OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
87 
already employed by Mr. Babbage and Mr. Herschel, and they are sufficiently 
explanatory. In the case of magnetic energy induced in a body by any of 
the ordinary methods, it may be observed as a general fact, that the energy 
is acquired in somewhat less time than that in which it is again lost. This is 
particularly the case in soft iron rendered magnetic, by an electro-magnetic 
spiral ; a simple contact between a magnet and a mass of soft iron will fre- 
quently convey to it an attractive force, which it retains for a comparatively 
long space of time. In regard to the susceptibility of different substances, it 
is found to vary considerably ; and depends on some peculiar property not yet 
explained ; it seems to be in some inverse ratio of their retentive power. Thus 
it is not without difficulty that hardened steel is made permanently magnetic, 
whilst its retentive power is considerable : soft iron, on the contrary, is observed 
to be comparatively feeble in its retentive power, whilst its susceptibility of 
magnetic change is great. It may be hence inferred, that in non-ferruginous 
masses, the mere susceptibility of magnetic change, is in fact also conside- 
rable, but then their retentive power is so feeble, that the subsequent attraction 
does not ensue to any great extent. This probably arises from some peculiar 
state of the particles of these substances, which allows what may be termed 
the new magnetic distribution, to tend more rapidly to the previous state of 
neutrality, immediately the tension passes a certain point. In every observed 
instance, however, more time seems necessary to restore perfectly, the original 
state of the body, than was required to disturb it, the former being the result 
of a progressive operation, whilst the latter is effected by a sudden and con- 
centrated force. 
34. The attraction, as usually observed, between a magnet and a mass of 
iron, is invariably preceded by this new magnetic distribution in the iron ; 
and unless such new distribution can occur, it seems, as in the analogous 
operation of electrical action, that no attractive effect can ensue ; indeed 
this is made evident by the repulsive efforts of two similar magnetic poles ; 
the repulsion may be conceived to be really the immediate consequence of 
an inductive effort, since an attempt is made to reverse the magnetic dis- 
tribution in the opposed bars, which action the already existing polarities re- 
sist : when, however, as in the case of opposing either pole of a very powerful 
