97_b_. WIEDEMANN AND EBERT’S EXPERIMENT. LIGHT DISCHARGES CATHODE, BUT HAS NO INFLUENCE UPON ANODE, NOR AIR-GAP. DIFFERENT GASES AND DIFFERENT PRESSURES. _Wied. Ann._ XXXIII, p. 241. 1888. From notes by N. D. C. Hodges.—The arc light was used in place of the active spark of Hertz. Principal result was that the effect depended on the illumination of the cathode (§ 99.) The illumination of the anode or of the spark-gap did not influence the discharge. The very character of the charge was altered by the action of light upon the cathode. The influence of the illumination of the cathode did not consist solely at the starting of the spark, but lasted as long as the sparks continued to pass. With decreasing pressure of surrounding gas, the effect first increased (§ 97_a_) to a maximum, and then decreased (§ 54). The illumination had an effect on the path of the sparks, the path being perpendicular to the rays of light. The best results were obtained with carbonic acid gas. Hydrogen was next, and then air. They were contained in the tubes surrounding the poles. The character of the gas also had an influence on the rays which would produce the effect, with carbonic acid gas the effect showing itself even with the visible rays. 98. ELSTER AND GEITEL’S EXPERIMENT. NEGATIVELY CHARGED BODIES DISCHARGED BY LIGHT. _Wien. Berichte._ Vol. CI, p. 703, ’92. _Wied. Ann._ Vols. XXXVIII, XXXIX, XLI, XLII, XLIII, XLIV, XLVI, XLVII, LII. _Nature_, Lon., Sept. 6, ’94, p. 451.—The elements employed for carrying on the experiment consisted of a delicate electroscope and certain metals, including aluminum, amalgamated zinc, magnesium, rubidium, potassium and sodium. Some of the experiments were made on the top of Mount Sonnblick, the same being 3,100 m. high, where the discharging power of light was found to be about twice as great as at Wolfenbuttel, which was at the level of 80 m. The whole time for the discharge was only a matter of a few seconds. The greater rapidity of discharge at the higher level was attributed to the greater proportion of ultra-violet rays (Hertz), which are the most easily absorbed by the atmosphere, according to Langley. All metals are not discharged alike by the action of light. The law follows the electro-positive series in such a way that the more electro-positive the metal, the longer the wave length of light necessary to produce the discharge. In experiments with potassium, sodium and rubidium, they made them successively, the cathode in a bulb of rarefied hydrogen. In this case it was found that the light of a candle, even at so great a distance as 7 m., would cause the discharge. Rubidium was sensitive in this respect to the red light from a heated rod of glass. Elster and Geitel were able also to discharge, by light, some non-metallic bodies, like calcic sulphide, when so prepared that it had the property of phosphorescing, and also darkly colored fluorites. Independently, the phenomenon is of importance, because Elster and