INORGANIC RESPONSE 97 
The amplitude of a given vibration can thus be pre- 
determined by the adjustment of the sliding stop. In 
this way we can obtain either uniform or definitely 
eraduated stimuli. 
Considerations showing that electric response is due 
to molecular disturbance.—The electromotive variation 
varies with the substance. With superposition of 
stimul, a relatively high value is obtained in tin, 
amounting sometimes to nearly half a volt, whereas 
in silver the electromotive variation is only about ‘01 of 
this value. The intensity of the response, however, 
does not depend on the chemical activity of the sub- 
stance, for the electromotive variation in the relatively 
chemically inactive tin is greater than that of zine. 
Again, the sign of response, positive or negative, is 
sometimes modified by the molecular condition of the 
wire (see Chap. XII). 
As regards the electrolyte, dilute NaCl solution, 
dilute solution of bichromate of potash &c. are normal 
in their action, that is to say, the electric response in 
such electrolytes is practically the same as with water. 
Ordinarily I use tap-water as the electrolyte. Zinc 
wires in ZnSO, solution give responses similar in 
character to those given by, for example, Pt or Sn in 
water. 
Test experiment.—It may be urged that the E.M. 
effect is due in some way (1) to the friction of the 
vibrating wire against the liquid; or (2) to some 
unknown surface action, at the point in the wire of 
the contact of liquid and air surfaces. This second 
objection has already been completely met in experi- 
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