340 Lord Kelvin on the 



of the elongation. In fact simple elongation in any direction 

 perpendicular to the principal axis of a quartz crystal pro- 

 duces electro-polarization ; and it is only when the lines of 

 two simple elongations are coincident with one another, or are 

 perpendicular to one another, that the resultant of their 

 electro-polarizations can he zero, 



§ 13. A most important contribution to our knowledge of 

 the electric properties of crystals has been made by Rontgen"*, 

 and by Friedel and J. Curie f, in independent investigations 

 proving that the irregular electrifications of the corners of 

 quartz crystals, which had been observed by many observers 

 as consequences of heatings and of returns to lower tempe- 

 ratures, are wholly due to mechanical stresses developed by 

 inequalities of temperature in different parts of the crystal. 

 Those phenomena are therefore truly piezo-electric, and are not 

 at all ' ' pyro-electric " like the electric property of tourmaline 

 which is due to change from one temperature to another, each 

 the same throughout the crystal. The very important and 

 interesting discovery thus made by Rontgen and by Friedel 

 and Curie, is, as they have pointed out, available also to 

 explain the perplexing and seemingly paradoxical statements 

 regarding positive and negative electrifications of corners and 

 hemihedral facets at opposite ends of the four long diagonals 

 of crystals of the cubic class, and of cubes of boracite, which 

 had been given by previous observers and writers, and 

 which have not yet disappeared from elementary treatises on 

 Mineralogy, Electricity, and General Physics. 



Appendix. 



[Extract from a pamphlet published by the 'f Societe Centrale de 

 Produits Chimiques^ 42, 43 Rue des Ecoles, Paris.] 



Quartz Piezo-Electrique de MM. J. et P. Curie. 



Cet instrument se compose essentiellement d ? une lame de 

 quartz, sur laquelle on exerce des tractions a l'aide de poids 

 places dans un plateau. Cette action mecanique provoque un 

 degagement d'electricite sur les faces de la lame. 



* " Ber. der Oberrh. Ges. f. Natur- und Heilkunde, xxii." [of date, 

 between December 1882, and April 30, 1883]. 



f " Bulletin de la Societe Miner alogique de France, t. v. p. 282, 

 decembre 1882." 



And Comptes Rendus of French Academy of Sciences, April 30, and 

 May 14, 1883. 



