1883.] 



on the Ultra- Violet Spectra of tlie Elements. 



251 



C£esiiim ; and the indigo line of calcium is represented by a blue line 

 in the spectrum of strontium, and by a green line in the spectrum of 

 barium. 



We may observe something of the same kind in regard to the 

 haloid elements : the absorption band which in the case of the element 

 of lowest atomic weight, namely chlorine, is altogether ultra-violet, is 

 shifted towards the less refrangible side in the case of bromine, and 

 lies altogether in the visible region in the case of iodine, the element 

 of highest atomic weight. 



Fig. 3. 



ro 



o 

 o 



o 



CD 



ro 



ro 



i>3 ro (o 



01 01 O' 

 CO 00 CO 

 OOP 



r\} ro ro 



>J -VI CD 



o ^ - 



No. 

 No. 

 No. 

 No. 



6. 

 7. 

 8. 

 9. 



J5 



No. 10. 



Absorption of chlorine. 



bromine vapour, 

 iodine vapour, 

 bromine liquid, 

 iodine solution. 



It is remarkable that bromine in the liquid state and iodine in 

 solution show absorptions quite different from those of their vapours. 

 A thin film of liquid bromine between two quartz plates is transparent 

 for a band which ends just where the transparency of the vapour 

 begins, while the film is opaque for rays both above and below this 

 band. Iodine dissolved in carbon disulphide is also transi^rent for a 

 certain distance, but the band is shifted to a less refrangible region 

 lying between G and H. 



Compound gases and vapours show, as might be expected, various 

 absorptions of ultra-violet rays. The absorbent action of coal-gas 

 begins at about the wave-length 2680, and above 2580 it is nearly 

 complete. Sulphurous acid has an absorption band extending from 

 about R (3179) to rays of wave-length 2680, with a weaker absorption 

 extending some way beyond these limits on both sides. Sulphuretted 

 hydrogen produces a pretty complete obliteration of all rays above 

 wave-length 2580. Vapour of carbon disulphide in very small quantity 

 produces an absorption extending from P to T, shading away at each 

 end. With more vapour this band widens, and a second absorption 

 band begins at about the wave-length 2580. Chlorine peroxide gives 



