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No. 841.- JANUARY 7, 1876.

because little, if anything, has been added to that know-
ledge for nearly twenty years, and because the statements

in some of the acknowledged text-books are not only in. INFLUENCE OF LIGHT ON THE ELECTRICAL sufficient, but often discordant with the results obtained

CONDUCTIVITY OF THIS ELEMENT.* by the savants to whom we owe all that up to this time By HARRY N. DRAPER, F.C.S., and RICHARD J. MOSS, F.C.s. 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 In a paper read before the Society of Telegraph En- upon our investigation, it must be understood that we gineerst on February 12th, 1873, Mr. Willoughby Smith note here only those hitherto observed and not widely announced the remarkable fact, that a bar of crystalline known characters of selenium which seem to us to be in selenium, through which a current of electricity passes, ntimate relation to the phenomena we have made the has its conductivity increased 15 to 100 per cent when iobjects of experiment. the bar is exposed to light. The light from an ordinary Selenium, discovered by Berzelius in 1817, was carefully. gas-burner placed at a distance of several feet increased studied by that chemist, and it is through his researches, the conductivity 15 to 20 per cent. Mr. Smith satisfied ; and those of Regnault, Mitscherlich, and Hitiorf, that we himself that alterations in temperature in no way affected have almost all our knowledge of the physical characters this result, by placing the selenium in water, in such a of this element. It is upon their authority that the fol. manner that the light from burning magnesium ribbon lowing statements are made. held some inches above the bar passed through about an Selenium may exist in several different forms: inch of water before falling upon the selenium. Under

1. As a vitreous mass, with conchoidal fracture. these circumstances, the conductivity of the bar was

2. As a red amorphous powder precipitated from sefound to increase more than two-thirds, returning to the

enious acid or selenites by the action of reducing normal conductivity when the light was withdrawn.

agents. Lieutenant Sale, in a communication made to the Royal

3. In the form of minute crystals deposited from its Society, describes a series of experiments undertaken solution in bisulphide of carbon. with the object of ascertaining the relative effect upon the electric resistance of selenium of the light in different selenides exposed to the air.

4. In crystals deposited from solutions of the alkaline parts of the spearum. He found that in the solar specErum the conductivity is least in the violet, and increases metallic cobalt or cast-iron, and obtained by the heating

5. As a granular body resembling, almost completely, as the red is approached, attaining its maximum in a

and slow cooling of either of the three first mentioned position just on the outside edge of the red rays at the red

forms. side. The conductivity in this position is greater than in

The first three modifications, it should be mentioned, diffuse daylight, but very considerably less than when the selenium is exposed to full sunlight. Mr. Sale observed resemble one another in their physical and chemical rela: that the effect of light is apparently instantaneous, but tions, and must be regarded as different conditions of the that the return in darkness to the normal resistance is not same allotropic form of selenium. so rapid. He corroborates the statement of Mr. Wil.

It is here only necessary to speak of vitreous selenium loughby Smith, already cited, that the varying resistance and of its heat-produced allotropic modification, the is in no way due to alteration of temperature of the se.

granular variety, or as it has been felicitously called by lenium. §

Regnault, metallic selenium. Soon after the publication of Mr. Smith's observations,

Vitreous selenium has no definite point of fusion. At we undertook a series of experiments with the object of, temperatures exceeding 60° C. it softens, becoming gradu. if possible, determining the precise molecular state of ally softer with increased heat, and being perfecily fluid selenium, which exhibited this phenomenon of diminished

at 250°. When rapidly cooled from this temperature, it electrical resistance under the action of light, and the

reiurns to its original condition. At normal temperatures conditions necessary for its production.

it may be kept without change of state for many years, It would here appear necessary to give a brief resumé and is probably under these conditions perfectly stable. of the state of our knowledge of the physical properties It is, though very sparingly, soluble in bisulphide of car: and relations of selenium. This is of the more importance bon. In thin films, it appears by transmitted light of a

beautiful ruby red colour. Its specific gravity is, accord* From the Proceedings of the Royal Irish Academy, vol. i., Ser. II. ing to Schaffgottsch, 4'276. (Sci.). + Nature, vol. vii., p. 203.

When this vitreous nium is maintained for some time ; Proceedings of the Royal Society of London, vol. xxi., p. 283. at any temperature between 949 and 200° C., and is then $ No experiments are adduced in support of this statement. slowly cooled, it is found to have assumed a metallic ap.

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