Ultra-violet Spectra of Magnesium and Selenium. 457 



Selenium. 



1. Spark Spectrum Experiments. — The most important 

 work on the emission spectrum of selenium appears to have 

 been done by Messerschmidt * and by Berndt f. The 

 electrical conductivity of metallic selenium, as is well known^ 

 is exceedingly low, and as a consequence it is impossible to 

 produce an arc or a spark in the usual manner. In his 

 experiments Messerschmidtused a strong condensed discharge 

 through a quartz Geissler tube containing a small bead of 

 selenium. 



Berndt investigated the spark spectrum by melting small 

 globules of selenium on the tips of platinum wires about 

 1*2 mm. in diameter and then passing a condensed discharge 

 across the terminals. The selenium metal was vaporized by 

 the heat of the spark, and its spectrum was obtained as well 

 as that of the spark spectrum of platinum. According to 

 Kayser's Handbuch der Spectroscopies the lowest limit reached 

 was about \ = 2340 A.U. 



In the present investigation the selenium spark spectrum 

 was obtained superimposed upon that of carbon. Two com- 

 mercial solid carbons were used. The lower one was cratered 

 and filled with a bead of gray vitreous selenium metal, 

 while the upper electrode was pointed and placed centrally 

 over the lower one. When the condensed discharge from a 

 Clapp-Eastham half-kilowatt transformer of 10,000 volts 

 was passed across the gap, it gave a cone of light reaching 

 from the tip of the upper electrode to the periphery of the 

 crater of the lowrr one. Owing to the fact that carbon is a 

 poor conductor of heat, the energy in the discharge was 

 sufficient to boil the selenium in the crater and the vapour 

 passed out through the cone discharge. The spectrum of 

 the carbon spark alone was first photographed and then that 

 of the carbon and selenium combined. As stated already 

 the spectrograms were taken with a quartz Hilger spectro- 

 graph type A. In all the photographs of the spectrum 

 taken in this way no lines due to selenium were obtained of 

 wave-length longer than 2200 A.U. Berndt's method was 

 also tried with aluminium wires in place of platinum ones, 

 but with the same result. This lack of lines in the longer 

 wave-lengths was probably due to the way the spectrum 



* J. Messerschmidt, Dissertation, Bonn, 1907; Zs. Wiss. Photoyraphie. 

 t. p. 249 (1907). 



f G. Berndt, Ann. der Phys. xii. p. 1115 (1903). 



