79. When a vacuum is produced under the receiver of an air-pump, it is held
down so forcibly that it is impossible to raise it, owing to the weight of the superin-
cumbent column of air. Let the air into the receiver, and it is raised with the greatest
ease, because the air beneath it acts as a counterpoise to the air above it. If the air
beneath the receiver be now compressed, it will acquire a density greater than that of
the air above, and the receiver will be raised, in spite of gravitation ; and, if the current
of air be rapid and violent, it will be sustained in space without any visible support, in
the same way as are the little figures of pith that are made to dance on a jet of water.
Why then should not the universal fluid, which is the constituent element of all matter,
when accumulated around a table, have the property of diminishing or of augmenting
its natural weight, just as air affects the receiver of the air-pump, and as hydrogen
affects balloons, without any derogation from the law of gravitation? Are you aware of
all the properties of all the forces of this fluid? No; then do not deny a fact because you
are as yet unable to explain it.