The Electron Theory of Matter. 173 
the average no magnetic moment at all. If, however, the 
O O 7 7 
electrons are subjected to impulses irregularly distributed at 
comparatively long interval-, magnetic effects may be pro- 
duced (Yoigt) : or if the electrons are subject to sufficiently 
rapid dissipation of energy, so that their orbits continually 
diminish in area, an average magnetic moment results and 
the system is paramagnetic (J. J. Thomson). Xeither 
hypothesis is quite satisfactory : for Yoigt's hypothesis 
requires either very large numbers of independent electrons 
in the atom, or very powerful impulses, to produce an appre- 
ciable effect (§ 22), while that of J. J. Thomson requires very 
great dissipation of energy, and besides leaves little scope for 
accounting for diamagnetism (§ 22). As a matter of fact 
we shall see that neither hypothesis is necessary ; for a 
system of electrons in orbital motion must group themselves 
in rings, if their radiation is to be sufficiently small for per- 
manence : and a ring, as opposed to a system of independent 
electrons, possesses magnetic properties (§ 21). 
§ 3. The magnetic effects to be explained may be con- 
veniently grouped as follows : — 
(1) Effects due to the mutual interaction of the molecules 
of the substance; these are usually taken to include hysteresis 
and coercive force, and the numerous interactions between 
strain and magnetization. It should, however, be borne in 
mind that some of these effects, such as the variation of 
magnetic susceptibility with magnetic force, can be explained 
without intermolecular action (Theory of Stefan * and 
W. Siemens t)- 
(2) Effects due to the actions between the atoms of the 
molecule. The smallness of the magnetism of oxides and 
salts of iron and cobalt in proportion to the amounts of iron, 
or cobalt, contained in them, is an effect of this kind. 
(3) Effects due to the constitution of the atom itself. The 
fact that the amalgams of iron and cobalt studied by Xao-aoka t 
are as strongly magnetic, m proportion to their concentration, 
as the pure metals, coupled with the usual supposition, made 
on chemical grounds, that the molecule of a metal is mon- 
atomic, can hardly be explained except by supposing that 
magnetism is, in the last resort, an atomic property ; and that 
the largeness of the magnetism of iron and its congener> is 
due to some peculiarity of atomic structure. This conclusion 
is supported by the experiments of St. Meyer § and others, 
from which it results that atomic magnetism is a periodic 
* Stefan, Wien. Ber. Ixix. (2) p. 165; Vwedernann, Elektrizitdt, iii. 
p. 515. 
t W. Siemens, JJ'ied. Ann. xxiv. p. 113. 
% Xagaoka, JJ'ied. Ann. lix. p. 66. 
§ St. Meyer, JJ'ied. Ann. lxviii. p. 325 : lxix. p. 236 
