THE SUBLIMATION CURVE FOR SELENIUM CRYSTALS 

 OF THE HEXAGONAL SYSTEM 



L. E. DODD 



The type of apparatus used in the present work on the vapor 

 tension of selenium, as well as the method, is due originally to 

 Knudsen.^ The apparatus in the form in which Knudsen used it 

 was modified somewhat by Egerton.^ Egerton's experimental 

 sc'heme has been followed essentially in the present work. 



The selenium was placed in the lower chamber of a glass tube, 

 which in turn was set in place in a Freas electric oven, after the 

 temperature had been adjusted. Tap water was run through the 

 condensation tube for cooling. The apparatus was exhausted by 

 oil pump, Gaede mercury pump, and Langmuir condensation pump, 

 all in series, so that a good vacuum was obtained, as shown on the 

 McLeod gauge. The saturated selenium vapor passed up through 

 apertures in a platinum partition. This partition, as well as the 

 tube itself, ofifered resistance to the vapor flow, which was of the 

 molecular type described by Knudsen. The resistance is a constant 

 of the tube, which includes partition with apertures. Its value was 

 determined by measurements of the dimensions and calculation 

 therefrom by the use of Knudsen's eqviations for molecular flow, 

 and the calculated value was then checked by experimental value "> 

 obtained with mercury as the standardizing material. 



The vapor tension was computed from the Knudsen equation, 

 p_pi=W/VP'-G/T, 

 where p is the vapor tension whose value is sought, p^ the vapor 

 tension at the temperature of the condensation tube, regarded here 

 as negligible, W is the tube resistance, G the amount in grams of 

 selenium passing over in time T, and P' the vapor density at unit 

 pressure and at temperature of the tube. 



A considerable amount of data was taken on the vapor tension 

 of selenium during the past year. One of the latest and best of the 

 sublimination curves for selenium crystals of the hexagonal system 

 is presented. The measurements, which will be found in Table I, 



lAnnalen der Physik, 29, p. 179, 1909 



^Eg-erton, Philosophical Magazine, V. 33, 193, p. 33. 



