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172 of recovery had been uniform, there would have been complete recovery in about 21 minutes, but in the later stages, the recovery is very slow. In (b) is shown the effect of increased molecular mobility due to cautious warming. There was now semi-recovery in 60 seconds; the quickness of recovery went on improving, and after a while the recovery was completed in 30 seconds.

Negative Type.—Fig. 34 exhibits the effect on the negative type as exemplified by Sn and Pb coated with Br. It appears that warmth is favourable to quick recovery; for in India with Pb coated with Br the self-recovery was obtained with the greatest ease. In the colder climate of England it takes a longer time for

the receiver to become adjusted to the condition of self-recovery. At first the flash of radiation produces a sudden increase of resistance, from which there is no immediate recovery, but an inspection of the galvano-