58 Mr. E. J. Evans on the Absorption Spectra 



would be mixed with mercuric-chloride vapour, and the 

 continuous spectrum from the carbon arc is very faint 

 below X 2800. 



The Bromide, Iodide, and Chloride of Cadmium. 



In the experiments with cadmium bromide, which melts 

 at 571° C. and boils at 809° C, *07 gram of the salt was 

 placed in an evacuated quartz tube, and it was calculated 

 that the pressure of the v;ipour was slightly in excess of 

 1 atmosphere at 1000° C. 



Two sets of experiments were carried out at temperatures 

 varying from 600° C. to 900° C, and several photographs of 

 the absorption spectrum were taken. The first series of 

 photographs showed no trace of any cadmium absorption 

 line, but the second series even at 600° showed the presence 

 of the cadmium absorption line at X 3261. The line was, 

 however, not strong on any of the films, showing that the 

 cadmium bromide had only undergone slight decomposition 

 at the higher temperatures. Apart from this line there were 

 no other absorption lines or bands on the photographs. At 

 600° C. there was evidence of general absorption below 

 X3100, and at 900° C. all wave-lengths between X 2500 and 

 X 3800 were completely absorbed by the cadmium-bromide 

 vapour. In later experiments *065 gram of metallic cndmium 

 were placed in the quartz tube, and a photograph of the 

 absorption spectrum was taken at 1000° C. The quantity of 

 tree cadmium present in this experiment was much greater 

 than in the experiments with the cadmium bromide, but 

 even then there was complete absorption only below X 3000. 

 These experiments, therefore, strongly point to the conclu- 

 sion that cadmium-bromide vapour shows general selective 

 absorption in the ultra-violet. The absorption dne to the 

 cadmium-bromide vapour is shown in PI. II. photograph 1 

 («) and (b) ; (a) giving the spectrum of the positive pole of 

 the carbon arc, and (b) the same spectrum after the light has 

 passed through the vapour at 900° C. The latter photo- 

 graph shows a general absorption of light of shorter wave- 

 length than 3800 A.U. 



Experiments were also carried out on the absorption of 

 light by the vapour of cadmium iodide, which melts at 

 404° C. and boils at 714° 0. The iodide was produced by 

 the combination of metallic cadmium with iodine contained 

 in a quartz tube, which was heated to a temperature of about 



