854 



LORD KELVIN. 



above the leftwards. The geometrical centre of the electrions within 

 A is displaced rightwards to a distance, l, from C eqnal to — 2 . 



§ 33. Imagine now a crystal or a solid of any shape built up of 

 parallel rows of atoms such as those of § 32. The amonnt of the dis- 

 placing force on each quartet of electrions will be soraewhat altered by 

 mntnal action between the rows, but the gênerai character of the resuit 

 will be the same; and we see that throughout the solid, except in a 

 thin superficial layer of perhaps five or ten atoms deep, the whole 

 interior is in a state of homogeiieous electric polarization, of which the 

 electric moment per unit of volume is 4 e NI-, where N is the number 

 of atoms per unit volume, and l is the displacement of the geometrical 

 centre of each quartet from the centre of its atom. This is the interior 

 molecular condition of a di-polar pyro-electric crystal, which I des- 

 cribed in 1860 *) as probably accounting for their known pyro-electric 

 quality, and as in accordance with the free electro-polarities of fractured 

 surfaces of tourmaline discovered by Canton 2 ). If a crystal, which we 

 may imagine as given with the electrions wholly undisturbed from their 

 positions according to § 32, is dipped in water and then allowed to 

 dry, electrions would by this process be removed from one part of its 

 surface and distributed over the remainder so as to wholly annul its 

 external manifestation of electric quality. If now either by change of 

 température or by mechanical stress the distances between the atoms 

 are altered, the interior electro-polarization becomes necessarily altered; 

 and the masking superficial electrification got by the dipping in water 

 and drying will now not exactly annul the electrostatic force in the air 



x ) Collected Mathematical and Physical Papers", vol. 1. p. 315. 



2 ) Wiedemann („Die Lehre von der Elektricitàt", Second Edition 1894 vol. 2. 

 § 378) mentions an experiment without fully describing it by which a null 

 resuit, seemingly at variance with Canton's expérimental discovery and con- 

 demnatory of my suggested theory, was found. Interesting experiments might 

 be made by pressing together and reseparating fractured surfaces of tourmaline, 

 or by pressing and rubbing polished surfaces together and separating them It 

 would be very difficult to get trustworthy resiilts by breakages, because it would 

 be almost impossible to avoid irregular electrifications by the appliances used for 

 makiug the breakage. The mode of electric measurement followed in the expe- 

 riment referred to by Wiedemann is not described. 



