Conroy on the Absorption-Spectra of Iodine. 71 



Iodine in Solution. 



As 1 have before mentioned, the liquids in which iodine is soluble 

 may be divided into two classes : — first, those with which it forms 

 brownish-red solutions, as alcohol, ether, ethyl bromide, Dutch liquid, 

 benzole, glycerine, potassium iodide in an aqueous solution, hydro- 

 gen chloride, &c. ; second, those with which it forms violet solu- 

 tions : this latter class is less numerous, and as far as I have hither- 

 to been able to ascertain by actual experiment, only consists of 

 the following substances — the bisulphide, tetrachloride, and mono- 

 chloride of carbon, chloroform, phosphorous terchloride, tin tetra- 

 chloride, and under certain circumstances, as will be hereafter men- 

 tioned, hydrogen sulphate. In Watts's ' Dictionary of Chemistry/ 

 vol. i. p. 881, it is stated that the solution of iodine in chloral has a 

 purple colour. 



In order to obtain solutions of iodine of a known strength, some 

 of the liquids in which it is soluble being very volatile, a small 

 glass weighing-tube closed with a cork was nearly filled with iodine 

 which had been crushed small with the edge of a platinum spatula. 

 A stoppered flask holding 25 c. c. was filled with the liquid in 

 which the iodine was to be dissolved ; and the tube having been 

 weighed, a small quantity of iodine was shaken out of it, and the 

 tube again weighed, and so on, till the desired amount had been 

 added to the liquid. 



In this way solutions of iodine in bisulphide and tetrachloride of 

 carbon, chloroform, 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 solu- 

 tion of potassium iodide, of different strengths were then prepared, 

 and the absorption produced by layers of these solutions, varying 

 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 gra- 

 dual one, it is somewhat difficult to determine the exact point at 

 which the absorption ends. Kg. 2 represents the mean results ob- 

 tained 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 resem- 

 bles 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. 



Tig. 1 represents the mean result of eighty-one measurements of 

 the position of the absorption produced by different thicknesses of 



