when the stimulus is below the critical intensity, so also it is found that a feeble mechanical stimulus often produces an effect opposite to the normal. Thus with strain cell made of lead, I found that whereas the acted wire became cuproid with an amplitude of vibration of 4°, the same wire when vibrated through 45° became zincoid. Thus in a Pb cell (50,000 ohms in circuit)
| Amplitude of vibration. | Deflection. | Result. | |
| 4° | 5 divisions to right | Acted wire C. | |
| 45° | 70„left | „Z. |
This effect I have often noticed. It was too frequent to be accidental, but it did not occur invariably. On the occasions when it happened, this negative effect disappeared after continued vibration. Thus on taking a record of effect of continued vibration, there is produced a negative twitch, which is converted later into a positive deflection, just as in the curves of effect of light in fig. 17.
Reversal produced by Continued Stimulation.—After the maximum effect is reached, if the vibration is still continued, there is a tendency for the curve to descend to the neutral line. In the case of nickel I have even found the curve reversed, that is to say, there was a complete reversal of electromotive force.
I have described the various molecular effects produced by mechanical stimulus under varying conditions, and shown how very similar in every detail they are to the effects produced by electric radiation and light. How striking these similarities are, will be seen from the following tabular statement and comparison of different curves.
9. Molecular Effects common to Electric Radiation, Light, and Mechanical Vibration.
1. The molecular effect produced may be detected either by conductivity or electromotive variation methods.
2. Substances when not overstrained exhibit recovery, but the recovery is delayed when there is overstrain.
3. Response modified by previous history, and the influence of the surrounding conditions. Slight rise of temperature and annealing generally favourable to increased sensitiveness and quick recovery.
4. Under the action of electric radiation, light, and mechanical vibration, two opposite effects are exhibited; by the conductivity variation method this is seen in the diminution or increase of resistance; by the electromotive variation method we get positive or negative variation.
5. In the curve of response, in all the above cases, the ascending portion is abrupt, whereas the fall during recovery is at first rapid,
