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Page 25
_Gravitation and Cohesion._--Unlike the attraction of aggregation, or
cohesion, which acts only between particles separated from each other
by spaces that are imperceptible, gravitation takes effect at distances
which transcend conception, but it diminishes in force as the distance
increases. The law according to which it does so is expressed thus; its
intensity decreases with the square of the distance; that is to say, at
twice the original distance it is 1-4th; at thrice, 1-9th; at four
times, 1-16th, for 4, 9, 16 are the squares respectively of 2, 3, and 4.
To take an instance, a ball which weighs 144 lb. at the surface of the
earth will weigh 1-4th of that, or 36 lb., when it is twice as far from
the centre as it is at the surface; and 1-9th, or 16 lb. when it is
thrice as far; and 1-16th, or 9 lb. when it is four times as far. The
attraction of cohesion, on the other hand, as we say, acts only when the
particles seem almost in contact, and it ceases altogether when once, by
mechanical or other means, the bond is broken, in consequence of the
particles being forced too near, or sundered too far from, one another.
One distinguishing difference between the attraction of gravitation and
that of cohesion is, that whereas the former is uniform, the latter is
variable; that is, under gravitation the attraction of any one particle
to any other is the same, but under cohesion, some sets of particles
are more forcibly drawn together than others. For instance, a particle
of iron and a particle of cork gravitate equally, but particles of iron
and particles of cork among themselves do not cohere equally. And it is
just because those of the former cohere more than those of the latter,
that a piece of iron feels harder and weighs heavier than a piece of
cork.
Further, the attraction of gravitation is unaffected by change in the
condition of bodies, while that of cohesion is. It makes nothing to
gravitation whether a piece of metal is as cold as ice, or heated with a
sevenfold heat. Not so to the power of cohesion; withdraw heat, and the
particles under cohesion cling closer; add it, and both the spaces grow
wider and the attraction feebler. Thus, for example, you may suspend a
weight by a piece of copper-wire, and the wire not break. But apply heat
to the wire, and its cohesion will be lessened; the force of gravitation
will overpower it, rupture the wire, and cause the weight to fall.
_Cohesion_.--That the action of the attraction of cohesion depends on
the contiguity of the particles in the cohering body, may be shown by
an illustration. Take a ball of lead, divide it into two hemispheres,
smooth the surfaces of section, then press them together, and you will
find it requires some force to separate them; thus proving the
dependence of cohesion on contiguity, although the effect in this case
may be due in some degree to the pressure of the atmosphere as well as
the power of cohesion.
Heat is the principal agent in inducing cohesion, as well as in relaxing
its energy; for by means of it you can weld the hardest as well as the
softest substances into one, and two pieces of iron together, no less
than two pieces of wax. It is possible, indeed, by heat to unite two
sufficient waxed corks to one another, so as to be able by means of the
one to draw the other out of a bottle: such, in this case, is the force
of cohesion induced by heat.
The power of cohesion exists between the particles of liquids as well as
those of solids, the only difference being that in solids the particles
are relatively fixed, while in liquids they move freely about one
another, unless indeed when they are attracted to the surface of a
solid--a fact we are familiar with when we dip our finger into a vessel
of water. The cohesive power of liquids is overcome by heat as well as
that of solids, only to a much greater degree, for under it they assume
a new form, acquire new properties, and expand immensely in volume. They
pass into the form of vapour, occupy a thousand times larger area, and
possess an elasticity of compressibility and expansibility they were
destitute of before.
There is a beautiful phenomenon which accompanies the expansion of ether
under the influence of heat. Placed in a flask to which heat is applied,
the ether will go off in vapour; and as the heat increases, the vapour
will gradually light up into a lovely flame. The expansibility of air,
which is vapour in a permanent form, can be shown by experiment. If we
tie up an empty or collapsed bladder, and place it in a vessel over an
air-pump, we may see, as we withdraw the air from the vessel, and so
diminish its pressure, the bladder gradually expand and swell as it does
under inflation.
The cohesive power of water is beautifully illustrated. Have a small
barrel or bucket so constructed as to be fitted with gauze at the top;
immerse it exactly, so that the water may form a film between the
meshes, and then open the tap at the bottom: the water will not flow
till the meshes at the top are broken by blowing on their surface. The
adhesion of the particles in a soap-bubble is another illustration, no
less beautiful, as well as more familiar; for the soap, which might be
supposed to be the cause of the phenomenon, serves only to prevent the
intrusion of dust between the particles, but by no means to intensify
their attractive power.
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