1876.] 



Absorptiori-Spectra of Iodine. 



4^7 



first, those with which it gives reddish-brown solutions, like alcohol ; 

 and secondly, those with which it gives violet ones, as bisulphide of 

 carbon ; and also that the colours of these two solutions correspond 

 respectively with the colour of solid iodine, when seen by transmitted 

 light, and with that of iodine vapour. 



Andrews (Bnt. Assoc. Eeport, 1871) has also remarked that iodine 

 vapour and the solution of iodine in bisulphide of carbon are dichroic, 

 while such is not the case with its solution in alcohol. 



As I am not aware of any other observations on the absorption of 

 light by iodine in solution or in the solid state having been published, 

 I have the honour of having an account of some experiments I have 

 recently made on this subject communicated to the Eoyal Society. 



For these observations I have used one of Browning's spectroscopes 

 with a single dense-glass prism of 60°, as with a greater amount of dis- 

 persive power it became more difficult to observe the beginning and end 

 of the absorption. The spectroscope was firmly screwed to the wall of 

 the rooiQ, with the collimator pointing vertically downwards, the light 

 from a parafhn-lamp being reflected along it by a mirror — the width of 

 the slit and the position of the mirror and lamp remaining unaltered 

 during the course of the experiments, in order that the different absorp- 

 tion-spectra should, as far as possible, be comparable with each other. 

 The solution whose absorption was to be observed was contained in a 

 small beaker, supported by the ring of a retort-stand between the mirror 

 and the slit of the collimator. 



This arrangement was adopted in order to be able to observe the 

 absorption through various thicknesses of the same solution, without 

 having to use a wedge-cell, as some of the hquids in which iodine is 

 soluble act very quickly on the cement with which such cells are 

 fastened together. 



A vertical scale was attached to the beaker, so that by gradually 

 pouring a solution into it, the absorption through different knox^n thick- 

 nesses could be observed, the solutions of iodine in bisulphide and tetra- 

 chloride of carbon being covered with a thin layer of water to prevent 

 their evaporating. 



"When the absorption-spectra of solid and liquid iodine were to be 

 observed, the beaker was replaced by a large cork which fitted the ring 

 of the retort-stand, and through Avhich a hole had been bored in a 

 line with the axis of the collimator, and the glass slips between which 

 the iodine had been melted laid on this. In the case of the liquid 

 iodine, the low conductive power for heat of the cork retarded the 

 cooling of the glass, and facilitated the observation of the absorption. 



The telescope of the spectroscope, the eyepiece of which was fur- 

 nished with cross wires, was carried by an arm moving over a divided 

 arc ; and the position of 10 of the principal lines in the solar spectrum 

 having been observed, from these measurements, and from the wave- 



