Selenium was discovered by Berzelius'in 1817, who found it associated with tellurium, named for the earth. Selenium is found in a few rare minerals, s'uch as crooksite and clausthalite. 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.