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+ 3o' C., was undertaken to determine which of them seem to be related to the piezo-electric effect, which increases rapidly between -2o and -x5' C. and decreases rapidly between + 2o and + 3o 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 wavelengths from 4SS4 to 6Soo A. and at temperatures of -7o, -II. S, o, + 2x. 3 and + 4o' C. The variation with temperature, however, is linear, the average coefficient being S9 X xo ' per degree. The thermal expansion betweenxo and + 2o' C. the piezo-electric changes.Nevvman's Principle of Symmeh y 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 a 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 Sg p. 64.0 JOSEPH VA I.A SEK, t SECOND SERIES.' Valasek, PHvs. Rmv. , XIX., 478, Igz2.

Residual charge and fatigue effects for Rochelle salt condenser.-The throw of a ballistic galvanometer connected to a condenser with Rochelle salt crystal as dielectric depends on the time of charging and the time of discharging. It increases to a maximum as the charging time is increased to 2 sec. (for crystal plates .15 cm. thick) and then decreases slowly, reaching for a time of 24 hours a value about half the original. The decrease of the throw for .03 sec. charging, due to fatiguing for 24 hours at 100 volts, was found to be nearly independent of the temperature from -15°C to -r-20°C. The first increase is probably due to residual charges which are easily displaced to a more or less definite limit and which account for the high dielectric constant in this temperature range. The subsequent decrease is due to a fatigue effect which may be likened to an electrolytic polarization of the internal displacement current. These effects are associated with the w T ater of crystallization, since dessication decreases them while previous soaking in alcohol accelerates them. To explain the limited temperature range of the effects it is supposed that at -20°C there is a loosening of the water ions enough to permit a slight motion of about 10~9 cm with reference to the Rochelle salt molecules, while at temperatures above +25°C further loosening allows electrolytic conduction to increase greatly.Thermal changes in Rochelle salt.-There is apparently an evolution of heat starting at 24°C and persisting to 54°C where a very strong absorption of heat takes place.