A new method of measuring the principal magnetic susceptibilities of crystals.-The crystal to be measured is immersed in a solution, the susceptibility of which is varied, and the orientation of the crystal adjusted till there is no movement of the crystal due to the magnetic field. The susceptibilities of the solutions are then measured. From these values the principal susceptibilities can then be easily obtained. The method does not require any preparation of crystal sections, measurement of the magnetic field or gradient of the field. The range of application of this method is for volume susceptibilities of -0.9×10-6 to +70×10-6. The principal magnetic susceptibilities of certain crystals.-The above method is applied to the measurement of principal susceptibilities of 14 crystals. Of these eleven are paramagnetic, and belong to the monoclinic double sulphate hexahydrate isomorphous series, MeSO4.R2SO4.6H2O. With these crystals the results seem to indicate that, unlike the optical and crystallographic properties, the principal susceptibilities and their relative magnitudes depend almost entirely on the paramagnetic ion alone. The influence of the alkali or ammonium ion is secondary. In the series containing copper the greatest difference in maximum and minimum susceptibilities is 28% of the average susceptibility, in the nickel series 4%, in the Co series 32%, in the Fe series 16% and in the Mn series 1%. Of the diamagnetic crystals, NaNO3 and KNO3, which are similar in their crystallographic properties to calcite and aragonite respectively, show similar magnetic properties.