THE MOLECULAR THEORY OF EXCITATION 597 



effect on the cessation of the force. But, under intense 

 magnetisation, the molecular transformation is more or less 

 persistent, and we observe an after-effect in the induced 

 permanent magnetisation. To revert here to the illustration 

 of sulphur, it is only because the persistent terminal change 

 is the most easily distinguishable that we single it out for the 

 name of ' allotropic change.' As a matter of fact we see 

 that this is but the climax of a series of changes, and so 

 incomplete a view has been made current by the fact that 

 we had no means of recording the intermediate changes 

 while they were in progress. 



The next question is as to the possibility of making such 

 records of molecular transformations, or of induced varia- 

 tions in the state of molecular aggregation, while they are 

 taking place. This may be accomplished, as 1 shall show, 

 by the concomitant variation of electrical conductivity. It 

 is to be borne in mind that the state of molecular aggrega- 

 tion plays an important part in determining the conductivity 

 of a substance, and as an example we may take the case 

 of carbon, which exhibits wide differences of conductivity in 

 its two allotropic conditions of graphite and diamond. Let 

 us imagine a piece of carbon in an intermediate or neutral 

 state between these two. We may suppose that an external 

 force distorts it to a small extent towards the more con- 

 ducting state of graphite. This distortion would be attended 

 by an increase of conductivity, from which latter the extent 

 of molecular distortion or upset involved might be inferred. 

 Now, during the distortion from the equilibrium position, a 

 force of restitution will tend to restore the carbon to its 

 original neutral condition. If the distortion does not 

 proceed beyond the elastic limit, then, on the cessation of 

 the external stimulus, it will recover its original state, and 

 this will be evidenced by the restoration of its original con- 

 ductivity. But if the distortion be of a sub-permanent or 

 permanent type, the recovery will be very much protracted, 

 or will not take place at all. Such more or less permanent 

 distortion, known as allotropic transformation due to stimulus 



