6$ lloyal Society: — 



removed. The conductivity of the bar again increased from day 

 to day, and is still steadily but slowly increasing (eleven days after 

 the second sealing of the tube). Although the bar of selenium now 

 possesses a comparatively low resistance, I cannot detect the 

 slightest alteration in the size of the minute globule of mercury 

 which has supplied the material for the conducting-filin, extending 

 over a surface one thousand times greater than that of the globule. 



The granular modification produced by subjecting vitreous sele- 

 nium to a temperature of 100° C. for three hours also acquires a 

 great increase of conductivity when exposed to the vapour of mer- 

 cury in the Sprengel vacuum. 



As it is possible at any moment to arrest the formation of these 

 conducting-films, bars of selenium of any given high resistance 

 may be obtained in this way with great certainty and accuracy. 



May 18.— Dr. J. Dalton Hooker, C.B., President, in the Chair. 



The following communication was read : — 



"Absorption-Spectra of Iodine." By Sir John Conroy, Bart., M.A. 



Iodine, as is well known, when in very thin layers, appears red 

 by transmitted light ; and when in solution the colour of the liquid 

 depends not only on the amount of iodine contained in it, but also 

 on the nature of the liquid in which it is dissolved. 



Schultz-Sellack has pointed out (Pogg. Ann. vol. cxl. p. 334) 

 that the liquids in which iodine is soluble may be divided into two 

 classes : — 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 (Brit. Assoc. Report, 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 absorp- 

 tion of light by iodine in solution or in the solid state having been 

 published, I have the honour of having an account of some ex- 

 periments I have recently made on this subject communicated to 

 the E-oyal Society. 



Por these observations I have used one of Browning's spectro- 

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

 amount of dispersive power it became more difficult to observe the 

 beginning and end of the absorption. The spectroscope was firmly 

 screwed to the wall of the room, with the collimator pointing ver- 

 tically downwards, the light from a paraffin-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 absorption-spectra should, 

 as far as possible, be comparable with each other. The solution 

 whose absorption was to be observed was contained in a small 



