1902.] accompanying Mechanical Disturbance in Metals. 291 



enhancement of response thereby produced. A further quantity of 

 KHO was now added so as to increase the strength to 3 parts in a 

 hundred. This caused (c) a complete abolition of response. (Refer to 

 fig. 2, c, d.) 



I shall now briefly mention some of the interesting points in connec- 

 tion with the action of chemical reagents. (1.) The effect of reagent is 

 not only to increase or diminish the height of response, but also to 

 modify the time relations. By the action of some the quickness with 

 which the maximum effect is reached is enhanced, others produce a pro- 

 longation of the period of recovery. Curious effects of this in producing 

 diphasic variation have already been mentioned. (2.) In a sensitive 

 annealed wire the further enhancement of response by Na2C03 is not 

 so great as in a fresh wire. The effect produced by a reagent is thus seen 

 to depend to some extent on the previous condition of the wire. (3.) A 

 certain time is required for the full development of this effect. With 

 some the maximum effect takes place almost instantaneously, while 

 with others it takes place gradually. Again, the effect may with some 

 reach a maximum, after which there is a slight decline. The after- 

 effect of some reagents is transitory while that of others is very per- 

 sistent. 



It is difficult to say how much of this modification of responsiveness 

 by various reagents is due to " physical " and how much to " chemi- 

 cal " cause. It has been shown that the responsive power does not 

 depend on the chemical activity of the substance. Tin is more re- 

 sponsive than zinc. Pt in distilled water shows response. Strong 

 acids and alkalies abolish response, but very dilute KHO enhances 

 response. Oxalic acid, even in minute quantities, abolishes it. Neutral 

 Na 2 C0 3 enhances it, but dilute NaCl produces no change in the 

 normal response. The responses of the same wire under different 

 physical modifications are different, and under certain molecular 

 modification the sign of response is even reversed. But continued 

 vibration makes the response normal. Again, the enhancement of 

 response produced by Na2C0 3 can be closely imitated by the effect 

 of continued vibration. The stimulating effect of this reagent 

 gradually attains a maximum. The after-effects of some chemical 

 reagents persist even after all traces have been removed. For 

 example, in a certain experiment the A and B wires gave each a 

 response of 23-5 divisions. The wires were lifted from the cell, 

 and the A wire touched with dilute oxalic acid. It was then 

 rubbed under tap-water with a piece of cloth, so as to remove all 

 traces of the acid. On replacing the wire on the cell, the responsive- 

 ness of the untouched B was found unchanged, but that of A had 

 undergone an abolition. The depressing action is often so persistent 

 and deep that I have on many occasions failed to revive the response 

 even after the surface layers had been removed by rubbing the wire 



x 2 



