138 Prof. W. C. Rontgen on the Change in the Double 



to the other end. The double refraction diminishes, on the 

 other hand, when the distribution of the communicated elec- 

 tricities is the opposite. 



If we adopt the view that a piezoelectric crystal is in a 

 condition of electric polarization, whose direction, in the 

 particular case of quartz, seems to coincide with the direction 

 of a secondary axis, and that the observed piezoelectricity is 

 a consequence of the change of polarization produced by 

 pressure, then simple reasoning shows that the end which 

 becomes negative upon increase of pressure is that to- 

 wards which the negative side of the electrically polarized 

 molecules is turned. But we found before that the double 

 refraction increases when positive electricity is communicated 

 to this end and negative electricity to the other ; the polari- 

 zation must be strengthened by this electrification ; and we 

 consequently obtain the result, that the double refraction of 

 quartz increases or decreases according as the natural polari- 

 zation is increased or weakened by external electric forces. 



Having thus found the first of the consequences mentioned 

 at the beginning confirmed by experiment, I proceeded fur- 

 ther to put the second to the experimental test. The experi- 

 ments described had shown me that, as far as intensity was 

 concerned, there was at any rate no great difference between 

 the increase and the decrease of the double refraction pro- 

 duced by equal electric forces ; thence I concluded that it must 

 be possible to find a direction in the quartz possessing the 

 property that electric forces acting in this direction would 

 produce no perceptible change in the double refraction. 

 From what has been said above, this direction was to be 

 sought for in an axis of no piezoelectricity — consequently in, 

 or at any rate in the neighbourhood of, an intermediate axis 

 of the crystal. I obtained therefore from Messrs. Steeg and 

 Reuter a square plate of quartz of 1*5 centim. in the side and 

 0*25 centim. thickness, which was cat accurately parallel to a 

 side face, and consequently at right angles to an intermediate 

 axis. The principal axis is parallel to a side of the square; a 

 secondary axis is consequently parallel to one of the sides at 

 right angles to the first named. The four narrow side faces 

 are polished. 



First of all I examined whether the intermediate axis of 

 this crystal was really an axis of no piezoelectricity. The 

 result obtained was that even great changes of pressure in the 

 direction of the intermediate axis gave rise to no perceptible 

 quantities of electricity at the points of pressure, and that 

 this direction is consequently an axis of no piezoelectricity. 

 It may be remarked that a pressure parallel to the principal 



