Magnetic Balance of MM. P. Curie and C. Cheneveau. 357 



arenaceous group. The former may be taken as approxi- 

 mating to 1*3 x 10 " 5 , making allowance for some small 

 amount in the alkaline solutions; the latter to 0*6 x 10~ 5 gram 

 per gram. 



Accepting the estimate cited by F. W. Clarke ("A Preli- 

 minary Study of Chemical Denudation," Smithsonian Miscel- 

 laneous Collections, vol. lvi. No. 5, June 1910) that the 

 calcareous rocks compose 5 per cent., the arenaceous 15 per 

 cent., and the argillaceous 80 per cent, of the sedimentaries, 

 .my results on thorium measurements (assuming O06 x 10~ 5 

 to represent the mean for the calcareous rocks) give for the 

 sedimentary rocks generally a thorium content of 1*16 x 10r& 

 gram per gram. 



July 11th, 1910. 



XXXIV. The Magnetic Balance of MM. P. Curie and 

 C. Chnereau. By C. Cheneveau, with an Appendix 

 by A. C. Jolley *. 



THIS apparatus is intended for the measurement of the 

 coefficient of specific magnetization, or the suscepti- 

 bility or permeability of feebly paramagnetic or diamagnetic 

 bodies f . 



Principle and Theory of the Apparatus. 



The body whose magnetic properties are to be determined 

 is suspended from one end of the arm of a torsion balance. 

 By means of this balance the force is measured, which 

 is experienced by the body when placed in a non-uniform 

 magnetic field, produced by a permanent magnet whose lines 

 of force cross the space occupied by the body. The method 

 of calculating this force will first be briefly indicated. 



Suppose that the body is placed at a point O in a field of 

 direction Oy and of intensity H y . The force/ which tends 

 to move the body will be normal to the direction of the field, 



* Communicated by the Physical Society : read April 22, 1910. 



t The coefficient of specific magnetization K. is the ratio of the 



intensity of magnetization &= — (where M is the magnetic moment 



m v c 



and m the mass of the body) to the magnetizing field. The magnetic 



susceptibility k = KA, where A is the density of the body, and the 



permeability is obtained from the susceptibility bv the relation 



u=l-f4w«c. 



