48 Sir John Com-ov on the [May 18, 



lengths of the same lines, as determined by Angstrom, a cnrve was con- 

 structed, by means of which the readings of the spectroscope were reduced 

 to wave-lengths. 



Solid Iodine. 



Layers of iodine sufficiently thin to be transparent can be readily 

 obtained, as Schultz-Sellack has remarked, by squeezing melted iodine 

 between two pieces of flat, well-polished glass : it is only necessary to 

 place a small fragment of iodine between two pieces of glass which 

 have been previously well cleaned with alcohol, and heat them over a spirit- 

 lamp till the iodine melts, and then press them together. I have obtained 

 the best results by heating the iodine till it just melts, placing the pieces 

 of glass on a smooth block of wood and squeezing them together with 

 a flat cork. 



The layers of iodine thus obtained are not usually of uniform thick- 

 ness ; and, in addition to this, they contain so little iodine that I was 

 unable to determine their thickness by ascertaining the weight and area 

 of the film. "WTien seen, however, by reflected and transmitted light, 

 the iodine film usually appears surrounded by coloured rings ; and as 

 these alter their position and shape when the glass slips are pressed 

 together, they must be due to a thin layer of air, and not to any sub- 

 stance adhering to the glass ; and consequently the layers of iodine are 

 probably less than -00004 inch, or -001016 mm., in thickness. 



When seen by transmitted light, these layers of iodine vary in colour 

 from a deep brownish red, through drfferent shades of brown, to a more 

 or less pure yellow, according to the thickness and nature of the film ; 

 for, as is shown in the paper " On the Polarization of Light by Crystals 

 of Iodine" (infm, p. 51), the colour of the transmitted light apparently does 

 not depend solely on the thickness of the layer of iodine through which it 

 passes. These films correspond in colour with alcoholic solutions of iodine of 

 different strength, and the absorption- spectra are very similar — the whole 

 of the blue end of the spectrum being cut o:ff, and the absorption extend- 

 ing further and further towards the less refrangible end of the spectrum, 

 as the thickness of the film increases, till at length only light having a 

 wave-length of about 650 (in " tenth-metres "), or slightly more refran- 

 gible than the C line, passes through ; and a very slight increase in the 

 thickness of the film is sufficient to stop this also. 



Fig. 4 shows the appearance with a film of medium thickness, the 

 strong absorption ending at a point about one third of the distance 

 between E and D, together with a certain amount of darkening extend- 

 ing to about D, and represents the mean results from measurements of 

 the absorption-spectra of seven different films of iodine. 



Liquid Iodine. 



"With a little careful management it is usually possible to melt one of 

 these thin layers of iodine by heating it over the flame of a spirit-lamp, 



