39 
upon a single subtile fluid in excess in the bodies 
said to be positively electrified, in deficiency in the 
bodies said to be negatively electrified. A second 
class suppose the effects to be produced by two 
different fluids, called by them the vitreous fluid 
and the resinous fluid; and an hypothesis has 
been advanced in which they are considered as 
affections or motions of matter, or an exhibition 
of attractive powers, similar to those which pro- 
duce chemical combination and decomposition ; 
but usually exerting their action on masses. 
The power which gives to a bar or needle of 
steel the property of directing itself to two points 
of the globe, called north and south poles, depends 
upon what is called magnetism. It agrees with 
electricity in many of its laws, but as far as our 
researches have hitherto gone, it is most active in its 
operation on metals and certain of their combinations. 
Iron, nickel, and cobalt, are most susceptible of 
magnetic impressions, and in the harder compounds 
of iron these impressions produce permanent ef- 
fects ; but the recent experiments of M. Arago 
show, that copper metals in general, and, probably, 
all other substances, receive very weak and evanes- 
cent magnetism, which seems to differ in intensity 
for everybody. Magnetism is capable of being com- 
municated from bodies endowed with it to others 
that do not possess it, and is produced whenever 
concentrated electricity passes through space, its 
sphere of action or communication being at right 
angles to the course of the electricity. Thus a bar 
of steel, placed transversely over a wire, convey- 
ing an electrical shock, becomes a magnet. The 
d 4 
