THE RARE METAL* 283 



resists corrosion. These properties ought to lead to new uses of 

 run mid-rial significance. 



A small tonnage of tantalum-bearing minerals is produced by 

 tin- Wot mi 1! -duct ion Company of Omaha, Neb. The source 

 of tho ore was near Keystone, South Dakota. 



Selenium 



Properties. Selenium is a non-metallic element closely allied 

 to sulphur. Its association with copper, silver, lead, mercury, 

 bismuth and thallium, together with its relation to tellurium, a 

 semi-metallip element, has led to its consideration in this work 

 on the metallics. Selenium is known in four allotropic modifi- 

 cations: (1) A brick-red amorphous powder; (2) a black crystal- 

 line powder; (3) in dark red translucent monoclinic crystals; 

 and (4) a black, shining, brittle, amorphous mass. It is a con- 

 ductor of electricity. The conductivity is twice as great in 

 the presence of light as in the dark. The melting-point of selen- 

 ium is 217; it boils at 680 C., and burns with a blue flame to SeO 2 . 

 Its specific gravity varies from 4.26 to 4.8, and its atomic weight 

 is 79.2. 



Ores of Selenium. Native selenium, Se. 



Sekn-sulphur, SeS, an orange-red or reddish-brown mineral, 

 consisting of mixtures of selenium and sulphur in unknown 

 proportions. 



Selen-tellurium, SeTe, a blackish-gray mineral with metallic 

 luster, consisting of selenium and tellurium in the ratio of 2: 3. 



Clausthalite, PbSe, a selenide of lead. 



Naumannite, PbSe, 13Ag 2 Se. Another variety gives 5PbSe, 

 Ag 2 Se; a third variety is Ag 2 Se, with 73.15 per cent, of Ag. 



Guanajuatite, Bi 2 Se 3 , a selenide of bismuth. 



Berzelianite, Cu 2 Se, a selenide of copper. 



Lehrbachite, (PbHg 2 ) Se, a selenide of lead and mercury. 



Eucairite, Cu 2 Se, Ag 2 Se, a selenide of copper and silver. 



Crookesite, (Cu, Ag, Tl) 2 Se, a selenide of copper, silver, and 

 thallium. 



Zorgite, a mixture of the selenides of silver, lead, and copper. 



Origin of the Ores. The majority of the selenides are primary 

 minerals; only a few are of secondary origin. Native selenium 

 may be a product of volcanic emanation, like its associate 

 sulphur. Selen-sulphur occurs in crusts with sal-ammoniac on 



