Page:On the Continuity of Effect of Light and Electric Radiation on Matter.djvu/5

158 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 loss of sensibility. In substances which are nearly neutral, pressure variation may even cause reversal of response.

Again, the same receiver may, owing to some molecular modification, exhibit a response opposite in sign to the normal. But subjection to this continued stimulation of radiation, generally speaking, converts the abnormal response into normal. Exactly parallel instances will be noticed in the case of response to mechanical stimulus and to light.

We thus see how the response is dependent on the molecular condition, and how a change of this condition may even give rise to 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 used for detection, are the factors which are instrumental in the modification of the final response. I have already shown how the increasing effect of continuous radiation may produce molecular reversal. There may thus be one or more reversals. It is probable that the other variables may also produce similar reversals.

3. Recording Appatatus.

In the following investigations, the electrical effects, either the conductivity or the electromotive variations, due to external disturbances of various durations, have to be observed. It is also necessary to note the time-relations of the after-recovery from these effects. The conductivity and electromotive variations can be deduced from the observed varying deflections of the galvanometer. When the variations are rapid, the observation requires great alertness and is very fatiguing. This difficulty is still greatly enhanced when simultaneous time-observations have to be taken. It thus becomes necessary to have at least two observers; the process of observation is made slow and tedious, and the accumulation of results by this method is very tardy. But in the apparatus now to be described, the mode of procedure has been very much simplified, affording facilities for quick and highly accurate observations.

The apparatus is a modified railway myograph. (See fig. 1.) The moving platform carries squared paper (divided into $1⁄10$ inch) on which the record is made. The platform moves uniformly by clockwork, and the rate of travel of the paper may be roughly adjusted by means of different-sized pulleys, or more finely by the clockwork governor. The usual rate is 1 inch in 30 seconds, and one small division of the paper measured horizontally is thus equal to 3 seconds.