120 CHEMISTRY 



Becquerel's photo- voltaic effect, which Samsonov 1 has recently produced in chlorophyll, 

 and in uranyl and quinine sulphates, by illuminating one of the two translucent plat-*- 

 inum electrodes. 



Selenium. The electronic theory of the light-action of selenium, first advocated by 

 C. Ries, has acquired an additional value since both the photo-sensitiveness and the 

 variation of resistance with the voltage (the " potential effect "). have been discovered 

 on a smaller scale in other substances as well. Quite recently (1912) Ries announces 

 that the potential effect is a general property of photo-sensitive substances. The con- 

 ductivity of selenium has been invested with a special interest since Kaempf- succeeded 

 in producing in it a saturation current with a gradient of 12,000 volts per cm. This is 

 the first time that a saturation current has been observed in a solid. 



Selenium photometry, at which Stebbins, Pfund, and others have been working for 

 some time, achieved a notable success when Stebbins (1911) discovered a slight lumi- 

 nosity of the " dark " companion of Algol by its means. 



Another new practical application of selenium is Fournier d'Albe's " optophone," 3 

 which enables totally blind persons to recognise light by means of the ear, the light 

 producing a change in a selenium resistance forming one arm of a Wheatstone bridge, 

 and the resulting current being periodically interrupted and thus made audible in a 

 telephone. 



WV M. Thornton, who regards the eye as an electrical organ, gives (1910) 23 micro- 

 amperes per cm 2 of the retina as the minimum current required to produce visual sen- 

 sation. He concludes that the eye can detect one-twentieth of the energy perceptible 

 to the .ear. 



Magnetons. Ampere's "molecular magnets " have acquired a new significance by a 

 remarkable discovery made by P. Weiss, 4 viz., that the molecular magnetic moments of 

 bodies differ from each other in simple ratios expressible by small whole numbers. Ow- 

 ing to thermal agitation, these molecular moments do not co-operate except at absolute 

 zero, but measurements can be made in liquid hydrogen, or even at higher ranges of 

 temperature . when temperature coefficients obey a well-defined law. The common 

 constituent of the magnetic molecular magnetic moments is called a "magneton," and 

 Us 'Value is- 0.174 X i o". 28 in cm-webers. The magneton-number is for iron n> for 

 nickel 3, and closely . approaches whole numbers in a great variety of bodies. 



N-ickel-on-Glass .Reflectors. The silver-on-glass mirror, itself a great advance on 

 speculum metal, is likely to give way before a nickel-on-glass mirror both as regards 

 durability and capacity of reflecting ultra-violet light. R. W. Wood 5 gives. instructions 

 for depositing the nickel, which is best done electrically from a solution. of a double sul- 

 phate of nickel and ammonia. In this connection it should be noted that the thinnest 

 optically effective laysr of lead peroxide on a platinum mirror is 0.84 pn thick, and must 

 have the thickness of two molecules (Koenigsberger and Miiller, 1911). 



F used-Silica Skmdard. Owing to its low thermal hysteresis, fused silica is a suit- 

 able material for standards of length. G. W. C. Kaye 6 describes those adopted in the 

 Bureau of Standards. They have a very small expansion coefficient, and are light and 

 inexpensive. They must be annealed for eight days at 450 C. and cooled slowly for 

 fourteen days. Annealing caused a shrinkage of about half a micron in the metre 

 standard. (E. E. FOURNIER D'ALBEL) 



CHEMISTRY 7 



The increasing significance of chemistry as a factor in modern industrial and social 

 progress is obvious to anyone who gives consideration to the matter. It is true that 

 a great deal of the chemist's work seems to have no bearing on the practical side of life, 

 but every now and then a convincing demonstration is given that laboratory research 



1 Zeitschrift fur Wissenschaftliche Photographic, n, p. 33 (1912). 



2 Physikalische Zeitschrift, 13, p. 689 (1912). 3 Ibid., 13, p. 942 (1912). 



4 Comptes Rendus, 152, p. 187 (1911). 6 A strophysical Journal, 34, p. 404 {1911). 



6 Royal Society, Proceedings, A85, p. 430 (1911). 



7 See generally E. B. vi., 33 et. seq. ; and the articles on particular chemical subjects and 

 substances, as enumerated in E. B. Index Volume, p. 892. 



