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that both permanent and electro-magnets, under certain 
conditions, induce electrical currents. The circumstances 
under which a magnet or an electric current induces another 
current, have been very fully investigated, but we do not 
find any instance in which the direction of the induced 
current is affected by the nature of the matter in which 
the induction takes place. If it were only necessary to 
account for those magnetic phenomena in which the matter 
subjected to electrical or magnetic force becomes affected 
by such polarity that it is attracted by the pole of the 
magnet to which it is opposed, we might have no great 
difficulty in the general application of Ampere's theory ; 
for we may easily conceive that currents circulating around 
the poles of an electro-magnet induce other currents in a 
neighbouring mass of iron, under the influence of which 
the latter is affected by such induced electrical currents as 
to be attracted by the electro-magnet ; but I am not aware 
of any extension of Ampere's theory by which it can be 
explained how it happens that when, in lieu of the piece 
of iron a piece of bismuth is substituted, the latter is re- 
pelled, instead of being attracted. 
The theory of Ampere may be applied to explain the 
magnetic or dia-magnetic state of some bodies, for instance, 
platinum and bismuth, by supposing that in the platinum 
a current is induced in one direction, and in the bismuth 
in the opposite ; but with regard to the phenomena exhibited 
by lead, the difficulty is much greater. We may suppose 
that the magnetic condition of lead is caused by a current 
in one direction, and (passing over the difficulty of account- 
ing why a feeble magnetic force should induce a current in 
one direction, and a greater degree of magnetic force a 
current in another direction) that the dia-magnetic condi- 
tion is caused by a current in the contrary direction to 
the assumed current in the magnet. But we have the 
