PHYSICS. 4G7 



H. Becquerel has examined the emissiou-spectra of metallic vapors 

 iu the infra-red region, making use of a suitable phosphorescent sub- 

 j^iance, previously rendered luminous by insolation, as the screen for 

 receiving these spectra. Under the influence of the infra-red rays a 

 tu^wporary excitation preceding- extinction is developed, and the lines 

 otherwise invisible appear brilliant. Certain of these phosphorescent 

 powders, notably calcium sulphide, were so sensitive as to allow the wave- 

 lengths of the more brilliant lines of potassium, sodium, and cadmium 

 to be determined at once by means of a Entherfurd grating. In other 

 cases the spectra were obtained by means of a carbon disulphide prism, 

 the wave-lengths being determined by interpolation. The wavelengths 

 of 4 lines of potassium, 1 of sodium, 5 of strontium, 2 of calcium, 4 of mag- 

 nesium, 2 of aluminum, 2 of zinc, 1 of cadmium, 5 of lead, 2 of silver, 

 2 of tin, and 1 each of bismuth and thallium are given. (0. i2., xcix, 

 374, August, 1884 ; Phil. Mag., October, 1884, V, xviii, 38G ; Am. J. Sci., 

 December, 1884, III, xxviir, 457.) 



Becquerel has applied the same method to the investigation of the 

 infra-red region of the solar spectrum. A beam of light reflected from 

 a diflraction grating was concentrated by a lens and traversed a car- 

 bon disulphide prism whose sides were normal to the slit and the lines 

 of the grating, and formed on the phosphorescent surface a series of 

 oblique spectra, in which the radiations with spectra of different orders 

 were juxtaposed and not superposed. The slit was so narrow that the 

 principal lines of the luminous spectra could be seen, so that by com- 

 paring the lines and bands in the infra-red of the first spectrum with 

 the known lines of the second and third their wavelengths could be 

 obtained. The author claims that this method allows of a further ex- 

 ploration than the photographic one, and is not exceeded by the bolom- 

 eter or thermopile. Indeed, if the phosphorescent agent is sensitive, 

 the details can be made out far better than with the bolometer. The 

 wave-lengths measured are given in the paper. (0. B., xcix, 417, Sep- 

 tember, 1884 ; Fhil. Mag., ISTovember, 1884, V, xviii, 4G5 ; Am. J. ScL, 

 III, XXVIII, 391, 459.) 



Soret and Sarasin, members of a commission appointed by the Natiiral 

 History Society of Geneva to study the properties of the water of their 

 lake, have observed a distinct absorption band in its spectrum, which 

 is visible when the light passes through a layer of the water only 2'" 

 in thickness. This baud is a little less refrangible than D, being 

 located in the orange, about one-fifth the distance between D and C, 

 corresponding to a wave-length of about GOO. This band was observed 

 in distilled water even when carefully purified by distillation m ith per- 

 manganate in a vessel of platinum. (C. B., March, 1884, xcviii, G24.^ 



Soret has published a later paper. In which the question of the color of 

 water is discussed with especial reference to the color of the Lake of 

 Geneva. {J. Fhjjs., October, 1884, II, in, 427.) 



Abney and Festing have shown that, when light, traverses a thick- 



