416 Intelligence and Miscellaneous Articles, 



oxychloride of zinc is formed, all the gallium remains insoluble * 

 — under the form of oxychloride, I suppose. 



(17) The spectrum is brighter with a spark of average length 

 than with a very short one. — Comptes Mendus de V Academic des 

 Sciences, Sept. 20, 1875, pp. 493-495. 



ON THE INFLUENCE OF LIGHT UPON THE CONDUCTIVITY OF 

 CRYSTALLINE SELENIUM. BY WERNER SIEMENS. 



The property of crystalline selenium first described by Willoughby 

 Smith, and more closely studied by Salet, viz. that in the illumi- 

 nated condition it is a better conductor of electricity than in dark- 

 ness, I have further investigated, and established the correctness of 

 the facts. The specific conductivity, however, of selenium made 

 crystalline by heating to 100° or 150° 0. is very slight and extra- 

 ordinarily variable ; the augmentation of the conductivity by illu- 

 mination is also very inconstant ; so that it was impossible to de- 

 monstrate a definite dependence of the conductivity upon the illu- 

 mination. But by protracted heating of amorphous selenium up 

 to the temperature of 210° 0., and also by cooling melted selenium 

 to the same temperature of 210° C. (at which, with longer duration of 

 it, the selenium passes into a coarsely granular crystalline state), I 

 succeeded in producing another modification of crystalline selenium, 

 which possesses and permanently retains a considerably greater 

 conductivity, and conducts electricity after the manner of metals, 

 its cond acting-power diminishing with rise of temperature. Also 

 the action of light upon it is far greater, and appears to be per- 

 fectly constant. By fusing into coarsely granular selenium two 

 flat spirals of wire, at the distance of about 1 millim. from each 

 other, between two thin plates of mica, I produced an extraordina- 

 rily sensitive photometer. With it obscure heat-rays are without 

 direct influence upon the conductivity, which is diminished by heat- 

 ing the selenium ; while diffused daylight doubles it, and direct 

 sunlight in some circumstances raises it to more than tenfold. The 

 augmentation of the conductivity of the coarsely granular selenium 

 by illumination proceeds with extraordinary rapidity. Likewise 

 the lessening of it on shutting off the light appears to commence 

 instantaneously ; yet a longer time elapses before the state corre- 

 sponding to darkness is completely restored. The increment of the 

 conductivity is not proportional to the light-intensity, but a func- 

 tion of it which comes nearer to the ratio of the square root of that 

 intensity. 



I defer communicating details on this interesting property until 

 the conclusion of my experiments, merely remarking that I hope 

 to be able to utilize them for the construction of a reliable photo- 

 meter. — Monatsbericht der Icon.prems. Alcademie der}Vissenschaften 

 zu Berlin, 1875, p. 280. 



* But readily soluble in a few drops of chlorhydnc acid. 



t Proc. Royal Society, vol. xxi. p. 283 ; Pogg. Ann. vol. cl. p. 333. 



