Page:Collected Physical Papers.djvu/187

Rh material which is almost on the line of demarcation; it passes easily from one condition to another under the influence of external variations.

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 previous history of the substance, (2) by temperature, and (3) by pressure.

Influence of previous history.—As regards the first, I have already shown in a previous paper that a substance strained by radiation often exhibits opposite or reversal effects. Freshly powdered particles often show erratic results, but the effect becomes consistent after annealing, which also increases its sensibility evidently by increasing the molecular mobility. 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, similar effect is observed. The receiver generally speaking, improves gradually with working; but after long-continued action it gets overstrained and exhibits fatigue.

Influence of temperature.—As regards temperature, I have often found that on excessively cold days the receivers exhibit a diminution of sensibility, which becomes restored by warming. Various reactions which were very strongly exhibited in the warm climate of India, I found to be much diminished in Europe. Cautious application of heat often increases, not only the sensibility, but also the power of self-recovery.