TRANSACTIONS OP THE SECTIONS. l7 



A cube of sulphate of iron was attracted in one direction by a force of 43, and in 

 another direction by a force of 36-3. A spheie of carbonate of iron was attracted 

 in the direction of the optic axis by a force of 43, and in a direction perpendicular 

 thereto by a force of 30-5. When these crystals are suspended between two poles, 

 these lines of chief attraction approach the poles, and finally set axial. 



Thus we see that the phaenoraena exhibited by crystals in the magnetic field, are 

 to be referred to a modification of magnetism or diamagnetism depending, no doubt, 

 upon the peculiar structure of the crystal. Let us endeavour to penetrate this my- 

 stery of structure. Our next inquiry is. What direction is that which is chosen 

 by the respective forces for the manifestation of their greatest energ)'? To this 

 question the author imagines that the following reply is returned by experiment : " If 

 the arrangement of the component particles of any body be such as to present differ- 

 ent degrees of proximity in different directions, then the line of closest proximity, 

 other circumstances being equal, will be that of strongest attraction in magnetic 

 bodies, and of strongest repulsion in diamagnetic bodies." 



The torsion balance furnishes us with the means of submitting this conclusion to 

 a direct test. A quantity of bismuth was ground to dust in an agate mortar, gum- 

 water was added, and the mass was kneaded into a stiff paste. This was placed be- 

 tween two glasses and pressed together. From the mass, when dried, two cubes 

 were taken, the line of compression being perpendicular to two of the faces of each 

 cube, and parallel to the other four. Suspended by a silk fibre in the magnetic field, 

 upon closing the circuit the line of compression turned strongly into the equatorial 

 position, exactly as the plane of most eminent cleavage in the case of the crystal. 

 The cubes were placed one upon each end of the torsion balance, first, with the line 

 of compression parallel to the axis of the cores, and secondly, perpendicular thereto ; 

 the repulsion in the former case was to the repulsion in the latter in the ratio of 

 53 to 30. A greater differential action was thus exhibited in the case of the model 

 than in the case of the crystal. 



A pair of cubes constructed in the same manner from powdered carbonate of iron 

 exhibited an analogous predominance of attraction in the line of compression. 



Against this mode of experiment an objection was urged, during the meeting of the 

 British Association at Edinburgh last year, by Prof. W. Thomson of Glasgow. 

 " You have," he said, " reduced the mass to pov<rder, but you have not thereby de- 

 stroj'ed the crystalline form ; your powder is a collection of smaller crystals, and 

 the pressing of the mass together gives rise to a predominance of axes in a certain 

 direction, so that the repulsion and attraction of the line of compression, which you 

 refer to closeness of aggregation, is after all a product of crystalline action. Besides, 

 we know that compressed isinglass exhibits the same optical phcenomena as crystals, 

 and you are unable to prove that the action is not due to a quasi-crystalline struc- 

 ture induced in the gum by compression." 



The following experiment will set this point at rest. It will not only show the 

 influence of compression apart from the mere arrangement of the axes or from the 

 influence of the gum, for none will be used ; but it will also demonstrate the nullity 

 of this presumed axial force when opposed to the influence of compression. To this 

 experiment the author was conducted by the following accident. 



The investigation was conducted in Berlin, and the great electro-magnet of the 

 university was beside him at the time. Some notion of the power of this magnet 

 may be gathered from the fact, that the copper helices alone surrounding the iron 

 pillars, which composed the magnet, weighed 243 pounds. On the top of the pillars, 

 two moveable masses of soft iron were placed, each weighing about 25 pounds, and 

 between these the substance to be examined was suspended. Afine cube of bismuth 

 crystal was suspended on one occasion between these moveable poles, and on closing 

 the circuit the planes of most eminent cleavage receded to the equator. Scarcely, 

 however, was this attained when the poles were observed moving towards each other, 

 and, before the circuit could be broken, they had rushed together and clenched their 

 iron jaws upon the crystal. The latter was reduced by the pressure to about three- 

 fourths of its primitive thickness, and immediately suggested the thought, that if the 

 theory of proximity were true it ought to tell here. The pressure brought the par- 

 ticles' of the crystal in the line of compression more closely together, and hence a 

 modification, if not an entire reversion, of the former action might be anticipated. 

 1851. '^ 



