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. [36] In this apparatus the mercury amalgams of K and Na are run through a fine funnel, so that the freshly-formed surface of the drops may be illuminated. Under these circumstances, while pure mercury fell from -185 to -175 volts in 30sec., amalgam of zinc fell from -195 to -116 in 15sec., amalgam of sodium fell from -195 to 0 in 10sec., and an amalgam of potassium fell from -195 to 0 in 5sec. [Illustration: FIG. 59. _Explanation of Fig. 59._--B′ is a brightly polished amalgamated zinc plate attached to the negative pole of a Holtz machine, with the positive knob from 6 to 10 centimetres distant. The source of a light is a strip of burning magnesium ribbon 30 to 50 centimetres away. Whenever the spark is able just to choose the path B B′, light shining on the zinc plate checks it and transfers the spark to A A′.] _Wied. Ann., 39, p. 332.--On a Checking Action of Illumination on Electric Spark and Brush Discharge._ If sparks are just able to occur between a brass knob and a clean amalgamated zinc cathode, illumination of the latter by ultra-violet light tends to check them. (This is a curious inversion of Hertz’s fundamental experiment on the subject. It is an effect I have not yet observed; but Elster and Geitel’s arrangement differs from mine[37] in that the surfaces are at a steady high potential before the spark, so that light can exert its discharging influence, whereas in mine the surfaces were at zero potential until the spark-rush occurred. Hertz’s arrangement was more like mine, inasmuch as he illuminated the knobs of an induction coil on the verge of sparking. It appears, then, that whereas the action of light in discharging negative electricity from clean oxidisable metallic surfaces is definite enough, its influence on a spark discharge differs according to the conditions of that discharge--in cases of “steady strain” it tends to hinder the spark; in cases of “sudden rush” it tends to assist it.--O. J. L.) [37] _See_ Fig. 7, page 9. _Wied. Ann., 41, p. 161.--On the use of Sodium-Amalgam in Photo-electric Experiments._ Elster and Geitel have repeated some of Righi’s experiments on the discharge of negative electricity from metals in rarefied air, and find, in agreement with him, that a reduction of pressure to about one millimetre increases the discharge velocity about six or seven times. They proceed to try sodium-amalgam exposed to daylight in exhausted tubes, and describe apparatus for the purpose. Such an arrangement simply cannot hold a negative charge in bright daylight, even although it be unprovided with quartz windows. Even paraffin lamps and sodium flames exert some action.