Sir John Conroy on the Absorption- Spectra of Iodine. 69 



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 liquids 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 gra- 

 dually pouring a solution into it, the absorption through different 

 known thicknesses could be observed, the solutions of iodine in bi- 

 sulphide and tetrachloride 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 which 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 obser- 

 vation of the absorption. 



The telescope of the spectroscope, the eyepiece of which was 

 furnished with cross-wires, was carried by an arm moving over a 

 divided arc ; and the position of 10 of the principal Hues in the 

 solar spectrum having been observed, from these measurements, 

 and from the wave-lengths of the same lines, as determined by 



o 



Angstrom, a curve was constructed, 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 

 thickness ; 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. "When 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 substance 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 



