1876.] 



On the Polarization of Light. 



51 



alter ou cooling ; after standing 24 hours the upper layer of acid for a 

 depth of about one centimetre from the surface had become colourless ; 

 but at the end of five months the acid in the lower part of the tube \^'as 

 still pink, the upper half having become colourless, and a small quantity 

 of a black powder having settled at the bottom. 



Only a very small quantity of iodine . can be held in solution by the 

 hydrogen sulphate when cold, as any excess separates out in minute 

 crystals. 



It does not appear probable that the di:fference in the colour of the 

 solutions which iodine forms with, liquids of these two classes depends 

 on any chemical fact, as both classes contain substances of very dis- 

 similar chemical composition. I have not, however, as yet been able to 

 ascertain any common property possessed by all the liquids of either 

 class, beyond (as, indeed, is obvious) that all those in which iodine forms 

 violet solutions are volatile liquids of high specific gravity. 



It has been shown by various observers (H. Morton, Pogg. Ann. vol. 

 civ. p. 573 ; Hagenbach, Pogg. Ann. vol. cxlvi. p. 533 ; Kraus, ' Ghloro- 

 phyllfarbstoffe,' p. 53) that the position of the absorption-bands of sub- 

 stances in solution vary to a certain extent with the liquid in which they 

 are dissolved ; but this would appear to depend on some other cause ; 

 for, in addition to the displacement being small, it differs in amount 

 with different liquids ; whilst in the case of iodine, as far as I have been 

 able to observe, the position of the absorption is the same for all the 

 liquids belonging to one of the two classes. The action on light of 

 iodine dissolved in alcohol greatly resembles the effect it produces when 

 in the solid state ; whilst the absorption of its solution in carbon bisul- 

 phide, and in other liquids of that class, bears, as has been pointed out 

 to me by Professor Stokes, the same relation to the absorption-spectrum 

 of the vapour as the spectrum of the solution of a coloured gas (nitrogen 

 peroxide for example) does to that of the gas. 



II. ^^On tlie "Polarization of Liglit by Crystals of lodine.^^ By 

 Sir John Conroy^ Bart._, M.A. Communicated by A. G. 

 Vernon Harcourt, Lee^s Reader in Chemistry in the Uni- 

 versity of Oxford. Received April 12, 1876. 



On examining by means of a Kicol prism the light reflected from the 

 surface of a layer of iodine, obtained by heating a fragment of that sub- 

 stance and then squeezing it between two plates of glass, as described 

 in the preceding paper, I found that the film did not appear of uniform 

 brightness, and that when the Kicol Avas rotated the relative brilliancy 

 of different parts of the film changed — a portion that had appeared dark 

 when the principal section of the Nicol was vertical, became bright when 

 it was horizontal, and vice vend; and, also, if instead of altering the 



■ e2 



