2 COMPARATIVE ELECTRO-PHYSIOLOGY 



changes, and it should be theoretically possible to detect and 

 measure this induced molecular change by recording such 

 concomitant variations. Thus the stimulus of light, for 



example, may induce a mole- 

 cular change which may in its 

 turn induce, say, a variation 

 in the permeability of the sub- 

 stance to liquid. Bichromated 

 gelatine becomes less perme- 

 able under the action of light. 

 The solubility of a substance 



FIG. 2. Response of India-rubber may a g a i n undergo variation 



Th Ttt S awo r m \ n rs ndat under external stimulus T sul- 



.phur, for example, usually 



soluble in carbon disulphide, is rendered insoluble under 

 the action of light. 



In order, then, to study the effect of a given stimulus 

 with accuracy, we should be able to detect and measure the 

 extent of the changes induced. The two effects which have 

 just been referred to are not, as will be seen, highly susceptible 

 of accurate measurement. But in the detection of molecular 

 changes by electrical means, we have at our disposal methods 

 for the measurement of such changes, the ease and delicacy 

 of which leave nothing to be desired. Two such methods 

 may be used that of Resistivity and that of Electro-motive 

 Variation. According to the method of resistivity variation, 

 the substance to be experimented on is placed in an electrical 

 circuit, including a delicate galvanometer and a suitable 

 electro-motive force, such as to cause a small deflection of 

 the galvanometer. The impact of the stimulus on the sub- 

 stance under examination now induces in it a molecular 

 change by which its resistance is made to undergo a variation, 

 which in the case of certain substances may be an increase, 

 or in that of others a diminution. On the cessation of external 

 stimulus, the substance shows recovery, with a corresponding 

 return to its original conductivity. Thus in the case of 

 selenium, for instance, the conductivity is increased, or the 



