Water percolates through the sides and roofs of caverns and grottoes,
and being impregnated with calcareous and other earths, forms
stalactites, or pendant protuberances, which present a curious
appearance.
(23.) _Compressibility._--That quality, in virtue of which a body
allows its volume to be diminished without diminishing its mass, is
called _compressibility_. This effect is produced by bringing the
constituent particles more close together, and thereby increasing the
density and diminishing the pores. This effect may be produced in
several ways; but the name “compressibility” is only applied to it
when it is caused by the agency of mechanical force, as by pressure or
percussion.
All known bodies, whatever be their nature, are capable of having their
dimensions reduced without diminishing their mass; and this is one of
the most conclusive proofs that all bodies are porous, or that the
constituent atoms are not in contact; for the space by which the volume
may be diminished must, before the diminution, consist of pores.
(24.) _Elasticity._--Some bodies, when compressed by mechanical agency,
will resume their former dimensions with a certain energy when relieved
from the operation of the force which has compressed them. This
property is called _elasticity_; and it follows, from this definition,
that all elastic bodies must be compressible, although the converse is
not true, compressibility not necessarily implying elasticity.
(25.) _Dilatability._--This quality is the opposite of compressibility.
It is the capability observed in bodies to have their volume enlarged
without increasing their mass. This effect may be produced in several
ways. In ordinary circumstances, a body may exist under the constant
action of a pressure by which its volume and density are determined. It
may happen, that on the occasional removal of that pressure, the body
will _dilate_ by a quality inherent in its constitution. This is the
case with common air. Dilatation may also be the effect of heat, as
will presently appear.
The several qualities of bodies which we have noticed in this chapter,
when viewed in relation to each other, present many circumstances
worthy of attention.
(26.) It is a physical law, of high generality, that an increase
in the temperature, or degree of heat by which a body is affected,
is accompanied by an increase of volume; and that a diminution of
temperature is accompanied by a diminution of volume. The exceptions
to this law will be noticed and explained in our treatise on HEAT.
Hence it appears that the reduction of temperature is an effect which,
considered mechanically, is equivalent to compression or condensation,
since it diminishes the volume without altering the mass; and since
this is an effect of which all bodies whatever are susceptible, it
follows that all bodies whatever have _pores_. (23.)
The fact, that the elevation of temperature produces an increase of
volume, is manifested by numerous experiments.
(27.) If a flaccid bladder be tied at the mouth, so as to stop the
escape of air, and be then held before a fire, it will gradually swell,
and assume the appearance of being fully inflated. The small quantity
of air contained in the bladder is, in this case, so much dilated by
the heat, that it occupies a considerably increased space, and fills
the bladder, of which it before only occupied a small part. When the
bladder is removed from the fire, and allowed to resume its former
temperature, the air returns to its former dimensions, and the bladder
becomes again flaccid.
(28.) Let A B, _fig. 2._ be a glass tube, with a bulb at the
end A; and let the bulb A, and a part of the tube, be filled with any
liquid, coloured so as to be visible. Let C be the level of the liquid
in the tube. If the bulb be now exposed to heat, by being plunged in
hot water, the level of the liquid C will rapidly rise towards B. This
effect is produced by the dilatation of the liquid in the bulb, which
filling a greater space, a part of it is forced into the tube. This
experiment may easily be made with a common glass tube and a little
port wine.
Thermometers are constructed on this principle, the rise of the liquid
in the tube being used as an indication of the degree of heat which
causes it. A particular account of these useful instruments will be
found in our treatise on HEAT.
(29.) The change of dimension of solids produced by changes of
temperature being much less than that of bodies in the liquid or
aeriform state, is not so easily observable. A remarkable instance
occurs in the process of shoeing the wheels of carriages. The rim of
iron with which the wheel is to be bound, is made in the first instance
of a diameter somewhat less than that of the wheel; but being raised by
the application of fire to a very high temperature, its volume receives
such an increase, that it will be sufficient to embrace and surround
the wheel. When placed upon the wheel it is cooled, and suddenly
contracting its dimensions, binds the parts of the wheel firmly
together, and becomes securely seated in its place upon the fellies.
(30.) It frequently happens that the stopper of a glass bottle or
decanter becomes fixed in its place so firmly, that the exertion of
force sufficient to withdraw it would endanger the vessel. In this
case, if a cloth wetted with hot-water be applied to the neck of the
bottle, the glass will expand, and the neck will be enlarged, so as to
allow the stopper to be easily withdrawn.
(31.) The contraction of metal consequent upon change of temperature
was applied some time ago in Paris to restore the walls of a tottering
building to their proper position. In the _Conservatoire des Arts
et Métiers_, the walls of a part of the building were forced out of
the perpendicular by the weight of the roof, so that each wall was
leaning outwards. M. Molard conceived the notion of applying the
irresistible force with which metals contract in cooling, to draw the
walls together. Bars of iron were placed in parallel directions across
the building, and at right-angles to the direction of the walls. Being
passed through the walls, nuts were screwed on their ends, outside the
building. Every alternate bar was then heated by lamps, and the nuts
screwed close to the walls. The bars were then cooled, and the lengths
being diminished by contraction, the nuts on their extremities were
drawn together, and with them the walls were drawn through an equal
space. The same process was repeated with the intermediate bars, and
so on alternately until the walls were brought into a perpendicular
position.
(32.) Since there is a continual change of temperature in all bodies on
the surface of the globe, it follows, that there is also a continual
change of magnitude. The substances which surround us are constantly
swelling and contracting, under the vicissitudes of heat and cold. They
grow smaller in winter, and dilate in summer. They swell their bulk
on a warm day, and contract it on a cold one. These curious phenomena
are not noticed, only because our ordinary means of observation are
not sufficiently accurate to appreciate them. Nevertheless, in some
familiar instances the effect is very obvious. In warm weather the
flesh swells, the vessels appear filled, the hand is plump, and the
skin distended. In cold weather, when the body has been exposed to the
open air, the flesh appears to contract, the vessels shrink, and the
skin shrivels.
(33.) The phenomena attending change of temperature are conclusive
proofs of the universal porosity of material substances, but they are
not the only proofs. Many substances admit of compression by the mere
agency of mechanical force.
Let a small piece of cork be placed floating on the surface of water
in a basin or other vessel, and an empty glass goblet be inverted over
the cork, so that its edge just meets the water. A portion of air will
then be confined in the goblet, and detached from the remainder of
the atmosphere. If the goblet be now pressed downwards, so as to be
entirely immersed, it will be observed, that the water will not fill
it, being excluded by the _impenetrability_ of the air inclosed in it.
This experiment, therefore, is decisive of the fact, that air, one of
the most subtle and attenuated substances we know of, possesses the
quality of impenetrability. It absolutely excludes any other body from
the space which it occupies at any given moment.
But although the water does not fill the goblet, yet if the position
of the cork which floats upon its surface be noticed, it will be
found that the level of the water within has risen above its edge or
rim. In fact, the water has partially filled the goblet, and the air
has been forced to contract its dimensions. This effect is produced by
the pressure of the incumbent water forcing the surface in the goblet
against the air, which yields until it is so far compressed that it
acquires a force able to withstand this pressure. Thus it appears
that air is capable of being reduced in its dimensions by mechanical
pressure, independently of the agency of heat. It is _compressible_.
That this effect is the consequence of the pressure of the liquid will
be easily made manifest by showing that, as the pressure is increased,
the air is proportionally contracted in its dimensions; and as it is
diminished, the dimensions are on the other hand enlarged. If the
depth of the goblet in the water be increased, the cork will be seen
to rise in it, showing that the increased pressure, at the greater
depth, causes the air in the goblet to be more condensed. If, on the
other hand, the goblet be raised toward the surface, the cork will be
observed to descend toward the edge, showing that as it is relieved
from the pressure of the liquid, the air gradually approaches to its
primitive dimensions.
(34.) These phenomena also prove, that air has the property of
_elasticity_. If it were simply compressible, and not elastic, it would
retain the dimensions to which it was reduced by the pressure of the
liquid; but this is not found to be the result. As the compressing
force is diminished, so in the same proportion does the air, by its
elastic virtue, exert a force by which it resumes its former dimensions.
That it is the air alone which excludes the water from the goblet,
in the preceding experiments, can easily be proved. When the goblet
is sunk deep in the vessel of water, let it be inclined a little to
one side until its mouth is presented towards the side of the vessel;
let this inclination be so regulated, that the surface of the water
in the goblet shall just reach its edge. Upon a slight increase of
inclination, air will be observed to escape from the goblet, and to
rise in bubbles to the surface of the water. If the goblet be then
restored to its position, it will be found that the cork will rise
higher in it than before the escape of the air. The water in this case
rises and fills the space which the air allowed to escape has deserted.
The same process may be repeated until all the air has escaped, and
then the goblet will be completely filled by the water.
(35.) Liquids are compressible by mechanical force in so slight a
degree, that they are considered in all hydrostatical treatises
as incompressible fluids. They are, however, not absolutely
incompressible, but yield slightly to very intense pressure. The
question of the compressibility of liquids was raised at a remote
period in the history of science. Nearly two centuries ago, an
experiment was instituted at the Academy _del Cimento_ in Florence,
to ascertain whether water be compressible. With this view, a hollow
ball of gold was filled with the liquid, and the aperture exactly
and firmly closed. The globe was then submitted to a very severe
pressure, by which its figure was slightly changed. Now it is proved
in geometry, that a globe has this peculiar property, that any
change whatever in its figure must necessarily diminish its volume
or contents. Hence it was inferred, that if the water did not issue
through the pores of the gold, or burst the globe, its compressibility
would be established. The result of the experiment was, that the water
_did_ ooze through the pores, and covered the surface of the globe,
presenting the appearance of dew, or of steam cooled by the metal.
But this experiment was inconclusive. It is quite true, that if the
water _had not_ escaped upon the change of figure of the globe, the
_compressibility_ of the liquid would have been established. The escape
of the water does not, however, prove its _incompressibility_. To
accomplish this, it would be necessary first to measure accurately the
volume of water which transuded by compression, and next to measure
the diminution of volume which the vessel suffered by its change of
figure. If this diminution were greater than the volume of water which
escaped, it would follow that the water remaining in the globe had
been compressed, notwithstanding the escape of the remainder. But this
could never be accomplished with the delicacy and exactitude necessary
in such an experiment; and, consequently, as far as the question of
the compressibility of water was concerned, nothing was proved. It
forms, however, a very striking illustration of the porosity of so
dense a substance as gold, and proves that its pores are larger than
the elementary particles of water, since these are capable of passing
through them.
(36.) It has since been proved, that water, and other liquids, are
compressible. In the year 1761, Canton communicated to the Royal
Society the results of some experiments which proved this fact. He
provided a glass tube with a bulb, such as that described in (28), and
filled the bulb and a part of the tube with water well purified from
air. He then placed this in an apparatus called a condenser, by which
he was enabled to submit the surface of the liquid in the tube to very
intense pressure of condensed air. He found that the level of the
liquid in the tube fell in a perceptible degree upon the application of
the pressure. The same experiment established the fact, that liquids
are _elastic_; for upon removing the pressure, the liquid rose to its
original level, and therefore resumed its former dimensions.
(37.) Elasticity does not always accompany compressibility. If lead or
iron be submitted to the hammer, it may be hardened and diminished in
its volume; but it will not resume its former volume after each stroke
of the hammer.
(38.) There are some bodies which maintain the state of density in
which they are commonly found by the continual agency of mechanical
pressure; and such bodies are endued with a quality, in virtue of which
they would enlarge their dimensions without limit, if the pressure
which confines them were removed. Such bodies are called _elastic
fluids_ or _gases_, and always exist in the form of common air, in
whose mechanical properties they participate. They are hence often
called _aeriform fluids_.
Those who are provided with an air-pump can easily establish this
property experimentally. Take a flaccid bladder, such as that already
described in (27.), and place it under the glass receiver of an
air-pump: by this instrument we shall be able to remove the air which
surrounds the bladder under the receiver, so as to relieve the small
quantity of air which is inclosed in the bladder from the pressure
of the external air: when this is accomplished, the bladder will be
observed to swell, as if it were inflated, and will be perfectly
distended. The air contained in it, therefore, has a tendency to
dilate, which takes effect when it ceases to be resisted by the
pressure of surrounding air.
(39.) It has been stated that the increase or diminution of temperature
is accompanied by an increase or diminution of volume. Related to this,
there is another phenomenon too remarkable to pass unnoticed, although
this is not the proper place to dwell upon it: it is the converse of
the former; viz. that an increase or diminution of bulk is accompanied
by a diminution or increase of temperature. As the application of heat
from some foreign source produces an increase of dimensions, so if the
dimensions be increased from any other cause, a corresponding portion
of the heat which the body had before the enlargement, will be absorbed
in the process, and the temperature will be thereby diminished. In the
same way, since the abstraction of heat causes a diminution of volume,
so if that diminution be caused by any other means, the body will _give
out_ the heat which in the other case was abstracted, and will rise in
its temperature.
Numerous and well-known facts illustrate these observations. A smith by
hammering a piece of bar iron, and thereby compressing it, will render
it _red hot_. When air is violently compressed, it becomes so hot as
to ignite cotton and other substances. An ingenious instrument for
producing a light for domestic uses has been constructed, consisting
of a small cylinder, in which a solid piston moves air-tight: a little
tinder, or dry sponge, is attached to the bottom of the piston, which
is then violently forced into the cylinder: the air between the bottom
of the cylinder and the piston becomes intensely compressed, and
evolves so much heat as to light the tinder.
In all the cases where friction or percussion produces heat or fire,
it is because they are means of compression. The effects of flints, of
pieces of wood rubbed together, the warmth produced by friction on the
flesh, are all to be attributed to the same cause.
CHAP. III.
INERTIA.
(40.) The quality of matter which is of all others the most important
in mechanical investigations, is that which has been called _Inertia_.
Matter is incapable of spontaneous change. This is one of the earliest
and most universal results of human observation: it is equivalent to
stating that mere matter is deprived of life; for spontaneous action
is the only test of the presence of the living principle. If we see a
mass of matter undergo any change, we never seek for the cause of that
change in the body itself; we look for some external cause producing
it. This inability for voluntary change of state or qualities is a
more general principle than inertia. At any given moment of time a
body must be in one or other of two states, rest or motion. _Inertia_,
or _inactivity_, signifies the total absence of power to change this
state. A body endued with inertia cannot of itself, and independent of
all external influence, commence to move from a state of rest; neither
can it when moving arrest its progress and become quiescent.
(41.) The same property by which a body is unable by any power of its
own to pass from a state of rest to one of motion, or _vice versâ_,
also renders it incapable of increasing or diminishing any motion which
it may have received from an external cause. If a body be moving in a
certain direction at the rate of ten miles per hour, it cannot by any
energy of its own change its rate of motion to eleven or nine miles an
hour. This is a direct consequence of that manifestation of inertia
which has just been explained. For the same power which would cause a
body moving at ten miles an hour to increase its rate to eleven miles,
would also cause the same body at rest to commence moving at the rate
of one mile an hour; and the same power which would cause a body moving
at the rate of ten miles an hour to move at the rate of nine miles in
the hour, would cause the same body moving at the rate of one mile
an hour to become quiescent. It therefore appears, that to increase
or diminish the motion of a body is an effect of the same kind as to
change the state of rest into that of motion, or _vice versâ_.
(42.) The effects and phenomena which hourly fall under our observation
afford unnumbered examples of the inability of lifeless matter to put
itself into motion, or to increase any motion which may have been
communicated to it. But it does not happen that we have the same
direct and frequent evidence of its inability to destroy or diminish
any motion which it may have received. And hence it arises, that while
no one will deny to matter the former effect of inertia, few will at
first acknowledge the latter. Indeed, even so late as the time of
KEPLER, philosophers themselves held it as a maxim, that “matter is
more inclined to rest than to motion;” we ought not, therefore, to be
surprised if in the present day those who have not been conversant with
physical science are slow to believe that a body once put in motion
would continue for ever to move with the same velocity, if it were not
stopped by some external cause.
Reason, assisted by observation, will, however, soon dispel this
illusion. Experience shows us in various ways, that the same causes
which destroy motion in one direction are capable of producing as
much motion in the opposite direction. Thus, if a wheel, spinning on
its axis with a certain velocity, be stopped by a hand seizing one of
the spokes, the effort which accomplishes this is exactly the same as,
had the wheel been previously at rest, would have put it in motion in
the opposite direction with the same velocity. If a carriage drawn
by horses be in motion, the same exertion of power in the horses is
necessary to stop it, as would be necessary to _back_ it, if it were
at rest. Now, if this be admitted as a general principle, it must be
evident that a body which can destroy or diminish its own motion must
also be capable of putting itself into motion from a state of rest,
or of increasing any motion which it has received. But this latter is
contrary to all experience, and therefore we are compelled to admit
that a body cannot diminish or destroy any motion which it has received.
Let us enquire why we are more disposed to admit the inability of
matter to produce than to destroy motion in itself. We see most of
those motions which take place around us on the surface of the earth
subject to gradual decay, and if not renewed from time to time, at
length cease. A stone rolled along the ground, a wheel revolving on
its axis, the heaving of the deep after a storm, and all other motions
produced in bodies by external causes, decay, when the exciting
cause is suspended; and if that cause do not renew its action, they
ultimately cease.
But is there no exciting cause, on the other hand, which thus gradually
deprives those bodies of their motion?--and if that cause were
removed, or its intensity diminished, would not the motion continue,
or be more slowly retarded? When a stone is rolled along the ground,
the inequalities of its shape as well as those of the ground are
impediments, which retard and soon destroy its motion. Render the
stone round, and the ground level, and the motion will be considerably
prolonged. But still small asperities will remain on the stone, and on
the surface over which it rolls: substitute for the stone a ball of
highly-polished steel, moving on a highly-polished steel plane, truly
level, and the motion will continue without sensible diminution for
a very long period; but even here, and in every instance of motions
produced by art, minute asperities must exist on the surfaces which
move in contact with each other, which must resist, gradually diminish,
and ultimately destroy the motion.
Independently of the obstructions to the continuation of motion arising
from friction, there is another impediment to which all motions on the
surface of the earth are liable--the resistance of the air. How much
this may affect the continuation of motion appears by many familiar
effects. On a calm day carry an open umbrella with its concave side
presented in the direction in which you are moving, and a powerful
resistance will be opposed to your progress, which will increase with
every increase of the speed with which you move.