ty
of the surfaces of liquids, but to the fatty part of the soap which he
supposed to separate itself from the other constituents of the solution,
and to form a thin skin on the outer face of the bubble.

In 1787 Gaspard Monge (_Memoires de l'Acad. des Sciences_, 1787, p. 506)
asserted that "by supposing the adherence of the particles of a fluid to
have a sensible effect only at the surface itself and in the direction
of the surface it would be easy to determine the curvature of the
surfaces of fluids in the neighbourhood of the solid boundaries which
contain them; that these surfaces would be _linteariae_ of which the
tension, constant in all directions, would be everywhere equal to the
adherence of two particles, and the phenomena of capillary tubes would
then present nothing which could not be determined by analysis." He
applied this principle of surface-tension to the explanation of the
apparent attractions and repulsions between bodies floating on a liquid.

In 1802 John Leslie (_Phil. Mag._, 1802, vol. xiv. p. 193) gave the
first correct explanation of the rise of a liquid in a tube by
considering the effect of the attraction of the solid on the very thin
stratum of the liquid in contact with it. He did not, like the earlier
speculators, suppose this attraction to act in an upward direction so as
to support the fluid directly. He showed that the attraction is
everywhere normal to the surface of the solid. The direct effect of the
attraction is to increase the pressure of the stratum of the fluid in
contact with the solid, so as to make it greater than the pressure in
the interior of the fluid. The result of this pressure if unopposed is
to cause this stratum to spread itself over the surface of the solid as
a drop of water is observed to do when placed on a clean horizontal
glass plate, and this even when gravity opposes the action, as when the
drop is placed on the under surface of the plate. Hence a glass tube
plunged into water would become wet all over were it not that the
ascending liquid film carries up a quantity of other liquid which
coheres to it, so that when it has ascended to a certain height the
weight of the column balances the force by which the film spreads itself
over the glass. This explanation of the action of the solid is
equivalent to that by which Gauss afterwards supplied the defect of the
theory of Laplace, except that, not being expressed in terms of
mathematical symbols, it does not indi