Page:On the Continuity of Effect of Light and Electric Radiation on Matter.djvu/4

1901.] and diamagnetic classes we also come across characteristic differences, though these divisions do not coincide with the classes giving the positive and negative electric touch. There is, however, one similarity; for just as the paramagnetic effect is more intense, so is the conductivity variation associated with the positive, generally speaking, much stronger than the effect on the negative class. From the strongly positive substance like Fe Mg to the pronounced negative like K there are numerous gradations. In silver we have a material which is almost on the line of demarcation; it passes easily from one condition to another under the influence of external circumstances.

2. External Influences.

If the response, positive or negative, is really an expression of some changed molecular condition, we may expect it to be modified not only by the chemical nature of the substance, but also (1) by the previous history of the substance, by the temperature, (3) by pressure.

Influence of previous History.—As regards the first, I have already shown in my previous paper that a substance strained by radiation often exhibits opposite or reversal effects. Freshly powdered particles often show erratic results, but the effects become consistent after annealing; this often increases the sensibility also, by increasing the molecular mobility. Fresh particles are sometimes found to exhibit very little sensitiveness. At first I thought that this might be due to some kind of fatigue; the following, however, showed that such could not be the case. I found that in these cases the sensibility was increased by subjecting the substance to strong radiation or even by passing a few induction shocks. The increase of sensitiveness thus produced appears to be due to the removal of molecular sluggishness. The improvement in sensitiveness often obtained by shaking of the particles is no doubt due to the same cause. In the various types of molecular receivers, whether responding to electric radiation, light, or mechanical vibration, the same effect is noticed. Generally speaking, the receiver in the first place improves gradually with working. But as it gets overstrained it exhibits fatigue.

Influence of Temperature.—As regards temperature, I have in many instances found that on excessively cold days some receivers exhibit a diminution of sensibility, removed by warming. Several effects which were very strongly exhibited in the warm climate of India, I found to be much diminished here. Cautious application of heat often increases, not only the sensibility, but also the power of self-recovery. But excess of temperature produces erratic behaviour by causing violent molecular disturbance.