Page:On the Continuity of Effect of Light and Electric Radiation on Matter.djvu/7

160 the stimulus is strong. The difference exhibited by various substances as regards self-recovery is one merely of degree. I give below typical cases which exhibit the gradual transition from so-called non-recovery to complete recovery.

The galvanometer used is a dead-beat D'Arsonval, in which the period taken for the maximum excursion and the return to zero from that position is less than half a second.

In these experiments the receiver was appropriately fixed on a heavy base. This rested in turn on a steady pedestal with one or two sets of pneumatic tyres interposed.

Positive Type.—In fig. 2 (a) is shown the effect of radiation on Fe$3$O$4$ when cold. Only the upper portion of the curve is given; the flash of radiation produced a deflection of the galvanometer of sixty-four

divisions. It will be observed that it had recovered to the extent of three divisions in the course of 60 seconds; if the rate of recovery had been uniform, there would have been complete recovery in about 21 minutes; but in the later stages, as we shall see, recovery is rather slow. In (b) is shown the effect of increased molecular mobility due to cautious warming. Now there was semi-recovery in 60 seconds; the quickness of recovery went on improving, and after a while the recovery was completed in 30 seconds.