1904 - 5 .] Magnetic Quality in Molecular Assemblages. 1049 
of the shape of those deduced from the present theory, fig. 7, with 
the observed shape, seems to be quite decisive in favour of the 
view here taken. The existence of the crossing point in the three 
curves, fig. 1, which is not present in all cases, and the apparently 
abnormal shape of the curve Q, must, if they are really abnormal 
conditions, have an effect tending to bring about the coincidence. 
On any view, the coincidence is remarkable, and, in conjunction 
with the apparent absence of transverse magnetisation, or variation 
of parallel magnetisation, in the case of the field lying in a plane 
perpendicular to a ternary axis (§ 17), fully justified Wallerant in 
connecting the quantities as he did. 
19. Condition at a Boundary. — Residual Magnetisation. — The 
expressions representing the force due to the assemblage of 
magnets show that the magnetic effect of a given distribution of 
molecular magnets is, at any given point, external or internal, 
proportional to the magnetic moment per unit volume. There- 
fore, when a bounding surface exists, the effect of the magnetisa- 
tion is representable in the usual way as being due to an imaginary 
surface distribution of magnetism. Yet it is necessary, on the 
molecular theory, to obtain a definite view of the mechanism of 
self-demagnetisation, residual magnetisation, and hysteresis. 
The existence of coercive force, in the case of magnetisation of 
an unbounded assemblage magnetised along a ternary axis, has 
already been found (§ 13). In such an assemblage, magnetised so, 
at the absolute zero of temperature, there would be no demagnetisa- 
tion on withdrawal of the external field if we suppose the magnets 
to have constant magnetic moment. But, when molecular motions 
are taken into account, the magnetic moment would be appreciably 
smaller unless infinite external field acted. Considerable residual 
magnetisation would be left, since self-demagnetisation from sur- 
face effects do not take place in the unbounded medium. In the 
case of magnetisation along a binary axis in an unbounded medium, 
withdrawal of the external field would be followed by alignment 
of the molecules along the two nearest ternary axes. The effective 
component along the binary axis is therefore reduced to two-thirds 
of its original value, apart from possible effects of molecular 
motions. In the case of magnetisation along a quaternary axis, 
