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./cm.2 under ordinary conditions, this being about 13.3 × 10-21 e.s.u./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 dDdE against E. Accordingly they show maxima displaced from the origin by the effect of permanent polarization.