Experiments also on differently-coloured lights. Summary of results. The photo-electrically active metals arrange themselves in the following order--Pure K, alloys of K and Na, pure Na. Amalgams of Rb, K, Na, Li, Mg (Tl, Zn); the same as their voltaic order. With the most sensitive term of the series a candle six metres off can be detected, and the region of spectral red is not inactive. The later terms of the series demand smaller waves, and even for potassium blue light gives a much greater effect than red. No discharge of positive electricity is observable with these substances. _Wied. Ann. 44, p. 722.--On the Dissipation of Electric Charge from Mineral Surfaces by Sunlight._ Hitherto only Balmain’s paint powder has been observed to be active among non-metallic substances. Now they try other phosphorescent bodies, and arrive at the following results:-- Fluorspar is conspicuously photo-electric, both in sunlight and daylight, especially the variety of fluorite called stinkfluss. Freshly-broken surfaces discharge much more rapidly than old surfaces. Blue waves, and not alone the ultra-violet, have a perceptible effect on fluorspar. In a vacuum the mineral loses its photo-electric sensibility and its conductivity too. Contact with damp air restores its sensibility. Moistening with water weakens but does not destroy the sensitiveness. On the other hand, igniting the mineral destroys both its photo-electric power and its exceptional phosphorescent property. Distinct traces of photo-electric power are shown by the following minerals also: Cryolite, heavy spar, celestine, arragonite, strontianite, calcspar, felspar and granite. The hypothesis that the power of phosphorescing when illuminated is approximately a measure of the discharging power of light has been verified in many cases; the exceptions can probably be explained by the influence which the electrical conductivity of the illuminated substance exerts on the rate of discharge of electricity from its surface. This agreement confirms the view expressed by us on the occasion of experiments with Balmain’s paint, that, during electrical discharge by light, actions take place which are analogous to those of resonance. Messrs. Wiedemann and Ebert had previously been led by other considerations to the same conclusion. We are compelled by the results of the present experiments to conclude that a more rapid discharge of electricity into the atmosphere takes place in sunlight than in darkness from the surfaces of the earth, which is composed of mineral particles charged, as the positive sign of the slope of atmospheric potential indicates, with negative electricity. [Illustration: FIG. 63.