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XIX. On the Change in the Double Refraction of Quartz pro- 

 duced by Electric Forces. By Prof. W. C. Rontgen*. 



IT is well known that Sir W. Thomson has endeavoured to 

 explain pyroelectric phenomena by assuming that the 

 interior of pyroelectric crystals is constantly in a condition of 

 electric polarization ; the external action of this polarization 

 is neutralized by a constant electric charge of the surface, 

 so long as the polarization remains unaltered. Changes in the 

 temperature of the crystals alter this condition; and the pyro- 

 electric phenomena observed are the consequence of this 

 change. 



This view is supported by the phenomena recently observed 

 by Messrs. J. and P. Curief, confirmed by Hankel |, and dis- 

 tinguished as piezoelectric, as well as by the experiments 

 made by J. and P. Curie § on the changes of form produced 

 by electricity in pyroelectric crystals ; at least these pheno- 

 mena are naturally explained by the hypothesis in question. 



I will not now dwell upon the difficulties which, in my 

 opinion, militate against the acceptance of this hypothesis, but 

 will simply explain how this view induced me to make the ex- 

 periments here described, the results of which are certainly in 

 themselves worthy of attention. The consideration from which 

 I started was the following : — If an electric polarization were 

 constantly present in a pyroelectric crystal in definite direc- 

 tions, and if it is allowable to conclude from the recently dis- 

 covered effects of statical electricity on the optical properties 

 of singly-refracting media that not only the polarization pro- 

 duced by external electric forces, but also any natural polari- 

 zation already existing, would exert an influence on the 

 vibrations of light transmitted through the crystal, then the 

 optical properties of a pyroelectric crystal would be affected 

 in different ways, according as the natural polarization was 

 weakened or strengthened by the action of external electric 

 forces. 



Taking a quartz crystal as example, the result of piezo- 

 electric experiments with it may be described as follows, at 

 least for normal crystals of simple formation: — A section of 

 the crystal at right angles to the principal axis may be divided 

 by three straight lines intersecting each other in any point 



* Translated from a separate impression from the Berichte der Oberh. 

 Ges. fur. Natur- und Heilkunde, communicated by the Author. 



t Compt. Rend. xci. pp. 294, 383 (1880) ; xcii. pp. 186, 350 ; xciii. 

 p. 204 (1881). 



X Abhandl. der kdn. sacks. Gesellsehaft. vol. xii. p. 459 (1881). 



§ Compt. Rend, xciii. p. 1137 (1881). 



