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



intensity on photographs taken when the temperatures o£ 

 the vapour were 650° C, 840° C, and 980° 0. In addition 

 to the 3261 absorption line, photographs taken at tempera- 

 tures above 800° C. showed the presence of more diffuse 

 absorption lines of shorter wave-length, but later experiments 

 indicated that the lines could not possibly be attributed to 

 the chloride. These lines became more distinct as the 

 temperature increased, but their investigation was rendered 

 difficult by the general absorption in the same region pro- 

 bably due to the chloride vapour. 



In the second series of experiments, greater care was 

 taken to remove moisture from the chloride, and two photo- 

 graphs taken at 700° C. and 830° G. showed no trace of the 

 cadmium absorption line. There was, however, distinct 

 evidence of general absorption in the ultra-violet, but it was 

 less intense than the absorption by CdBr 2 and Cdl 2 . Even 

 in these experiments a slight decomposition of the vapour 

 was obtained at a higher temperature (940° C), and the 3261 

 absorption line together with the other lines of shorter wave- 

 length appeared on the photographic film. At this tempera- 

 ture all wave-lengths between X 3100 and X2500 were 

 almost completely absorbed. 



The absorption of cadmium-chloride vapour is illustrated 

 by photograph (2) (a) and (b), where (a) is the continuous 

 spectrum due to the positive pole of the carbon arc, and 

 (6) the same spectrum after passing through CdCl 2 vapour 

 at 940° C. It shows a general absorption of light of shorter 

 wave-length than 3100 A.U., and also the presence of 

 absorption lines at 3261, 3171, 3162, and 3152. The 3261 

 line is the well-known absorption line of cadmium vapour, 

 and the origin of the other lines will be discussed later. 



Experiments on the Absorption of Cadmium Vapour. 



The two specimens of cadmium used in these experiments 

 were analysed spectroscopically to test their purity. The 

 cadmium was placed in the positive pole of the carbon arc, 

 and two photographs were taken with the concave grating. 

 A careful examination of the photographs showed that the 

 two specimens did not contain any impurities. 



Wood * found that the absorption spectrum of cadmium 

 vapour consisted of two lines at X 228S'l and X 3261*2, which 

 can also be obtained as emission lines. The former line, 



* Wood, Astrophys. Journal, xxix. pp. 211-223 (1909). 



