A. G. Belli—Production of Sound by Light. 307 
The chemical properties of this new element were found to 
‘ resemble those of tellurium in such a remarkable degree shat 
Berzelius gave to the substance the name of “ selenium,” from 
the Greek word oedjvy, the moon, (“tellurium,” as is well 
al ere; derived from 9 the earth). Although tellu- 
and selenium are alike in many respects, they differ in 
their sida properties ; eiariont being a good conductor of 
oe and selenium, as Berzelius deeded a non-conductor. 
Knox* discovered in 1887, that selenium became a con- 
Sastor pee fused ; and Hittorff + in 1851, showed that it 
conducted ordinary temperatures when in one of its allo- 
tropic form 
When selenium is rapidly cooled — a fused condition it is 
anon-conductor. In this, its “vitreous” form, it is o 
rown color, sithodt black by reflected light, having an exceed- 
ingly brilliant surface. In thin films it is transparent, and 
appears of a beautiful ruby wel: ra transmitted light. 
hen selenium is cooled from a fused condition with extreme 
selenium ; or as Regnault called it, “ metallic” selenium. 
It was selenium of ‘this kind that Hittorff found to be a con- 
ductor of electricity at ordinary temperature. 
He also found that its resistance to the passage of an electri- 
cal current diminished continuously by heating up to the point 
of fusion ; and that the resistance suddenly increased in passing 
from the solid to the liquid condition. 
It was early ee that exposure to sunlight § hastens 
the change of selenium from one allotropic form to another; 
and this siiasiabiee is significant in the light of recent 
discoveries. 
times found to be in the metallic condition, but more usually 
~~ are in the vitreous or apie sco form 
dt occurred to Willoughby Smith that, on account of the 
high resistance of crystalline selenium, it might be usefully 
employed at the shore-end of a submarine cable, in his system 
* Trans. ea Trish Acad. (1839), xix, 147; also Phil. 8 II, xvi, 185. 
Pogg. Ann., Ixxxiv, 214; ee ee IV, iii, 546 
Soo Draper and Mess in cad., Nov . 1873, I, so a p. a 
Gmelin’s Handbook of ‘Chemisty (184s,) ii, 235; see also the 
Phil. Mag. (si2) 1, iii, 547. 
