530 
Proceedings of the Royal Irish Academy. 
taining the relative effect upon the electric resistance of selenium of 
the light in different parts of the spectrum. He found that in the 
solar spectrum the conductivity is least in the violet, and increases as the 
red is approached, attaining its maximum in a position just on the out- 
side edge of the red rays at the red side. The conductivity in this position 
is greater than in diffuse daylight, but very considerably less than when 
the selenium is exposed to full sunlight. Mr. Sale observed that the 
effect of light is apparently instantaneous, but that the return in dark- 
ness to the normal resistance is not so rapid. He corroborates the 
statement ofMr. Willoughby Smith, already cited, that the varying resis- 
tance is in no way due to alteration of temperature of the selenium.* 
Soon after the publication of Mr. Smith's observations, we under- 
took a series of experiments with the object of, if possible, determining 
the precise molecular state of selenium, which exhibited this pheno- 
menon of diminished electrical resistance under the action of light, 
and the conditions necessary for its production. 
It would here appear necessary to give a brief resume of the state 
of our knowledge of the physical properties and relations of selenium. 
This is of the more importance because little, if anything, has bee|^ added to 
that knowledge for nearly twenty years, and because the statements in 
some of the acknowledged text-books are not only insufficient^ but 
often discordant with the results obtained by the savants to whom we 
owe all that up to this time has been done in relation to the subject. 
As we would desire to avoid matter which is supplied by handbooks 
of chemistry, or details not directly bearing upon our investigation, it 
must be understood that we note here only those hitherto observed 
and not widely known characters of selenium which seem to us to be 
in intimate relation to the phenomena we have made the objects of 
experiment. 
Selenium, discovered by Berzelius in 1817, was carefully studied 
by that chemist, and it is through his researches, and those of Eegnault, 
Mitscherlich, and Hittorf, that we have almost all our knowledge of the 
physical characters of this element. It is upon their authority that 
the following statements are made. 
Selenium may exist in several different forms : — 
1°. As a vitreous mass, with conchoidal fracture. 
2°. As a red amorphous powder precipitated from selenious acid 
or selenites by the action of reducing agents. 
3°. In the form of minute crystals deposited from its solution in 
bisulphide of carbon. 
4°. In crystals deposited from solutions of the alkaline selenides 
exposed to the air. 
5°. As a granular body resembling, almost completely, metallic 
cobalt or cast iron, and obtained by the heating and slow cooling of 
either of the three first-mentioned forms. 
* No experiments are adduced in support of this statement. 
