72 Royal Society. 



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 ab- 

 sorption 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 unabsorbed through a considerable thickness of the solution ; 

 but a thickness 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 

 fragments 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 alter on 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 was still pink, the 

 upper half haviug 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 difference 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 dissimilar 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, 

 ' Chlorophyllfarbstoffe,' p. 53) that the position of the absorption- 

 bands of substances in solution vary to a certain extent with the 

 liquid in which they are dissolved ; but this would appear to de- 

 pend 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 posi- 



