A freshly prepared surface and a surface that has long been in contact with air at atmospheric pressure behave exactly alike with respect to the influence of a magnetic field. Temperature has a very different influence on the effect. Thus, for one and the same field of 7460 Gauss, the decrease in the photo-current reached 22% at 25° C and 50% at −79° C (solid carbon dioxide).

The author compares the effect he has found with the change in the conductivity and diamagnetic susceptibility of bismuth in a magnetic field, both of these phenomena being very strongly dependent on temperature. Evidently, in this case we are dealing with the action of the magnetic field on the free electrons inside the metal, which in bismuth possess a very considerable mean free path. Dember indicates as a possible cause of the effect he observed a change in the trajectories of the photo-electrons emerging from comparatively deep layers of the metal in the magnetic field.

S. Vavilov.

Fluorescence of Mercury Vapors.

Chr. Füchtbauer. Über eine neue Art der Erzeugung von Spektrallinien durch Einstrahlung (Fluoreszenz). Phys. Zeitschr. 21, 635 (1920).

When mercury vapors are illuminated by the light of a mercury arc, until now it has been possible to obtain only the so-called resonance radiation of the lines 253.7 and 184.9. In the paper reviewed, the author, together with A. Kröner and G. Joos, succeeded in producing in mercury vapors, under illumination, the semi-series and other lines not absorbed by mercury vapors in the ordinary state (fluorescence). This was achieved by increasing the energy of the exciting light. The exciting mercury lamp (quartz) had the form of a hollow cylinder, inside which a tube containing mercury vapors could be placed (also of quartz). A branch of this inner tube, containing a droplet of mercury, was immersed in water of the desired temperature. In this way it was possible to change the density of the mercury vapors inside the resonance tube. The resonance tube was closed at one end by a plane-parallel quartz plate, perpendicular to the slit of the spectrograph. To intensify the exciting light, the mercury lamp was surrounded externally by a palladium-glass envelope, inside which mercury was poured, serving as an electrode. The lamp was supplied with a current of 10–12 Amp. The fluorescence spectrum was so bright that an exposure of 15 sec. was sufficient to obtain on a photographic plate a distinct image of seven lines. The author exhaustively interprets the spectrum he observed from the standpoint of Bohr’s theory.

S. Vavilov.

Temperature Coefficient of the Decomposition of Chlorophyll by Light.

L. A. Ivanov. On the influence of temperature on the decomposition of chlorophyll by light. Journal of the Russian Botanical Society, vol. 4, p. 11, 1920.

The author measured the dependence of the initial rate of decomposition of a petroleum-ether and skimmed extract of chlorophyll from fresh leaves of Aspidistra on temperature. The temperature interval was 5°–40°; the light source was a 3000-candlepower Nitro lamp. The amount of decomposed chlorophyll was measured spectrophotometrically. Part of the experiments was carried out with collodion films stained with chlorophyll and deposited on glass. The results are given in the table.

TABLE I.

In addition, having measured the temperature coefficient of the decomposition of chlorophyll deposited on filter paper, and after illumination having washed the chlorophyll off the paper with alcohol, the author subjected the resulting solution to electrophotometric measurement. The temperature coefficient found by this method is 1.285. The indicated method of washing off the chlorophyll gives a figure somewhat lower. More accurate results could be obtained by electrophotometry of the paper in reflected light. On the basis of the figures obtained, the author concludes that the influence of the solvent medium on the temperature coefficient of the photochemical decomposition of chlorophyll is insignificant.

S. Vasiliev.

Continuous Ultraviolet Spectrum.

G. Gehlhoff. Ueber eine konstante Lichtquelle mit kontinuierlichem ultravioletten Spektrum. ZS. für techn. Phys. 10, p. 224, (1920).

Usually a continuous ultraviolet spectrum is obtained either by means of an arc or by an Angström spark (Al electrodes in water). If the first method is unsuitable because of its imperfection, then the second is unsuitable because of the considerable difficulty of the installation. Therefore the author proposes, as a constant source of continuous ultraviolet spectrum, a half-watt lamp heated by a metallic filament. He studied, in parallel, two lamps: one with a tantalum filament and a second with a tungsten spiral.

The distribution of energy in the spectrum was calculated by him according to Aschkinass’s formula for metals.

The ratio of the energy of the ultraviolet part of the spectrum from 0.3 μ to 0.4 μ to the energy of the visible spectrum (0.4 μ—0.8 μ) is 0.22%, and under an overload of 30%—1.23% for the tantalum lamp; the corresponding quantities for the tungsten lamp are 1.17% and 2.02%. When photographic plates are used, these ratios become still more favorable. In the same sense, the ratio of the energy in the spectral region 0.3 μ—0.4 μ to the energy in the region 0.4 μ—0.5 μ is expressed by the following quantities:

For the tantalum lamp, 7.2%; under an overload of 30%—17.7%.

For the tungsten lamp, 16.9%; under an overload of 30%—21.6%.

The author’s lamps consist of a sphere 10 cm in diameter, filled with nitrogen or argon. A tube is soldered to the sphere, to which a quartz plate or lens is attached by means of a binding. For ordinary absorption measurements, a tantalum strip 25 mm long and 2–3 mm wide is sufficient, with a voltage of 6–8 volts at a load of up to 20 amps. At the same voltage (6–8 v.) and the same load (15–20 amps), a tungsten spiral of length 10 mm is sufficient, with a wire thickness of 0.5 mm. The advantages of such a source of ultraviolet light are self-evident.

T. Molodoi.