598 SCIENCE PROGRESS 



small an order that the differential induction effect along 

 different directions within the crystal could certainly not give 

 rise to the mechanical movements observed. On our view of 

 the structure of a diamagnetic molecule, which we have seen 

 is strongly magnetic locally, but as a whole diamagnetic, this 

 difficulty disappears. The local force is large enough to explain 

 the deportment of the crystal in a magnetic field and also to 

 produce the molecular distortion which accompanies the tran- 

 sition from the liquid to the crystalline state. 



Tyndall examined more than one hundred different crystals. 

 The importance of his results can best be judged by quoting 

 one or two examples. He remarks (p. 24) : " If a prism of 

 sulphate of magnesia be suspended between the poles with its 

 axis horizontal, on exciting the magnet, the axis will take up 

 the equatorial position. This is not entirely due to the form 

 of the crystal ; for even when its axial dimension is shortest, 

 the axis will assert the equatorial position ; thus behaving like 

 a magnetic body, setting its longest dimension from pole to 

 pole. 



" Suspended from its end with its axis vertical, the prism 

 will take up a determinate oblique position. When the crystal 

 has come to rest, let that line through the mass which stands 

 exactly equatorial be carefully marked. Lay a knife-edge 

 along this line, and press it in the direction of the axis. 1 he 

 crystal will split before the pressure, disclosing shining surfaces 

 of cleavage. 1 his is the only cleavage the crystal possesses, 

 and it stands equatorial. 



" Sulphate of zinc is of the same form as sulphate of mag- 

 nesia, and its cleavage is discoverable by a process exactly 

 similar to that just described. Both crystals set their planes 

 of cleavage equatorial. Both are diamagnetic. 



" Let us now examine a magnetic crystal of similar form. 

 Sulphate of nickel is, perhaps, as good an example as we can 

 choose. Suspended in the magnetic field with its axis horizontal, 

 on exciting the magnet the axis will set itself from pole to pole ; 

 and this position will be persisted in, even when the axial dimen- 

 sion is shortest. Suspended from its end, the crystalline prism 

 will take up an oblique position with considerable energy. 

 When the crystal thus suspended has come to rest, mark the 

 line along its end which stands axial. Let a knife-edge be 

 laid along this line, and pressed in a direction parallel to the 

 axis of the prism. The crystal will yield before the edge, and 

 discover a perfectly clean plane of cleavage. 



"These facts are suggestive. The crystals here experi- 

 mented with are of the same outward form ; each has but 

 one cleavage ; and the position of this cleavage, with regard 

 to the form of the crystal, is the same in all. The magnetic 



