94 Prof. Challis on a Theory of Magnetic Force. 
of space, there would seem to be no reason why a magnet should 
not move various substances as an electrified body does. In the 
one case, as in the other, variation of pressure is produced, accord- 
ing to the theory, by etherial streams. What, then, is the reason 
that an electrified body acts indifferently on many kinds of sub- 
stances, whilst a magnet acts in a special manner on certain sub- 
stances, which from this peculiarity have been named magnetic ? 
18. In answering this question, it is first to be considered 
that, according to the hydrodynamical theory of electricity, an 
electrified body has had its superficial atoms forcibly disturbed, 
and that this state of disturbance gives rise to vibrations of the 
ether, the dynamic effect of which on a neighbouring body is to © 
put its superficial atoms also into a state of disturbance, and 
thus to induce electricity. It was explained in the theory referred 
to, that this state of induced electricity coexists with a gradation 
of internal density, which generates the secondary currents to 
which electric attractions and repulsions are attributable. But 
in a magnet there is nothing corresponding to that superficial 
disturbance, the internal gradation of density bemg an inde- 
pendent property of the body. Hence there are no vibrations 
to disturb the superficial atoms of adjacent bodies, the magnet 
acting dynamically wholly by currents, and consequently, as is 
known by experience, it is incapable of inducing electricity. At 
the same time, between a magnet and a body partially or induc- 
tively electrified, there must be interference of cetherial currents, 
and consequently mutual action, as is found experimentally to 
be the case. 
Still it may be argued that magnetic streams, varying from 
point to point of space as to density and velocity, must act 
dynamically on the atoms of bodies of all kinds, independently 
of their electric or magnetic states. That such action does 
really take place to a sensible amount under the influence of 
powerful magnets, will be made apparent by the following con- 
siderations applied to the class of facts discussed experimentally 
by Faraday in the Twentieth and Twenty-first Series of his ‘ Re- 
searches in Electricity’ (Phil. Trans. part 1. 1846). 
19, It has been already stated that, according to hydrody- 
namics, a single spherical atom, subject to the action of a uniform 
steady current, suffers no change thereby of a condition either 
of rest or of uniform motion, But this is no longer true if, the 
stream being steady, the lines of motion are not parallel, to each 
other, because in that case there will be inequality of the pres- 
sures on opposite hemispherical surfaces of the atom. Also when 
such a stream enters into any substance, the partial convergences 
or divergences of its course, which must result from its encoun- 
tering an aggregation of atoms, may materially influence its 
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