596 COMPARATIVE ELECTRO-PHYSIOLOGY 



the mechanical and magnetometric methods of studying 

 response in magnetic substances, there is also a third means 

 available, in the Induction or Electro-motive Method, to be 

 fully described at the end of the present chapter. 



I have now explained how the extent of molecular 

 distortion induced in a magnetic substance by an external 

 force can be gauged or measured by magnetometric or 

 electric indications. For the detection of similar changes, 

 however, in matter which is not pronouncedly magnetic, it 

 is necessary to devise a method of record of more universal 

 application. Such a method we have, as already said, in 

 the record by resistivity-variation. It is here desirable, 

 however, to give a more detailed account of this and the 

 principle involved. 



Our object being the detection of the molecular changes 

 induced by stimulus, let us briefly consider certain well- 

 known cases of molecular transformation induced by various 

 stimulating agencies. Thus, when sulphur is subjected for 

 a certain length of time to the action of light, there is no 

 visible sign of any change. Its solubility in carbon 

 disulphide, however, has been altered, and we can dis- 

 criminate the portions acted upon from those unacted, by 

 means of this 'developing' solution. But such discrimina- 

 tion is only possible when the molecular or allotropic 

 modification has gone so far as to be somewhat stable that 

 is to say, when the after-effect of stimulus is persistent. 

 The development of any after-effect would have been 

 impossible had the substance in the meanwhile exhibited 

 self-recovery. Between the original condition A, again, and 

 the terminal modification D, the substance must have passed 

 through many gradations of condition, of a more or less 

 impermanent stability. This case is analogous to that of a 

 piece of iron under the action of magnetising forces, with 

 their consequent molecular modifications. When the acting 

 force is moderate, and the specimen has the power of self- 

 recovery, the induced molecular distortion that is to say, the 

 induced magnetisation is fugitive, and there is no after- 



