350 ATAGXECRYSTALLIC ACTION. SECT. XXXIV. 



cleavage, and that the elective position of crystals depends more 

 upon the direction of these planes with respect to the electric 

 force, than upon the optic axis. The planes of principal cleavage 

 set themselves equatorially in diamagnetic, and axially in para- 

 magnetic substances : it was thence inferred that the phenomena 

 offered by crystals in the magnetic field is a particular case of the 

 general law, that the superior action of magnets upon matter in 

 a particular direction is due to the particles of the body being 

 closer together in that direction than in any other : in short, 

 the line of maximum density ; the force exerted being attractive 

 or repulsive according as the particles are paramagnetic or dia- 

 magnetic. 



It appears, however, that the set of crystals with regard to the 

 line of magnetic force does not depend solely upon their density 

 in particular directions. Professor Matteucci, of Pisa, has proved 

 that the diamagnetic force is inversely as the conducting power 

 of substances for electricity, that the conducting power is a 

 maximum in the planes of principal cleavage, and that a needle 

 of crystallized bismuth, in which the planes of cleavage are 

 parallel to its length, places itself equatorially with more force 

 when these planes are vertical, or at right angles to the force, 

 than when they are horizontal or parallel to it. Experiments had 

 hitherto been made only with diamagnetic or slightly paramag- 

 netic bodies, which induced M. de Roux to try the effect of mag- 

 netism on pulverized iron compressed by the hydraulic press, 

 which reduced the grains of iron to lamellae equivalent to planes 

 of cleavage. Cubes of this substance, suspended by a thread over 

 a horseshoe magnet, oscillated fora longer time when the lamella? 

 were perpendicular than when they were horizontal ; that is, the 

 force was stronger when the lamella? were equatorial than when 

 they were axial, exactly the same result as in Professor Mat- 

 teucci's experiment with the needle of bismuth. Thus the ver- 

 tical position of the cleavages, which increases the diamagnetism 

 of the bismuth, increases also the paramagnetism of the iron. 

 M. le Roux observes that these results are independent of the 

 influence of the currents of electricity induced in the oscillating 

 body, for the fundamental character of the phenomena of Arago's 

 discovery of rotation by induction is, that the oscillations diminish 

 rapidly in extent without any sensible diminution in their dura- 

 tion, while in his experiments the time of the oscillations varied. 



