of the liquid, and is independent
of the form of the lower part of the tube. He considered that the
suspension of the liquid is due to "the attraction of the periphery or
section of the surface of the tube to which the upper surface of the
water is contiguous and coheres." From this he showed that the rise of
the liquid in tubes of the same substance is inversely proportional to
their radii. Sir Isaac Newton devoted the 31st query in the last edition
of his _Opticks_ to molecular forces, and instanced several examples of
the cohesion of liquids, such as the suspension of mercury in a
barometer tube at more than double the height at which it usually
stands. This arises from its adhesion to the tube, and the upper part of
the mercury sustains a considerable tension, or negative pressure,
without the separation of its parts. He considered the capillary
phenomena to be of the same kind, but his explanation is not
sufficiently explicit with respect to the nature and the limits of the
action of the attractive force.

It is to be observed that, while these early speculators ascribe the
phenomena to attraction, they do not distinctly assert that this
attraction is sensible only at insensible distances, and that for all
distances which we can directly measure the force is altogether
insensible. The idea of such forces, however, had been distinctly formed
by Newton, who gave the first example of the calculation of the effect
of such forces in his theorem on the alteration of the path of a
light-corpuscle when it enters or leaves a dense body.

Alexis Claude Clairault (_Theorie de la figure de la terre_, Paris,
1808, pp. 105, 128) appears to have been the first to show the necessity
of taking account of the attraction between the parts of the fluid
itself in order to explain the phenomena. He did not, however, recognize
the fact that the distance at which the attraction is sensible is not
only small but altogether insensible. J.A. von Segner (_Comment. Soc.
Reg. Gotting_, i. (1751) p. 301) introduced the very important idea of
the surface-tension of liquids, which he ascribed to attractive forces,
the sphere of whose action is so small "ut nullo adhuc sensu percipi
potuerit." In attempting to calculate the effect of this surface-tension
in determining the form of a drop of the liquid, Segner took account of
the curvature of a meridian section of the drop, but neglected the
effect of the curvature in a plane at right angles to thi