himself saw them, having lost his sight in about 1840.

G.L. van der Mensbrugghe (_Mem. de l'Acad. Roy. de Belgique_, xxxvii.,
1873) devised a great number of beautiful illustrations of the
phenomena of surface-tension, and showed their connexion with the
experiments of Charles Tomlinson on the figures formed by oils dropped
on the clean surface of water.

Athanase Dupre in his 5th, 6th and 7th Memoirs on the Mechanical Theory
of Heat (_Ann. de Chimie et de Physique_, 1866-1868) applied the
principles of thermodynamics to capillary phenomena, and the experiments
of his son Paul were exceedingly ingenious and well devised, tracing the
influence of surface-tension in a great number of very different
circumstances, and deducing from independent methods the numerical value
of the surface-tension. The experimental evidence which Dupre obtained
bearing on the molecular structure of liquids must be very valuable,
even if our present opinions on this subject should turn out to be
erroneous.

F.H.R. Ludtge (_Pogg. Ann._ cxxxix. p. 620) experimented on liquid
films, and showed how a film of a liquid of high surface-tension is
replaced by a film of lower surface-tension. He also experimented on the
effects of the thickness of the film, and came to the conclusion that
the thinner a film is, the greater is its tension. This result, however,
was tested by Van der Mensbrugghe, who found that the tension is the
same for the same liquid whatever be the thickness, as long as the film
does not burst. [The continued coexistence of various thicknesses, as
evidenced by the colours in the same film, affords an instantaneous
proof of this conclusion.] The phenomena of very thin liquid films
deserve the most careful study, for it is in this way that we are most
likely to obtain evidence by which we may test the theories of the
molecular structure of liquids.

Sir W. Thomson (afterwards Lord Kelvin) investigated the effect of the
curvature of the surface of a liquid on the thermal equilibrium between
the liquid and the vapour in contact with it. He also calculated the
effect of surface-tension on the propagation of waves on the surface of
a liquid, and determined the minimum velocity of a wave, and the
velocity of the wind when it is just sufficient to disturb the surface
of still water.

THEORY OF CAPILLARY ACTION

When two different fluids are placed in contact, they may either diffuse
into each other or remain separate. In some cas