468 Mr. A. P. Chattock on an 



a value which is in striking agreement with that obtained from 

 the stress data. 



It may be well to refer here to an argument brought for- 

 ward by Wiedemann in his Electricitat * against Lord Kelvin's 

 theory, by which he seems to consider that the latter is 

 rendered untenable. According to him a pyro-electric crystal 

 when carefully broken at right angles to its axis does not 

 exhibit opposite charges on its new surfaces, as it should do 

 if polarized throughout. This argument would be, of course, 

 unanswerable if it were applied to magnetic polarization, and 

 it holds also in the corresponding electrical case provided the 

 separated electricities both occur within the dimensions of a 

 single atom. But if they occur in different parts of a complex 

 molecule (and under this head may be included the pairs of 

 oppositely charged molecules already referred to) they may be 

 separated by dissociation of the latter, and Wiedemann's result 

 seems to me to be just what should be expected. At the instant 

 of breaking, the molecules at the point of rupture will be 

 violently agitated. Some of them, in the conflict, will almost 

 necessarily become dissociated, and if their parts are oppositely 

 charged it follows that the infinitesimal gap between the newly 

 formed surfaces will, for the moment, be playing the part of a 

 nearly perfect conductor. Instantly, therefore, a rush of 

 negatively-charged particles will take place to the positively- 

 charged surface and vice versa-, and the surfaces will be ren- 

 dered more or less neutral, as Wiedemann found them to be. 

 Even without this action, the field in the gap would probably 

 be strong enough to start discharge in air. 



Cohesion. 



The agreement of the above estimations of the value of q 

 both with each other and with the ionic charge leads naturally 

 to the search for a possible connexion between the hypo- 

 thetical molecular charges and other physical properties of 

 the crystals. 



Imagine a mixture of two equal numbers of particles, 

 equally and oppositely charged, and uniformly spaced. The 

 mass will cohere in virtue of the lines of force which pass in 

 all directions from positive to negative particles. Draw an 

 imaginary surface through the mass, in such a way that it 

 cuts none of the particles, but passes midway between those 

 on either side of it, Provided the mixture of the opposite 

 charges is thorough, and their distance apart not too small, 

 this surface will everywhere cut lines of force, roughly speaking, 



* Vol. ii. pp. 338, 345. 



