THE

CHEMICAL NEWS

AND

JOURNAL OF PHYSICAL SCIENCE.

WITH WHICH IS INCORPORATED THE "CHEMICAL GAZETTE."

A Journal of Practical Chemistry

IN ALL ITS APPLICATIONS TO

PHARMACY, ARTS, AND MANUFACTURES.

EDITED BY

WILLIAM CROOKES, F.R.S., &c.

VOLUME XXXIII.-1876.

LONDON:

PUBLISHED AT THE OFFICE, BOY COURT, LUDGATE HILL, E.C.

AND SOLD BY ALL BOOKSELLERS.

MDCCLXXVI.

IRINTED БУ EDWIN JOHN DAVEY,

THE CHEMICAL NEWS.

VOLUME XXXIII.

EDITED BY WILLIAM CROOKES, F.R.S., &c.

No. 841.-JANUARY 7, 1876.

ON SOME FORMS OF SELENIUM,

AND ON THE

INFLUENCE OF LIGHT ON THE ELECTRICAL
CONDUCTIVITY OF THIS ELEMENT.*
By HARRY N. DRAPER, F.C.S., and RICHARD J. MOSS, F.C.S.

IN a paper read before the Society of Telegraph En-
gineerst on February 12th, 1873. Mr. Willoughby Smith
announced the remarkable fact, that a bar of crystalline
selenium, through which a current of electricity passes,
has its conductivity increased 15 to 100 per cent when
the bar is exposed to light. The light from an ordinary
gas-burner placed at a distance of several feet increased
the conductivity 15 to 20 per cent. Mr. Smith satisfied
himself that alterations in temperature in no way affected
this result, by placing the selenium in water, in such a
manner that the light from burning magnesium ribbon
held some inches above the bar passed through about an
inch of water before falling upon the selenium. Under
these circumstances, the conductivity of the bar was
found to increase more than two-thirds, returning to the
normal conductivity when the light was withdrawn.

Lieutenant Sale, in a communication made to the Royal Society, describes a series of experiments undertaken with the object of ascertaining 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 outside 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 darkness to the normal resistance is not so rapid. He corroborates the statement of Mr. Willoughby Smith, already cited, that the varying resistance is in no way due to alteration of temperature of the selenium.§

Soon after the publication of Mr. Smith's observations, we undertook a series of experiments with the object of, if possible, determining the precise molecular state of selenium, which exhibited this phenomenon 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 resumé of the state of our knowledge of the physical properties and relations of selenium. This is of the more importance * From the Proceedings of the Royal Irish Academy, vol. i., Ser. II. (Sci.).

+ Nature, vol. vii., p. 203.

Proceedings of the Royal Society of London, vol. xxi., p. 283. § No experiments are adduced in support of this statement.

because little, if anything, has been added to that knowledge for nearly twenty years, and because the statements sufficient, but often discordant with the results obtained in some of the acknowledged text-books are not only inby 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 ntimate relation to the phenomena we have made the iobjects of experiment.

Selenium, discovered by Berzelius in 1817, was carefully. studied by that chemist, and it is through his researches, and those of Regnault, 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:

I. As a vitreous mass, with conchoidal fracture. 2. As a red amorphous powder precipitated from seenious 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.

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 selenium.

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.

Vitreous selenium has no definite point of 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 temperatures it may be kept without change of state for many years, and is probably under these conditions perfectly stable. bon. In thin films, it appears by transmitted light of a It is, though very sparingly, soluble in bisulphide of carbeautiful ruby-red colour. Its specific gravity is, according to Schaffgottsch, 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 ap