. The evolution of forces . ns obtained by bodies placed in contact, and inthe dark, with a sensitive photographic plate. Agelatino-bromide plate is placed in a dark slideunder a strip of metal — zinc, aluminium, orplatinum. By interposing between the strip andthe metal a cross made of various substances, wegenerally obtain, after a few hours exposure to thesun or to a strong paraffin lamp, an image of theobject interposed, even when it is separated from themetal by a thin plate of mica. This experiment, and others of the same orderwhich in former years caused me to waste muchtime, succeed ver
. The evolution of forces . ns obtained by bodies placed in contact, and inthe dark, with a sensitive photographic plate. Agelatino-bromide plate is placed in a dark slideunder a strip of metal — zinc, aluminium, orplatinum. By interposing between the strip andthe metal a cross made of various substances, wegenerally obtain, after a few hours exposure to thesun or to a strong paraffin lamp, an image of theobject interposed, even when it is separated from themetal by a thin plate of mica. This experiment, and others of the same orderwhich in former years caused me to waste muchtime, succeed very irregularly, and at the end of afew days the metal no longer gives any image. These effects are in no way connected with phos- INVISIBLE PHOSPHORESCENCE 299 pliorescence, but with the radio-activity of the metal,and this is the reason that they can be increased byslight heat. The absence of action of the metal-its fatigue, if we may call it so—is likewise observed,I have shown, in the discharge of the electroscope. as. Fig. 29.—Thick metal plate pierced with holes ore)- xohich plates ofdifferent metals have been soldered. It is with this plate insertedin a photographic plate-carrier that my first researches on theradio-active properties of different metals were made in photographic method was soon aljandoned, because theimpressions were very irregular, as the metals, for the reasonsexplained, soon lose their peculiar properties. It is owing to the fact of the metal havincT ex-pelled, under the influence of slight heat, a provisionof radio-active substance, which cannot be regene-rated without a long rest, that it becomes inactive. These radio-active actions, which I confused atthe beginning of my experiments with those of the 300 THE EVOLUTION OF FORCES infra-red and invisible phosphorescence, were thecause of many researches before I could distinguishthem. From time to time different observers comeacross my early experiments, and, as Dr. Russell,Dr. Kahlbaum,
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