ADHESIVE ATTRACTION.
The term cohesion must not be confounded with that of adhesion, which
refers to the clinging to or attraction of bodies of a dissimilar kind.
The late Professor Daniell defines cohesion to be an attraction of
homogeneous ([Greek: _homos_], like, and [Greek: _genos_], kind) or
similar particles; adhesion to be an attraction subsisting between
particles of a heterogeneous, [Greek: _eteros_], different, and [Greek:
_genos_], kind.
There are numerous illustrations of adhesion, such as mending china, and
the use of glue, or paste, in uniting different surfaces, or mortar, in
building with bricks; it is also well shown at the lecture table by
means of a pair of scales, one scale-pan of which being well cleaned
with alkali at the bottom, may then be rested on the surface of water
contained in a plate; the adhesion between the water and the metal is so
perfect, that many grain weights may be placed in the other pan before
the adhesion is broken; and after breakage, if the pan be again placed
on the water, and a few grains removed from the other, so as to adjust
the two pans, and make them nearly equal, a drop of oil of turpentine
being added, instantly spreads itself over the water, and breaking the
adhesion between the latter and the metal, the scale-pan is immediately
and again broken away, as the adhesion between the turpentine and the
metal is not so great as that of water and metal. The adhesion of air
and water is well displayed in an apparatus recommended for ventilating
mines, in which a constant descending stream of water carries with it a
quantity of air, which being disengaged, is then forced out of a proper
orifice. The same kind of adhesion between air and water is displayed in
the ancient [Page 68] Spanish Catalan forge, where the blast is
supplied to the iron furnace on a similar principle, only, a natural
cascade is taken advantage of instead of an artificial fall of water
through a pipe.
The adhesion of air and water becomes of some value when a river flows
through a large and crowded city, because the water in its passage to
and fro, must necessarily drag with it, a continuous column of air, and
assist in maintaining that constant agitation of the air which is
desirable as a preventive to any accumulation of noxious air charged
with foetid odours, arising from mud banks or from other causes. The
fact of adhesion, existing between water and air, is readily shown, by
resting one end of a long glass tube, of at least one inch diameter, on
a block of wood one foot high. If water is allowed to flow down the
tube, so as to leave a sufficient space of air above it, the adhesion
between the two ancient elements becomes apparent, directly a little
smoke is produced, near the top end of the glass tube resting on the
block of wood. The smoke, which has a greater tendency to rise than to
fall, is dragged down the glass tube, and accompanies the water as it
flows from the higher to the lower level. The same truth is also
illustrated in horizontal troughs or tubes through which water is caused
to flow.
The adhesion between air and glass is so great, that it is absolutely
necessary to boil the mercury in the tubes of the best barometers; and
if this is not carefully attended to, the adhering air between the glass
and mercury gradually ascends to, and destroys, the Torricellian vacuum
at the top of the barometer tube. Even after the mercury is boiled, the
air will creep up in course of years; and in order to prevent its
passage between the glass and quicksilver, it has been recommended, that
a platinum ring should be welded on to the end of the glass tube,
because mercury has the power of wetting or enfilming the metal
platinum, and the two being in close contact, would, as it were, shut
the only door by which the air could enter the barometer tube.
[Illustration: Fig. 81. Model of the apparatus for drawing down air. A,
cistern of water, supplied by ball-cock, and kept at one level, so that
the water just runs down the sides of the tube, and draws down the air
in the centre, B C. The vessel to which the air and water are conveyed
by a gutta-percha tube, T. There is another ball-cock to permit the
waste water to run away when it reaches a certain level; the end of the
pipe always dips some inches into this water, whilst the air escapes
from the jet, D.]
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