1901.] between Radiation and Mechanical Strains. 183 



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 mechani- 

 cal 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 follow- 

 ing tabular statement and comparison of different curves. 



9. Molecular Effects common to Electric Radiation^ Light, and 

 Meclmnical Vibration. 



1. The molecular effect produced may be detected either by con- 

 ductivity 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 re- 

 sistance; 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, 



