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TIT. On the Refractive Indices of Gaseous Potassium, Zinc, Cadmium, Mercury, 



Arsenic, Selenium and Tellurium. 



liy C. CUTHBERTSOX and E. PARR METOAI.FK, H.Sc. 

 Communicntfd by Professor F. T. TROUTOX, F. It.S. 



Received October 23, Read November 18, 1906. 



WE have continued, with more perfect apparatus, the enquiry recorded in a previous 

 paper* on the refractive indices of the vapours of elements which are not gaseous at 

 ordinary temperatures. The instrument employed was the refractometer of JAMIN, 

 and the arrangement of the apparatus has been fully described in the paper just 

 cited, so that it requires only a brief recapitulation here. 



Two similar exhausted tubes are placed in the paths of the rays of monochromatic 

 light between the mirrors, and a known weight of the element is vaporised in one of 

 them, of which tlie volume is known. The tubes are heated by means of a simple 

 form of electric furnace, consisting of two coils of nickel wire embedded in asbestos. 



The number of interference bands which pass a fiducial mark in the field of the 

 observing telescope during the evaporation, or return during the condensation, 

 combined with the other data, give the index for the wave-length employed. 



The calculation is as follows : 



If N be the number of bands observed to pass the fiducial mark, X the wave-length 

 X the length of the tube, in the refractive index observed, and /i the refractive index 

 at the standard density selected, we have 



and 



p. 1 _ standard density 

 7j> 1 observed density 



_ '00009 x atomic weight_qf element volume of tube 



atomic weight of hydrogen weight of element volatilised 



Hence 



,\ _ NX '00009 x atomic weight of element x volume of tube 

 X atomic weight of hydrogen x weight volatilised 



* 'Phil. Trans.,' A, vol. 204, p. 323, 1905. 

 VOL. OCVII. A 415. j.07 



