216 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XVII. — The Molecular Theory of Magnetism in Solids. 
By Professor W. Peddie. 
(Read February 5, 1912. MS. received March 4, 1912.) 
1. It is well known that the molar theory of magnetism, in which the 
magnetic medium is regarded as being continuous in its ultimate structure 
and properties, is of very limited applicability to the elucidation of 
magnetic phenomena. The fact of the impossibility of isolating one kind 
of magnetism from the other by any process based on inductive action 
makes evident the essentially molecular nature of magnetism. This was 
recognised even in the early development of the theory by Poisson, who 
presumed that the molecules only became magnetic under the action of an 
external field. Weber recognised the necessity for the ascription of 
permanence to the magnetic quality of the molecule in order to account for 
the phenomena of magnetic saturation ; and his theory was modified by 
Maxwell so that it might also include in its range the phenomena of 
residual magnetisation. In Maxwell’s discussion of Weber’s theory, we have 
the first explicit statement of the distinctive feature of the modern theory 
of molecular magnetism. 
He says : “ Let us now suppose that a magnetic force X is made to act 
on the iron in the direction of the axis of x, and let us consider a molecule 
whose axis was originally inclined a to the axis of x. If this molecule is 
perfectly free to turn, it will place itself with its axis parallel to the axis of 
x, and if all the molecules did so, the very slightest magnetising force would 
be found sufficient to develop the very highest degree of magnetisation. 
This, however, is not the case. The molecules do not turn with their axes 
parallel to x, and this is either because each molecule is acted on by a force 
tending to preserve it in its original direction, or because an equivalent 
effect is produced by the mutual action of the entire system of molecules .” 
Maxwell then points out that Weber adopts the former postulate as 
being the simplest, and he next proceeds to make the modification above 
referred to. There can be no doubt that Maxwell’s own preference would 
have been for the latter postulate, but the former was sufficient for his 
purpose in connection with the magnetic data available at the time. The 
first application of his idea was made by Ewing, who explained by its 
means the general form of the magnetisation curve of iron, and showed 
that a magnetic body, consisting of groups of four molecules in square 
