ible distance. On these
assumptions his results are certainly right, and are confirmed by the
independent method of Gauss, so that the objections raised against them
by Poisson fall to the ground. But whether the assumption of uniform
density be physically correct is a very different question, and Poisson
rendered good service to science in showing how to carry on the
investigation on the hypothesis that the density very near the surface
is different from that in the interior of the fluid.

The result, however, of Poisson's investigation is practically
equivalent to that already obtained by Laplace. In both theories the
equation of the liquid surface is the same, involving a constant H,
which can be determined only by experiment. The only difference is in
the manner in which this quantity H depends on the law of the molecular
forces and the law of density near the surface of the fluid, and as
these laws are unknown to us we cannot obtain any test to discriminate
between the two theories.

We have now described the principal forms of the theory of capillary
action during its earlier development. In more recent times the method
of Gauss has been modified so as to take account of the variation of
density near the surface, and its language has been translated in terms
of the modern doctrine of the conservation of energy.[2]

J.A.F. Plateau (_Statique experimentale et theorique des liquides_), who
made elaborate study of the phenomena of surface-tension, adopted the
following method of getting rid of the effects of gravity. He formed a
mixture of alcohol and water of the same density as olive oil, and then
introduced a quantity of oil into the mixture. It assumes the form of a
sphere under the action of surface-tension alone. He then, by means of
rings of iron-wire, disks and other contrivances, altered the form of
certain parts of the surface of the oil. The free portions of the
surface then assume new forms depending on the equilibrium of
surface-tension. In this way he produced a great many of the forms of
equilibrium of a liquid under the action of surface-tension alone, and
compared them with the results of mathematical investigation. He also
greatly facilitated the study of liquid films by showing how to form a
liquid, the films of which will last for twelve or even for twenty-four
hours. The debt which science owes to Plateau is not diminished by the
fact that, while investigating these beautiful phenomena, he never