270 



Proceedings of the British Association 



pendicular thereto; the repulsion in the former case was to the repul- 

 sion in the latter in the ratio of 53 : 30. A greater differential action 



was thus exhibited in the case of the model than in the 



of the 



crystal. A pair of cubes constructed in the same manner from pow- 

 dered carbonate of iron, exhibited an analagous predominance of at- 

 traction 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. 

 VVm. Thomson, of Glasgow. "You have," he said, " reduced the 

 mass to powder, but you have not thereby destroyed the crystalline 

 form ; your powder is a collection of smaller crystals, — and the press- 

 ing of the mass together gives rise to a predominence of axes in a cer- 

 tain direction, so that the repulsion and attraction of the line of com- 

 pression, 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 phenomena 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 com- 

 pression apart from the mere arrangement of the axes or from the m- 

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

 the total nullity of this presumed axial force where opposed to the in- 



fluence of compression, 

 following accident. 



To this experiment I was conducted by the 



The investigation was conducted in Berlin, and 

 the great electro-magnet of the University was beside me at the lime. 

 Some notion of the power of this magnet may be gathered from the 

 fact, that the copper helices alone which surrounded the iron pillars 

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

 pillars two movable masse3 of soft iron were placed, each weighing 

 about 25 pounds, and between these the substance to be examined was 

 suspended. Before I had thoroughly made the acquaintance of the in- 

 strument, I hung a fine cube of bismuth crystal between these movable 

 poles; on closinj^ the circuit the planes of most eminent cleavage re- 



I 



ceded lo the equator. Scarcely, however, was this attained when 

 observed the poles moving towards each other, and before I could break 



the circuit, 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 it immediately occurred to me 

 that if the theory of proximity were true it ought to tell here. The 

 pressure brought the particles of the crystal in the line of compression 

 nriore closely together, and hence a modification, if not an entire rever- 

 sion, of the former action might be anticipated. Having liberated the 

 crystal, I boiled it in hydrochloric acid, so as to remove any impurity it 

 might have contracted by contact with the iron. It was again suspend- 

 ed between the poles, and completely verified the foregoing anticipa- 

 tion. The line of compression, that is, the magnecrystallic axis of the 

 crystal, which formerly set from pole to pole, set now equatorial. The 

 experiment was then repeated with a common vice ; various pieces of 

 bismuth protected by plates of copper were placed within its jaws, and 

 there pressed to the thickness of a shilling. The plates thus obtained 

 when suspended from their edges in the magnetic field, exhibited one 



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