8h THE POPULAR SCIENCE MONTHLY. 



In the year 1817 Berzelius and Gottlieb Gahn made an examination 

 of the method of preparing sulphuric acid in use at Gripsholm. Dur- 

 ing the course of this examination, they observed in the acid a sedi- 

 ment of a partly reddish, partly clear brown color, which, under the 

 action of the blowpipe, gave out a peculiar odor, like that attributed 

 by Klaproth to tellurium. As tellurium was a substance of extreme 

 rarity, Berzelius attempted its production from this deposit ; but he 

 was unable, after many expei-iments, to obtain further indications of 

 its presence. He found plentiful signs of sulphur mixed with mercury, 

 copper, zinc, iron, arsenic, and lead, but no trace of tellurium. It was 

 not in the nature of Berzelius to be disheartened by this result. In 

 science every failure advances the boundary of knowledge as well as 

 every success, and Berzelius felt that, if the characteristic odor that 

 had been observed did not proceed from tellurium, it might possibly 

 indicate the presence of some substance then unknown to the chemist. 

 Urged on by this hope he returned with renewed ardor to his work. 

 He collected a great quantity of the material, and submitted the whole 

 mass to various chemical processes. He succeeded in separating suc- 

 cessively the sulphur, the mercury, the copper, the tin, and the other 

 known substances whose presence had been indicated by his tests 

 and, after all these had been eliminated, there still remained a residue 

 which proved upon examination to be what he had been in search of 

 a new elementary substance. The chemical properties of this new 

 element were found to resemble those of tellurium in so remarkable a 

 degree that Berzelius gave to the substance the name of " selenium," 

 from the Greek word selene, the moon ("tellurium," as is well known, 

 being derived from tellies, the earth). 



Although tellurium and selenium are alike in many respects, they 

 differ in their electrical properties, tellurium being a good conductor 

 of electricity, and selenium, as Berzelius showed, a non-conductor. 

 Knox discovered in 1837 that selenium became a conductor when 

 fused ; and Hittorff in 1852 showed that it conducted at ordinary 

 temperatures, when in one of its allotropic forms. When selenium is 

 rapidly cooled from a fused condition, it is a non-conductor. In this 

 its vitreous form it is of a dark-brown color, almost black by reflected 

 light, having an exceedingly brilliant surface. In thin films it is trans- 

 parent, and appears of a beautiful ruby red by transmitted light. 

 When selenium is cooled from a fused condition with extreme slow- 

 ness, it presents an entirely different appearance, being of a dull lead- 

 color, and having throughout a granulated or crystalline structure, and 

 looking like a metal. In this form it is perfectly opaque to light even, 

 in very thin films. This variety of selenium has long been known as 

 " granular " or " crystalline " selenium, or, as Regnault called it, " metal- 

 lic " selenium. It was selenium of this kind that Hittorff found to be 

 a conductor of electricity at ordinary temperatures. He also found 

 that its resistance to the passage of an electrical current diminished 



