CONSTITUTION AND TEMPERATURE ONf MAGNETIC SUSCEPTIBILITY. 271 



precisely what the experiments with these crystals exhibit. Analogy as we have seen 

 justifies the assumption here made. It will, however, be of interest to enquire, 

 whether any discoverable circumstance connected with crystalline structure exists 

 upon which the difference of proximity depends and knowing which, we can pronounce 

 with tolerable certainty, as to the position which the crystal will take up in the 

 magnetic field. 



" The following experiments will perhaps suggest a reply. 



"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. The crystal will split before the 

 pressure, disclosing shining surfaces of cleavage. This is the only cleavage the 

 crystal possesses and it stands equatorial. Sulphate of zinc is of the same form as 

 sulphate of magnesia, 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 dimension 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 experimented 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 force, however, 

 at once discovers a difference of action. The cleavages of the diamagnetic specimens 

 stand equatorial; of the magnetic, axial. 



" A cube cut from a prism of scapolite, the axis of the prism being perpendicular to 

 two of the parallel faces of the cube, suspended in the magnetic field, sets itself with 

 the axis of the prism from pole to pole. 



" A cube of beryl of the same dimensions with the axis of the prism from which it is 

 taken also perpendicular to two of the faces, suspended as in the former case, sets itself 

 with the axis equatorial. Both these crystals are magnetic. 

 VOL. ccxx. A. 2 P 



