We may for this purpose examine some of the laws of common physical and
chemical phenomena, neglect of which has resulted in much needless
discomfort in daily life, and even more serious consequences. For
instance, the laws of expansion of gases and liquids with heat, and
their subsequent behaviour, are phenomena that are often imperfectly
realised. There is probably no person who is unacquainted with the law
of gravitation, but there are many persons who accept literally the
statements that smoke rises and that balloons ascend. A clear
understanding of what actually takes place when gases and material
masses appear to move in opposition to the law of gravitation is
essential to any scheme for warming and ventilating the house.
A very simple experiment will serve to reconcile the apparent
contradiction of the universal law by the observed fact. Suppose we have
two fluids, oil and water, of which oil is, bulk for bulk, lighter than
water. If the oil be poured into a glass beaker, it will be seen to rest
at the bottom of the beaker; if water be now poured into the same
beaker the water will go to the bottom of the beaker and will displace
the oil and lift it up so that the oil will float on the water; the oil
may be lifted to any height we please if sufficient water be poured in
to lift it to that height. If a single drop of oil be introduced into
the water by means of a pipette and be liberated at the bottom of the
beaker the water will close in under it, and lift it up to the surface.
In both cases the oil "rises" through the water. Oil, however, has no
tendency to "rise" by itself, and in this case it lay motionless until
it was lifted by the heavier fluid. We may use colloquial language when
describing phenomena if we bear in mind what is really taking place.
A balloon "rising" through the air is exactly analogous to the drop of
oil in the water. The balloon is, bulk for bulk, lighter than air; the
air therefore closes in under it and lifts it just as the water lifted
the bubble of oil.