Royal Institution. 305 



from this coarse experiment to one more delicate, where nature 

 herself has imposed the conditions of aggregation. A plate taken 

 from a mass of shale, picked up a few weeks ago in the coal district 

 of Blackburn, was suspended between the poles ; although strongly- 

 magnetic, it set its longest dimension at right angles to the line 

 joining the poles. This deportment was at once explained by re- 

 ference to the structure of the mass : it also, though apparently 

 compact, was composed of layers transverse to its length ; these 

 layers set from pole to pole, and hence the length equatorial. Let 

 us ascend to a case still more refined. A crystal of sulphate of 

 nickel was suspended between the poles, and on exciting the magnet 

 a certain determinate position was taken up by the crystal. The 

 substance was magnetic, still its shortest dimension set from pole to 

 pole. The crystal was removed from the magnetic field and the edge 

 of a penknife placed along the line which set axial ; a slight pressure 

 split the crystal and disclosed two beautiful surfaces of cleavage. 

 The crystal could in this way be cloven into an indefinite number of 

 magnetic layers ; these layers set from pole to pole, and hence the 

 longest dimension, which was perpendicular to the layers, equatorial. 

 Comparing all these experiments, — ascending from the gross case 

 where the laminse were plates of iron stuck together by wax, to 

 that in which they were crystaUine, the inference appears unavoidable, 

 that the unanimity of deportment exhibited is the product of a com- 

 mon cause ; and that the results are due to the peculiarities of material 

 aggregation. 



The beautiful researches of Pliicker in this domain of science are 

 well known. Pliicker's first experiment was made with a plate of 

 tourmaline. Suspended in the magnetic field with the axis of the 

 crystal vertical, it set its length from pole to pole, like an ordi- 

 nary magnetic body. Suspended with the axis horizontal, on 

 exciting the magnet, Pliicker found to his astonishment that the 

 largest dimension set equatorial. Let us see whether we cannot 

 obtain this deportment otherwise. Suspending the piece of shale 

 already made use of, so that its laminae were horizontal, on exciting 

 the magnet the longest horizontal dimension of the plate set axial : 

 moving the point of suspension 90° so that the laminae were vertical, 

 on exciting the magnet the length of the plate set equatorial. In 

 the magnetic field the deportment of the crystal was perfectly 

 undistinguishable from that of the shale. But it may be retorted 

 that tourmaline possesses no such laminae as those possessed by the 

 shale : true — nor is it necessary that it should do so. A number 

 of plates, bars, and discs, formed artificially from magnetic dust, 

 exhibited a deportment precisely similar to the tourmaline, — sus- 

 pended from one point they set their lengths axial, suspended from 

 another point the lengths set equatorial. Let us now turn to 

 what may be called the complementary actions exhibited by dia- 

 magnetic bodies. A homogeneous diamagnetic bar sets its length 

 equatorial. But bars were exhibited composed of transverse dia- 

 magnetic laminae which set their lengths axial. This experiment is 

 complementary to that of the shale, &c. ; the magnetic laminae set 



