magnetic field, and raised to incandescence by electricity. He then
examined the magnetic deportment of gases generally. Almost all of
these gases are invisible; but he must, nevertheless, track them in
their unseen courses. He could not effect this by mingling smoke
with his gases, for the action of his magnet upon the smoke would
have troubled his conclusions. He, therefore, 'caught' his gases in
tubes, carried them out of the magnetic field, and made them reveal
themselves at a distance from the magnet.

Immersing one gas in another, he determined their differential
action; results of the utmost beauty being thus arrived at. Perhaps
the most important are those obtained with atmospheric air and its
two constituents. Oxygen, in various media, was strongly attracted
by the magnet; in coal-gas, for example, it was powerfully magnetic,
whereas nitrogen was diamagnetic. Some of the effects obtained with
oxygen in coal-gas were strikingly beautiful. When the fumes of
chloride of ammonium (a diamagnetic substance) were mingled with the
oxygen, the cloud of chloride behaved in a most singular manner,--
'The attraction of iron filings,' says Faraday, 'to a magnetic pole
is not more striking than the appearance presented by the oxygen
under these circumstances.'

On observing this deportment the question immediately occurs to him,
--Can we not separate the oxygen of the atmosphere from its nitrogen
by magnetic analysis? It is the perpetual occurrence of such
questions that marks the great experimenter. The attempt to analyze
atmospheric air by magnetic force proved a failure, like the
previous attempt to influence crystallization by the magnet.
The enormous comparative power of the force of crystallization I
have already assigned as a reason for the incompetence of the magnet