meter spot at once makes it evident that some internal struggle has been going on; the spot trembles, as if to overcome some internal friction; the substance then exhibits a sudden recovery. After these preliminary molecular adjustments the recovery becomes perfectly automatic and instantaneous. Each flash of radiation then produces a responsive galvanometer twitch, immediately followed by recovery.
Some of the stages are well seen in the curves given. In (a) the recovery takes place in 50 seconds; in (d) in only 6 seconds; (e) and (f) show recovery in lead coated with bromine. The recovery gradually became very quick, from 4 seconds to 3 seconds; after that it was too quick for record.
In all the above cases it will be noted that the curve of recovery is convex to the abscissa; that is to say, it is at first very rapid, but in the later stage it becomes relatively slow.
5. A Self-recovering and Metrical Receiver
The most perfect type of self-recovering receiver that I succeeded in constructing was made of the strained variety of silver described in a previous paper. It was there shown that this variety of silver exhibits, under the action of radiation, an increase of resistance. I had with me a portion of this variety prepared more than a year ago, and it is probable that time had improved its quality. I made with it a receiver consisting of 3 mm. thickness of the powder between two electrodes; the pressure was adjusted by means of a micrometer screw which pressed one of the electrodes. The applied electromotive force was 0.4 volt, and the resistance of the receiver was equal to 20 ohms. The
