638 SUPPLEMENTARY. 



As this expression must be an exact differential, we deduce 

 from it 



(24) 



or, taking equations (21) and (23) into account, 



It follows from equation (22), that at a constant potential, the 

 charge of electricity increases with the stretching weight, and that 

 with a constant charge, the potential diminishes when the stretch- 

 ing weight increases that is to say, when the tube is elongated. 



Experiment further shows that the elongation is proportional 

 to the square of the difference of potential, and that, k being a 

 constant, 



A/=/b; 2 . 



It follows from this, that 



a/ 



a = ~ 



and, from equation (23), 



The capacity of the jar increases, therefore, proportionally to 

 the stretching weight. . 



It may be observed that the electrical attraction of the two 

 armatures of a condenser would also crush the intermediate layer, 

 and give analogous effects ; but it does not seem that this cause is 

 sufficient to explain the phenomena. 



654. COMPRESSION OF TOURMALINE. We shall analyze in the 

 same way a phenomenon recently discovered by MM. P. and 

 J. Curie. When a tourmaline is compressed along its axis, the 

 crystal becomes electrically polarized in the same direction as 

 that which would be produced by a rise in temperature. This 

 polarization is proportional to the compression, and disappears 

 with it. Other hemihedral crystals, such as quartz and topaz, 

 behave like tourmaline when they are compressed along a hemi- 

 hedral axis. 



