ogen, on the supervention of the magnetic force, the oxygen was
pulled towards the axis, the nitrogen being pushed out. By turning
the torsion-head they could be restored to their primitive position of
equidistance, where it is evident the action of the glass envelopes was
annulled. The amount of torsion necessary to re-establish equidistance
expressed the magnetic difference of the substances compared.

And then he compared oxygen with oxygen at different pressures. One of
his tubes contained the gas at the pressure of 30 inches of mercury,
another at a pressure of 15 inches of mercury, a third at a pressure
of 10 inches, while a fourth was exhausted as far as a good air-pump
renders exhaustion possible. 'When the first of these was compared with
the other three, the effect was most striking.' It was drawn towards
the axis when the magnet was excited, the tube containing the rarer gas
being apparently driven away, and the greater the difference between the
densities of the two gases, the greater was the energy of this action.

And now observe his mode of reaching a material magnetic zero. When
a bubble of nitrogen was exposed in air in the magnetic field, on the
supervention of the power, the bubble retreated from the magnet. A less
acute observer would have set nitrogen down as diamagnetic; but Faraday
knew that retreat, in a medium composed in part of oxygen, might be due
to the attraction of the latter gas, instead of to the repulsion of the
gas immersed in it. But if nitrogen be really diamagnetic, then a bubble
or bulb filled with the dense gas will overcome one filled with the
rarer gas. From the cross-piece of his torsion-balance he suspended his
bulbs of nitrogen, at equal distances from the magnetic axis, and found
that the rarefaction, or the condensation of the gas in either of the
bulbs had not the slightest influence. When the magnetic force was
developed, the bulbs remained in their first position, even when one
was filled with nitrogen, and the other as far as possible exhausted.
Nitrogen, in fact, acted 'like space itself'; it was neither magnetic
nor diamagnetic.

He cannot conveniently compare the paramagnetic force of oxygen with
iron, in consequence of the exceeding magnetic intensity of the latter
substance; but he does compare it with the sulphate of iron, and finds
that, bulk for bulk, oxygen is equally magnetic with a solution of this
substance in water 'containing seventeen times the weight of