Deafer & Moss — 
On some Forms of Selenium.. 
531 
The first three modifications it should be mentioned, resemble 
one another in their physical and chemical relations, and must be 
regarded as different conditions of the same allotropic form of sele- 
nium. 
It is here only necessary to speak of vitreous selenium, and of its 
heat-produced allotropic modification, the granular variety, or as it has 
been felicitously called by Regnault, metallic selenium. 
Yitreous selenium has no definite point ef fusion. At temperatures 
exceeding 60° C. it softens, becoming gradually softer with increased 
heat, and being perfectly fluid at 250°. "When rapidly cooled from 
this temperature, it returns to its original condition. At normal tem- 
peratures it may be kept without change of state for many years, and 
is probably under these conditions perfectly stable. It is, though 
very sparingly, soluble in bisulphide of carbon. In thin films, it 
appears by transmitted light of a beautiful ruby-red colour. Its specific 
gravity is, according to Schaff'gottsch, 4,276. 
When this vitreous selenium is maintained for some time at any 
temperature between 94° and 200° C, and is then slowly cooled, it 
is found to have assumed a metallic appearance, and to have a grey 
granular fracture. It is now, we ourselves find, perfectly opaque to 
light, even in the thinnest films. Its specific gravity has increased 
to 4"796. When heated it does not soften, but at 217° fuses without 
taking any intermediate pasty condition. At 250° it is perfectly 
fluid, even when the mass is considerable ; and when rapidly cooled 
returns, without any tendency to crystallize, to the vitreous, non- 
metallic modification. All that has up to the present been made known 
as to the electrical relations of selenium may be very shortly told. 
Solid vitreous selenium cannot, according to Berzelius,^' be rendered 
electrical by friction, but on the contrary Eonsdorff f states that when 
rubbed in very dry air it has this property. J Knox found that fused 
selenium conducted the current of a battery of sixty pairs. Hittorf § found 
that granular selenium at normal temperatures conducted sufficient of 
the current of one Grove's element to defiect the astatic needle of a gal- 
vanometer having 200 convolutions 17°, and that when the selenium 
was heated to 210° in a small crucible the needle marked 80°. But 
when the temperature reached 217° (the point of fusion of granular 
selenium), the needle went back suddenly to 20°. 
The action of light as probably effecting some change in the allo- 
tropy of selenium was not wholly unsuspected prior to Mr. Smith's 
observations. Gmelin mentions exposure to sunlight as a favourable 
condition for the precipitation of selenium from dilute solutions of 
* Gmelin— Handbook, ii., p. 236. 
f Gmelin 1. c. 
X Gmelin 1. c. 
§ Poggendorf Annalen der Physik und Chemic, Bd. Ixxxiv, p, 214, 
K.I. A. PROC. — VOL. I., SER. I., SCIENCE. 3 Z 
