Electric Hysteresis in Rochelle Salt.---On the doublet theory of dielectric action, the dielectric displacement $D$, electric intensity $E$, and the polarization $P$ are analogous to $B$, $H$, and $I$ in the case of magnetism. Rochelle salt shows an electric hysteresis in $P$ analogous to the magnetic hysteresis in the case of iron. The loops obtained are displaced from the origin by an amount which gives a measure of the permanent polarization in the natural state. The moment per unit volume in the natural state is of the order of 50 e.s.u./${\mathrm{cm}.}^{2}$ under ordinary conditions, this being about 13.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}21}$ e.s.u./${\mathrm{cm}.}^{2}$ per molecule.Piezo-electric Response of Rochelle Salt in an Electric Field.---Mechanical force and electric field are to a certain extent equivalent in producing a change in the piezo-electric moment. Measurements of the piezo-electric response in various applied fields therefore give curves of the same nature as the curves of $\frac{\mathrm{dD}}{\mathrm{dE}}$ against $E$. Accordingly they show maxima displaced from the origin by the effect of permanent polarization.