Prof. Challis on a Theory of Magnetic Force. 97 
positions,—a property to which it may perhaps owe the facility 
with which it conducts galvanic currents. Iron and bismuth, 
which are not good conductors of galvanism, present very dif- 
ferent phenomena, in consequence, probably, of a great degree 
of fixity of their atoms, by reason of which, and of atomic ar- 
rangement and constitution, the magnetic streams permeating 
them may be so modified as to become respectively attractive 
and repulsive. <A bar of copper, when first. acted upon by the 
magnetic streams, begins to move as if it were a magnetic body, 
this motion being probably due only to the variation of pressure 
from point to point of the streams. But quickly its atoms, on 
account of their mobility, will be moved by that pressure into 
new positions, which may be such as to render it for a brief 
interval diamagnetic, so that the incipient motion 1s stopped, the 
atoms finally settling into positions for which the bar is neither 
magnetic nor diamagnetic. These are positions of constraint, in 
which they are held by the dynamic action of the pressure of 
the magnetic streams. When the streams are interrupted, the 
atoms, by the proper molecular action of the copper, return to 
their normal positions; and in the mean time the reaction of 
the ether on the bar produces the observed revulsion. The 
magnetic streams continuing in action, if an impulse be given 
to the bar tending to move it into a position of greater mag- 
netic action, for the same reason as above an increment of dia- 
magnetic force is generated, and a momentary excess of diamag- 
netic action, arising from the atoms being carried by their 
momentum beyond the positions in which they finally settle, 
stops it; and if it be moved into a position of less magnetic 
action, a momentary defect of diamagnetic action, also due to 
_ the momentum of the atoms in taking up new positions, equally 
stops it. The sluggish motion of the bar may be thus accounted 
for. The movement which occurs when the streams commence, 
would not take place if the bar were placed in the axial or equa- 
torial direction, because the initial magnetic action would .be 
symmetrical with respect to each of these directions. If the 
copper were of the form of a globe, with its centre on the axial 
direction, the sluggishness would still exist, because any move- 
ment of it about a vertical axis would bring each point into a 
position of either greater or less magnetic action. 
24, Since it appears from the foregoing discussion that, inde- 
pendently of the dynamic effect of magnetic streams resulting 
from gradations of the density of the ether, they exert, when 
they enter a magnetic body, a particular moving force due to 
their divergence or convergence as affected by the atomic con- 
stitution of the body, this force ought to be taken into account 
in the theory of the magnet (art. 4), and in the explanation 
Phil, Mag. 8. 4, Vol. 21. No, 138, Feb, 1861 H 
