Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter. Selenium exists in several allo- tropic forms. Three are generally recognized, but as many as six have been claimed. Selenium can be prepared with either' an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder fonrr, or black, in vitreous form.
Natural selenium contains six stable iso- topes.
Fourteen other nuclides and isomers have been characterized. The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds. Selenium is also able to convert a.c. It is used in Xerography for re- producing and copying documents, letters, etc. It is used by the glass industry to decolorize glass and to make ruby- colored glasses and enamels.