The following is our summary of results:
Bright fresh surfaces of the metals zinc, aluminium, magnesium were discharged by both sun- and daylight when they were negatively charged; and they spontaneously acquired a positive charge, whose amount could be increased by blowing.[35] A still more notable sensitiveness to light is shown by the amalgams of certain metals, viz., in the order of their sensitiveness, K, Na, Zn, Sn. Since pure mercury shows no effect, the hypothesis is permissible that the active agent is the metal dissolved in the mercury. If so, the following are the most active metals:—
K, Na (Mg, Al), Zn, Sn.
All other metals tried, such as Sn, Cd, Pb, Cu, Fe, Hg, Pt, and gas carbon, show no action. The same is true of nearly all non-metallic bodies; but one of them—namely, the powder of Balmain’s luminous paint—acted remarkably well in sunlight. Of liquids, hot and cold water, and hot and cold salt solution were completely inactive; consequently wetting the surface of metals destroys their sensibility to light.
The illumination experiments can be arranged in either of two ways. For experiments in free space we use zinc, aluminium, or magnesium wires, or small amalgamated spheres of zinc provided with an iron rod. With these it can be easily shown that the illuminated surface of certain metals act in the same way as a flame collector.
For demonstration experiments the apparatus described[36] is better, and with this we show the following:—
Amalgamated zinc, negatively charged, discharges almost instantly in sunlight; and if near a positively-electrified body charges itself positively.
The same thing happens, though more slowly, in diffuse daylight. Red glass stops the action, but the following let some through:—Selenite, mica, window glass, blue (cobalt) glass.
Fig. 59.