Page:Collected Physical Papers.djvu/188

168 High temperature, however, produces erratic behaviour by causing violent molecular disturbance.

Influence of pressure.—Pressure also has pronounced effect on molecular response. Moderate increase of pressure increases the sensibility, but too great an increase may cause a loss of sensibility. In substances which are nearly neutral, pressure variation may even cause reversal of response.

The same receiver may, owing to some obscure molecular modification, exhibit a response opposite in sign to the normal. But under continued stimulation of radiation, the abnormal response becomes converted into the normal. Parallel instances will also be noticed in the case of response to mechanical stimulus and to light. It is thus seen how the response is dependent on the molecular condition, and how a change of this condition may culminate in a reversal of response, say, from a diminution to an increase of resistance. The nature of the chemical substance, the molecular condition, the intensity and duration of radiation, the pressure, the temperature, and even the electromotive force acting on the circuit, are the factors instrumental in the determination of the resultant response. I have already shown how the cumulative action of continuous radiation may produce molecular reversal. There may thus be one or more reversals.

In the following investigations, it is necessary to observe the conductivity or the electromotive variations, induced by external stimulus of various durations. It is also necessary to note the time-relations of direct and after-effects. The conductivity and electromotive vari-