242 
Proceedings of the Royal Society of Edinburgh. [Sess. 
dodecahedral foliation perpendicular to the face ABGF takes place, and 
if that parallel to AG exceeds that parallel to BF, a term involving cos 2d, 
with its maximum in the direction AG, as exhibited in fig. 6, will appear 
in the effect parallel to ABGF in weak fields. The diagram shows that 
it persists to fairly high fields, where it becomes relatively obliterated, 
presumably in consequence of electro-striction. If the effect be sufficiently 
strong, it may overbalance the opposite effect, already alluded to, due to a 
possible octahedral foliation. 
Now the equal foliations in the dodecahedral planes parallel to AC and 
BD produce unequal effects in the octahedral plane at A. For the former 
cuts that plane normally, while the latter does not. This gives a cos 2 0 
term with its maximum along the projection of the a axis. And the 
dodecahedral foliations parallel to AG and BF give one with its maximum 
along the projection of the c axis : the latter being stronger than the 
former if the foliation parallel to AG be in excess. The two effects 
together combine into a cos 2d effect with its maximum close to 120° 
(fig. 7) and its minimum near the projection of the b axis, as Quittner 
observed. 
Foliation perpendicular to the quaternary axes could annul the 
structural cos 4d effect in a cubic face in weak fields, and, by becoming less 
at high fields, could allow the structural effect to show. But any want 
of equality in this foliation would give rise to a cos 2d term with its 
maximum parallel to a quaternary axis, of which there is no experimental 
evidence. Nor can that foliation give a cos 4d term in an octahedral plane. 
But it gives, in an octahedral plate, a cos 6d term having its minima in 
the directions of the projections of the axes on the octahedral plane, 
agreeing 1 with the structural effect and the effect of octahedral foliation. 
The cos 6d term observed at low fields in the octahedral plane must be 
due to dodecahedral foliation, which gives maxima along the projections 
of the axes, and which, diminishing as the field increases, allows the 
structural effect to preponderate ultimately. 
Thus the normal and abnormal effects observed in magnetite are 
deducible from theory, the latter being due to foliation. 
19. The mineral pyrrhotine, which crystallises in hexagonal, or quasi- 
hexagonal, form, has been investigated experimentally by Weiss; and he 
has also given a theoretical explanation in terms of the action of an internal 
field. He found that a magnetic plane (§ 8) existed. Fig. 9, which is 
copied from his paper ( Journ . de Phys., iv., 1905), exhibits the general 
nature of his results. The upper curve represents the magnetisation 
parallel to the external field, and the lower curve represents the magnet isa- 
