THE MOLECULAR THEORY OF EXCITATION 599 



two electrodes. The torsion of the wire by which the 

 cylinder is suspended represents the force of restitution. 

 The galvanometer coil, by its deflections, exhibits indirectly 

 the molecular strain produced in the substance by the action 

 of stimulus. 



Let us suppose that we start with the substance in its 

 normal state A, with moderate conductivity, and let the corre- 

 sponding galvanometer deflection be 50. Let the substance 

 belong to the negative class which exhibits an increase of 

 conductivity, or diminution of resistance, under the action of 

 stimulus. The stimulus will therefore distort the substance 

 to a state of increased conductivity, the increased conductive 

 aspect B B being brought opposite the electrodes. The 

 enhanced current thus produced causes a deflection of, 

 say, 100 in the galvanometer. If the strain has not been 

 excessive, the substance will return, on the cessation of 

 stimulus, to its original position of equilibrium, and the 

 galvanometer deflection will fall from 100 to the original 

 value 50. If the substance belong to the positive class, the 

 distortion will be in the opposite direction, and the effect 

 of stimulus will be to induce a responsive increase of 

 resistance. 



The coil of the indicating galvanometer thus moves in 

 perfect response to the varying molecular strain induced in 

 the sensitive substance by the action of stimulus. The 

 invisible molecular distortions are thus revealed by the 

 visible deflections of the galvanometric indicator the effect 

 on one is merely the reflection of the effect on the other. 

 A curve of the molecular effect, induced by the action of 

 stimulus, may thus be obtained with the galvanometer de- 

 flection as ordinate, and the time as abscissa. It is thus 

 seen that these response-curves faithfully represent the in- 

 visible molecular strain-effect due to the stimulus, and the 

 subsequent recovery. 



I shall now describe how in practice, by this method of 

 resistivity variation, we obtain responses of various sub- 

 stances to the stimulus of visible or invisible radiation. The 



