_Capillary attraction is the attraction which causes the ascent
of fluids in small tubes._ This word is derived from the Latin
_capillus_—a hair.
The tubes must be less than one-tenth of an inch in diameter in order
to produce the most satisfactory results, and tubes whose bores are
no larger than a hair present the phenomenon the most strikingly. But
though the rise of water above its natural level, is most manifest
in small tubes, it appears, in a degree, in vessels of all sizes and
shapes, by a ring of water formed around the sides with a concavity
upward.
The following are the leading _facts_ respecting capillary attraction.
(1.) _When small tubes, open at both ends, are immersed perpendicularly
in any liquid, the liquid rises in them, to a height which is inversely
as the diameter of the bore._ Though tubes of glass are usually
employed in experiments on this subject, yet tubes made of any other
material exhibit the same property. Nor does the thickness of the solid
part of the tube, or its quantity of matter, make the least difference,
the effect depending solely on the attraction of the surface, and
consequently extending only to a very small distance.
(2.) _Different fluids are raised to unequal heights by the same tube._
Thus, according to experiment, a tube which will raise water 23 inches
will raise alcohol only 9 inches.
(3.) A tube 1/100 of an inch in diameter raises water 5.3 inches; and
since the height is reciprocally as the diameter, _the product of the
diameter into the height is a constant quantity_, namely, the .053th
part of an inch square.
(4.) _Fluids rise in a similar manner between plates of glass, metal,
&c._, placed perpendicularly in the fluids, and near to one another.
If the plates are parallel, the height to which a fluid will rise, is
_inversely as the distance between the plates_; and the whole ascent is
just _half that which takes place in a tube_ of the same diameter. If
the plates be placed edge to edge, so as to form an angle, and they be
immersed in water, with the line of their intersection vertical, the
water will ascend between them in a _curve_ having its vertex at the
angle of intersection. This curve is found to have the properties and
form of the _hyperbola_.
In the adjustment of the stems of barometers and the thermometer
an allowance is made to compensate for the influence of capillary
attraction.
[Illustration: FIGS. 94-96.]
Such are the leading facts ascertained respecting capillary attraction.
Various explanations of them have been attempted but that of La Place
is most generally received. According to this high authority, the
action of the sides of the tube draws up the film of fluid nearest to
it, and that film draws along with it the film immediately below it,
and so each film drags along with it the next below, until the weight
of the volume of fluid raised exactly balances all the forces which act
upon it. The fact that the elevation of the water between the parallel
plates is exactly _half_ that in a tube of the same diameter, clearly
indicates that the force resides in the surrounding body; and the
additional fact that the thickness or quantity of that body makes no
difference, proves that the force resides in the surface, and that the
action extends only to a very small distance.
NOTE.—Several familiar examples of capillary attraction may be
added. A piece of sponge, or a lump of sugar, touching water at
its lowest corner, soon becomes moistened throughout. The wick of
a lamp lifts the oil to supply the flame, to the height of several
inches. A capillary glass tube, bent in the form of a syphon, and
having its shorter end inserted in a vessel of water, will fill
itself and deliver over the water in drops. A lock of thread or of
candle-wick, inserted in a vessel of water in a similar manner, with
one end hanging over the vessel, will exhibit the same result. An
immense weight or mass may be raised through a small space, by first
stretching a dry rope between it and a support, and then wetting the
rope.
The several figures above need no explanation to the attentive reader.