Temperature variation of several physical properties of Rochelle salt, from - 30° to + 30° C., was undertaken to determine which of them seem to be related to the piezo-electric effect, which increases rapidly between - 20 and - 15° C. and decreases rapidly between + 20 and + 30° C. In this way it was hoped to get information as to the nature of the structure underlying piezo-electric phenomena. Refractive indices were measured for the three principal directions for six wave-lengths from 4554 to 6500 Å. and at temperatures of - 70, - 11.5, 0, + 21.3 and + 40°C. The variation with temperature, however, is linear, the average coefficient being 59 × ${10}^{-6\mathrm{}}$ per degree. The thermal expansion between - 10 and + 20° C. was measured by the Fizeau method for the three principal directions. The ratio of ($n^2-1\mathrm{}$) to $\rho$ is approximately constant, in agreement with the Sellmeir formula for dispersion, and if the change in refractivity is regarded as due to the change in density, reasonable values are obtained for the constants in the Lorentz formula. The specific rotatory power of an aqueous solution of the salt was found to change less than 0.4° in the interval from + 4 to 40° C., giving no indication of a change in molecular asymmetry in this interval. The value obtained at 25° C. is 25.9° for sodium light, lying between the results of previous observers. The electrical conductivity increases only from 2 × ${10}^{-14\mathrm{}}$ mhos/${\mathrm{cm}.\mathrm{}}^3$ at - 65° C. to 5.4 × ${10}^{-14\mathrm{}}$ at - 20° C. Between - 20° and + 30° C., however, it was found to depend on the direction of the current, being only half as great in one direction as in the other at 0° and at 20°, and above 20° it increases very rapidly to 5 × ${10}^{-9\mathrm{}}$ at 43°. At all temperatures, however, the current was found proportional to the applied field, at least up to 10,000 volts/cm. The reversible electro-optic rotation was observed in the direction of the field, using transparent alcohol electrodes and plane polarized light of wave-length 5737 Å. At 20° the rotation of the $\alpha$ and $\beta$ axes in their plane per unit field normal to them is 1.7 × ${10}^{-4\mathrm{}}$ degrees, and decreases for higher temperatures much as the dielectric constant does. The electro-optic constant $e_{41}$ was also computed. Although Rochelle is classed as a type of crystal that should show no pyro-electric effect, a "real" effect was observed in the ă crystallographic direction as the crystal was gradually heated at a constant rate. The coefficient changes sign just below room temperature and becomes very large.

Theory of piezo-electricity is discussed briefly in the light of these results. In view of the fact that none of the properties measured show changes with temperature which correspond with those of the piezo-electric effect it is probable that the elastic or piezo-optic constants must change in such a way as to account for the piezo-electric changes.

Newman's Principle of Symmetry in Crystal Optics.—Rochelle salt, if it belongs to the hemihedral class of the orthorhombic system, seems to form an exception to this principle since most of its electrical properties in the direction of the ă axis depend on the direction of the field.

Infra-red absorption band for Rochelle salt is computed from the constants of the dispersion formula to be at about 55 μ.

• Received 1 July 1922

DOI:https://doi.org/10.1103/PhysRev.20.639