CONSTITUTION AND TEMPERATUKE ON 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 equatoiial 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. 
o 
“ 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 
