Astronomical and Nautical Collections. 323 
interval for producing its greatest effect, on account of the coercive 
force of the hard steel of which the needle was made, However 
this may be, we must certainly conclude from the experiment, that 
the mutual action of the magnetic fluids, contained in the steel and 
the soft iron, is decidedly greater than the mutual action of the 
fluids belonging to the steel and the nickel. 
Perhaps it may be thought that this difference of the actions of 
the magnetic fluid, in the different substances containing it, may 
depend on the different quantity of the boreal and austral fluids 
contained in each of these substances when they are in the neutral 
state; the quantity being greater, for example, in iron than in nickel. 
But this view of the subject is contrary to the phenomena, the quan- 
tities of both fluids contained in each substance when neutral being 
without limit, as far as regards our experiments : that is to say, the 
forces which we command are never sufficient to exhaust or sepa- 
rate them by the process of magnetizing ; for when a body is mag- 
netized by the influence of a neighbouring magnet, it is admitted 
that the intensity of its magnetic state, as shown by the effects 
which it produces, increases without limit in proportion as we in- 
crease the force of the magnet employed; which implies evidently 
that we have not reached the limit of the decomposition or separa= 
tion of the neutral fluid which it contains, in the same manner as 
we find it impossible to separate completely the two electric fluids 
contained in a conductor of electricity. 
We must therefore necessarily suppose that the mutual action of 
two magnetic particles, belonging to different bodies, depends on 
the matter of each of these bodies. It is probable that this action 
varies also with their temperature ; which seems, indeed, to follow 
from an old observation of Mr. Canton, and from some more extensive 
and more accurate experiments left unpublished by Coulomb, and 
since inserted in the Traité de Physique of Mr. Biot. These expe- 
riments show the influence of heat in the developement of mag- 
netism; but having been made with magnetized bars, which were 
by no means free from coercive force, the effects observed were de- 
rived, without doubt, in part from the variation of their force, and in 
part from that of the magnetic action. It would therefore be 
