(1.) Of the various productions designed by nature to supply the wants
of man, there are few which are suited to his necessities in the state
in which the earth spontaneously offers them: if we except atmospheric
air, we shall scarcely find another instance: even water, in most
cases, requires to be transported from its streams or reservoirs; and
food itself, in almost every form, requires culture and preparation.
But if, from the mere necessities of physical existence in a primitive
state, we rise to the demands of civil and social life,–to say
nothing of luxuries and refinements,–we shall find that everything
which contributes to our convenience, or ministers to our pleasure,
requires a previous and extensive expenditure of labour. In most
cases, the objects of our enjoyment derive all their excellences, not
from any qualities originally inherent in the natural substances out
of which they are formed, but from those qualities which have been
bestowed upon them by the application of human labour and human skill.
In all those changes to which the raw productions of the earth are
submitted in order to adapt them to our wants, one of the principal
agents is _motion_. Thus, for example, in the preparation of clothing
for our bodies, the various processes necessary for the culture of the
cotton require the application of moving power, first to the soil, and
subsequently to the plant from which the raw material is obtained: the
wool must afterwards be picked and cleansed, twisted into threads, and
woven into cloth. In all these processes motion is the agent: to
cleanse the wool and arrange the fibres of the cotton, the wool must
be beaten, teased, carded, and submitted to other processes, by which
all the foreign and coarser matter may be separated, and the fibres or
threads arranged evenly, side by side. The threads must then receive a
rotatory motion, by which they may be twisted into the required form;
and finally peculiar motions must be given to them in order to produce
among them that arrangement which characterises the cloth which it is
our final purpose to produce.
In a rude state of society, the motions required in the infant
manufactures are communicated by the immediate application of the
hand. Observation and reflection, however, soon suggest more easy and
effectual means of attaining these ends: the strength of animals is
first resorted to for the relief of human labour. Further reflection
and inquiry suggest still better expedients. When we look around us in
the natural world, we perceive inanimate matter undergoing various
effects in which motion plays a conspicuous part: we see the falls of
cataracts, the currents of rivers, the elevation and depression of the
waters of the ocean, the currents of the atmosphere; and the question
instantly arises, whether, without sharing our own means of
subsistence with the animals whose force we use, we may not equally,
or more effectually, derive the powers required from these various
phenomena of nature? A difficulty, however, immediately presents
itself: we require motion of a particular kind; but wind will not
blow, nor water fall as we please, nor as suits our peculiar wants,
but according to the fixed laws of nature. We want an _upward_ motion;
water falls _downwards_: we want a _circular_ motion; wind blows in a
_straight_ line. The motions, therefore, which are in actual existence
must be modified to suit our purposes: the means whereby these
modifications are produced, are called _machines_. A machine,
therefore, is an instrument interposed between some natural force or
motion, and the object to which force or motion is desired to be
transmitted. The construction of the machine is such as to modify the
natural motion which is impressed upon it, so that it may transmit to
the object to be moved that peculiar species of motion which it is
required to have. To give a very obvious example, let us suppose that
a circular or rotatory motion is required to be produced, and that the
only natural source of motion at our command is a perpendicular fall
of water: a wheel is provided, placed upon the axle destined to
receive the rotatory motion; this wheel is furnished with cavities in
its rim; the water is conducted into the cavities near the top of the
wheel on one side; and being caught by these, its weight bears down
that side of the wheel, the cavities on the opposite side being empty
and in an inverted position. As the wheel turns, the cavities on the
descending side discharge their contents as they arrive near the
lowest point, and ascend empty on the other side. Thus a load of water
is continually pressing down one side of the wheel, from which the
other side is free, and a continued motion of rotation is produced.
In every machine, therefore, there are three objects demanding
attention:–first, The power which imparts motion to it, this is
called the _prime mover_; secondly, The nature of the _machine_
itself; and thirdly, The object to which the motion is to be conveyed.
In the steam engine the first mover arises from certain phenomena
which are exhibited when heat is applied to liquids; but in the
details of the machine and in its application there are several
physical effects brought into play, which it is necessary perfectly to
understand before the nature of the machine or its mode of operation
can be rendered intelligible. We propose therefore to devote the
present chapter to the explanation and illustration of these
phenomena.
(2.) The physical effects most intimately connected with the
operations of steam engines are some of the mechanical properties of
atmospheric air. The atmosphere is the thin transparent fluid in which
we live and move, and which, by respiration, supports animal life.
This fluid is apparently so light and attenuated, that it might be at
first doubted whether it be really a body at all. It may therefore
excite some surprise when we assert, not only that it is a body, but
also that it is one of considerable _weight_. We shall be able to
prove that it presses on every _square inch_[1] of surface with a
weight of about 15lb. avoirdupois.
[Footnote 1: As we shall have frequent occasion to mention this
magnitude, it would be well that the reader should be familiar with
it. It is a _square_, each side of which is an inch. Such as A B C D,
Fig. 1.]
(3.) Take a glass tube A B (fig. 2.) more than 32 inches long, open at
one end A, and closed at the other end B, and let it be filled with
mercury (quicksilver.) Let a glass vessel or cistern C, containing a
quantity of mercury, be also provided. Applying the finger at A so as
to prevent the mercury in the tube from falling out, let the tube be
inverted, and the end, stopped by the finger, plunged into the mercury
in _C_. When the end of the tube is below the surface of the mercury
in C (fig. 3.) let the finger be removed. It will be found that the
mercury in the tube will not, as might be expected, fall to the level
of the mercury in the cistern C, which it would do were the end B open
so as to admit the air into the upper part of the tube. On the other
hand, the level D of the mercury in the tube will be about 30 inches
above the level C of the mercury in the cistern.
(4.) The cause of this effect is, that the weight of the atmosphere
rests on the surface C of the mercury in the cistern, and tends
thereby to press it up, or rather to resist its fall in the tube; and
as the fall is not assisted by the weight of the atmosphere on the
surface D (since B is closed), it follows, that as much mercury
remains suspended in the tube above the level C as the weight of the
atmosphere is able to support.
If we suppose the section of the tube to be equal to the magnitude of
a square inch, the weight of the column of mercury in the tube above
the level C will be exactly equal to the weight of the atmosphere on
each square inch of the surface C. The height of the level D above C
being about 30 inches, and a column of mercury two inches in height,
and having a base of a square inch, weighing about one pound
avoirdupois, it follows that the weight with which the atmosphere
presses on each square inch of a level surface is about 15lb.
avoirdupois.
An apparatus thus constructed, and furnished with a scale to indicate
the height of the level D above the level C, is the _common
barometer_. The difference of these levels is subject to a small
variation, which indicates a corresponding change in the atmospheric
pressure. But we take 30 inches as a standard or average.
(5.) It is an established property of fluids that they press equally
in all directions; and air, like every other fluid, participates in
this quality. Hence it follows, that since the downward pressure or
weight of the atmosphere is about 15lb. on the square inch, the
lateral, upward, and oblique pressures are of the same amount. But,
independently of the general principle, it may be satisfactory to give
experimental proof of this.
Let four glass tubes A, B, C, D, (fig. 4.) be constructed of
sufficient length, closed at one end A, B, C, D, and open at the
other. Let the open ends of three of them be bent, as represented in
the tubes B, C, D. Being previously filled with mercury, let them all
be gently inverted so as to have their closed ends up as here
represented. It will be found that the mercury will be sustained in
all,[2] and that the difference of the levels in all will be the same.
Thus the mercury is sustained in A by the upward pressure of the
atmosphere, in B by its horizontal or lateral pressure, in C by its
downward pressure, and in D by its oblique pressure; and as the
difference of the levels is the same in all, these pressures are
exactly equal.
[Footnote 2: This experiment with the tube A requires to be very
carefully executed, and the tube should be one of small bore.]
(6.) In the experiment described in (3.) the space B D (fig. 3.) at
the top of the tube from which the mercury has fallen is perfectly
void and empty, containing neither air nor any other fluid: it is
called therefore a _vacuum_. If, however, a small quantity of air be
introduced into that space, it will immediately begin to exert a
pressure on D, which will cause the surface D to descend, and it will
continue to descend until the column of mercury C D is so far
diminished that the weight of the atmosphere is sufficient to sustain
it, as well as the pressure exerted upon it by the air in the space B
D.
The quantity of mercury which falls from the tube in this case is
necessarily an equivalent for the pressure of the air introduced, so
that the pressure of this air may be exactly ascertained by allowing
about one pound per square inch for every two inches of mercury which
has fallen from the tube. The pressure of the air or any other fluid
above the mercury in the tube, may at once be ascertained by comparing
the height of the mercury in the tube with the height of the
barometer; the difference of the heights will always determine the
pressure on the surface of the mercury in the tube. This principle
will be found of some importance in considering the action of the
modern steam engines.
The air which we have supposed to be introduced into the upper part of
the tube, presses on the surface of the mercury with a force much
greater than its weight. For example, if the space B D (fig. 3.) were
filled with atmospheric air in its ordinary state, it would exert a
pressure on the surface D equal to the whole pressure of the
atmosphere, although its weight might not amount to a single grain.
The property in virtue of which the air exerts this pressure is its
_elasticity_, and this force is diminished in precisely the proportion
in which the space which the air occupies is increased.
Thus it is known that atmospheric air in its ordinary state exerts a
pressure on the surface of any vessel in which it is confined,
amounting to about 15lb. on every square inch. If the capacity of the
vessel which contains it be doubled, it immediately expands and fills
the double space, but in doing so it loses half its elastic force, and
presses only with the force of 7-1/2lb. on every square inch. If the
capacity of the vessel had been enlarged five times, the air would
still have expanded so as to fill it, but would exert only a fifth
part of its first pressure, or 3lb. on every square inch.
This property of losing its elastic force as its volume or bulk is
increased, is not peculiar to air. It is common to all elastic fluids,
and we accordingly find it in steam; and it is absolutely necessary to
take account of it in estimating the effects of that agent.
(7.) There are numerous instances of the effects of these properties
of atmospheric air which continually fall under our observation. If
the nozzle and valve-hole of a pair of bellows be stopped, it will
require a very considerable force to separate the boards. This effect
is produced by the diminished elastic force of the air remaining
between the boards upon the least increase of the space within the
bellows, while the atmosphere presses, with undiminished force, on the
external surfaces of the boards. If the boards be separated so as to
double the space within, the elastic force of the included air will be
about 7-1/2lb. on every square inch, while the pressure on the
external surfaces will be 15lb. on every square inch; consequently, it
will require as great a force to sustain the boards in such a
position, as it would to separate them if each board were forced
against the other, with a pressure of 7-1/2lb. per square inch on
their external surfaces.
When boys apply a piece of moistened leather to a stone, so as to
exclude the air from between them, the stone, though it be of
considerable weight, may be lifted by a string attached to the
leather: the cause of which is the atmospheric pressure, which keeps
the leather and the stone in close contact.
(8.) The next class of physical effects which it is necessary to
explain, are those which are produced when heat is imparted or
abstracted from bodies.
In general, when heat is imparted to a body, an enlargement of bulk
will be the immediate consequence, and at the same time the body will
become warmer to the touch. These two effects of expansion and
increase of warmth going on always together, the one has been taken as
a measure of the other; and upon this principle the common thermometer
is constructed. That instrument consists of a tube of glass,
terminated in a bulb, the magnitude of which is considerable, compared
with the bore of the tube. The bulb and part of the tube are filled
with mercury, or some other liquid. When the bulb is exposed to any
source of heat, the mercury contained in it, being warmed or increased
in temperature, is at the same time increased in bulk, or expanded or
dilated, as it is called. The bulb not having sufficient capacity to
contain the increased bulk of mercury, the liquid is forced up in the
tube, and the quantity of expansion is determined by observing the
ascent of the column in the tube.
An instrument of this kind, exposed to heat or cold, will fluctuate
accordingly, the mercury rising as the heat to which it is exposed is
increased, and falling by exposure to cold. In order, however, to
render it an accurate measure of temperature, it is necessary to
connect with it a scale by which the elevation or depression of the
mercury in the tube may be measured. Such a scale is constructed for
thermometers in this country in the following manner:–Let us suppose
the instrument immersed in a vessel of melting ice: the column of
mercury in the tube will be observed to fall to a certain point, and
there maintain its position unaltered: let that point be marked upon
the tube. Let the instrument be now transferred to a vessel of boiling
water at a time when the barometer stands at the altitude of 30
inches: the mercury in the tube will be observed to rise until it
attain a certain elevation, and will there maintain its position. It
will be found, that though the water continue to be exposed to the
action of the fire, and continue to boil, the mercury in the tube will
not continue to rise, but will maintain a fixed position: let the
point to which the mercury has risen, in this case, be likewise marked
upon the tube.
The two points, thus determined, are called the _freezing_ and the
_boiling_ points. If the distance upon the tube between these two
points be divided into 180 equal parts, each of these parts is called
a _degree_; and if this division be continued, by taking equal
divisions below the freezing point, until 32 divisions be taken, the
last division is called the _zero_, or _nought_ of the thermometer. It
is the point to which the mercury would fall, if the thermometer were
immersed in a certain mixture of snow and salt. When thermometers were
first invented, this point was taken as the zero point, from an
erroneous supposition that the temperature of such a mixture was the
lowest possible temperature.
The degrees upon the instrument thus divided are counted upwards from
the zero, and are expressed, like the degrees of a circle, by placing
a small ° over the number. Thus it will be perceived that the freezing
point is 32° of our thermometer, and the boiling-point will be found
by adding 180° to 32°; it is therefore 212°.
The temperature of a body is that elevation to which the thermometer
would rise when the mercury enclosed in it would acquire the same
temperature. Thus, if we should immerse the thermometer, and should
find that the mercury would rise to the division marked 100°, we
should then affirm that the temperature of the water was 100°.
(9.) The dilatation which attends an increase of temperature is one of
the most universal effects of heat. It varies, however, in different
bodies: it is least in solid bodies; greater in liquids; and greatest
of all in bodies in the aeriform state. Again, different solids are
differently susceptible of this expansion. Metals are the most
susceptible of it; but metals of different kinds are differently
expansible.
As an increase of temperature causes an increase of bulk, so a
diminution of temperature causes a corresponding diminution of bulk,
and the same body always has the same bulk at the same temperature.
A flaccid bladder, containing a small quantity of air, will, when
heated, become quite distended; but it will again resume its flaccid
appearance when cold. A corked bottle of fermented liquor, placed
before the fire, will burst by the effort of the air contained in it
to expand when heated.
Let the tube A B (fig. 5.) open at both ends, have one end inserted in
the neck of a vessel C D, containing a coloured liquid, with common
air above it; and let the tube be fixed so as to be air-tight in the
neck: upon heating the vessel, the warm air inclosed in the vessel C D
above the liquid will begin to expand, and will press upon the surface
of the liquid, so as to force it up in the tube A B.
In bridges and other structures, formed of iron, mechanical
provisions are introduced to prevent the fracture or strain which
would take place by the expansion and contraction which the metal must
undergo by the changes of temperature at different seasons of the
year, and even at different hours of the day.
Thus all nature, animate and inanimate, organized and unorganized, may
be considered to be incessantly breathing heat; at one moment drawing
in that principle through all its dimensions, and at another moment
dismissing it.
(10.) Change of bulk, however, is not the only nor the most striking
effect which attends the increase or diminution of the quantity of
heat in a body. In some cases, a total change of form and of
mechanical qualities is effected by it. If heat be imparted in
sufficient quantity to a solid body, that body, after a certain time,
will be converted into a liquid. And again, if heat be imparted in
sufficient quantity to this liquid, it will cease to exist in the
liquid state, and pass into the form of vapour.
By the abstraction of heat, a series of changes will be produced in
the opposite order. If from the vapour produced in this case, a
sufficient quantity of heat be taken, it will return to the liquid
state; and if again from this liquid heat be further abstracted, it
will at length resume its original solid state.
The transmission of a body from the solid to the liquid state, by the
application of heat, is called _fusion_ or _liquefaction_, and the
body is said to be fused, _liquefied_, or melted.
The reciprocal transmission from the liquid to the solid state, is
called _congelation_, or _solidification_; and the liquid is said to
be _congealed_ or _solidified_.
The transmission of a body from the liquid to the vaporous or aeriform
state, is called _vaporization_, and the liquid is said to be
_vaporized_ or _evaporated_.
The reciprocal transmission of vapour to the liquid state is called
_condensation_, and the vapour is said to be _condensed_.
We shall now examine more minutely the circumstances which attend
these remarkable and important changes in the state of body.
(11.) Let us suppose that a thermometer is imbedded in any solid body;
for example, in a mass of sulphur; and that it stands at the ordinary
temperature of 60 degrees: let the sulphur be placed in a vessel, and
exposed to the action of fire. The thermometer will now be observed
gradually to rise, and it will continue to rise until it exhibit the
temperature of 218°. Here, however, notwithstanding the continued
action of the fire upon the sulphur, the thermometer will become
stationary; proving, that notwithstanding the supply of heat received
from the fire, the sulphur has ceased to become hotter. At the moment
that the thermometer attains this stationary point, it will be
observed that the sulphur has commenced the process of fusion; and
this process will be continued, the thermometer being stationary,
until the whole mass has been liquefied. The moment the liquefaction
is complete, the thermometer will be observed again to rise, and it
will continue to rise until it attain the elevation of 570°. Here,
however, it will once more become stationary; and notwithstanding the
heat supplied to the sulphur by the fire, the liquid will cease to
become hotter: when this happens, the sulphur will boil; and if it
continue to be exposed to the fire a sufficient length of time, it
will be found that its quantity will gradually diminish, until at
length it will all disappear from the vessel which contained it. The
sulphur will, in fact, be converted into vapour.
From this process we infer, that all the heat supplied during the
processes of liquefaction and vaporization is consumed in effecting
these changes in the state of the body; and that under such
circumstances, it does not increase the temperature of the body on
which the change is produced.
These effects are general: all solid bodies would pass into the
liquid state by a sufficient application of heat; and all liquid
bodies would pass into the vaporous state by the same means. In all
cases the thermometer would be stationary during these changes, and
consequently the temperature of the body, in those periods, would be
maintained unaltered.
(12.) Solids differ from one another in the temperatures at which they
become liquid. These temperatures are called their _melting points_.
Thus the melting point of ice is 32°; that of lead 612°; that of gold
5237°.[3] The heat which is supplied to a body during the processes of
fusion or vaporization, and which does not affect the thermometer, or
increase the temperature of the body fused or vaporized, is said to
become _latent_. It can be proved to exist in the body fused or
vaporized, and may even be taken from that body. In parting with it
the body does not fall in temperature, and consequently the loss of
this heat is not indicated by the thermometer any more than its
reception. The term latent heat is merely intended to express this
fact, of the thermometer being insensible to the presence or absence
of this portion of heat, and is not intended to express any
theoretical notions concerning it.
[Footnote 3: Temperatures above 650° cannot be measured by the
mercurial thermometer. They can be inferred only with probability by
pyrometers.]
(13.) In explaining the construction and operation of the steam
engine, although it is necessary occasionally to refer to the effects
of heat upon bodies in general, yet the body, which is by far the most
important to be attended to, so far as the effects of heat upon it are
concerned, is water. This body is observed to exist in the three
different states, the solid, the liquid, and the vaporous, according
to the varying temperature to which it is exposed. All the
circumstances which have been explained in reference to metals, and
the substance sulphur in particular, will, _mutatis mutandis_, be
applicable to water. But in order perfectly to comprehend the
properties of the steam engine, it is necessary to render a more
rigorous and exact account of these phenomena, so far as they apply to
the changes produced upon water by the effects of heat.
Let us suppose a mass of ice immersed in the mixture of snow and salt
which determines the zero point of the thermometer: this mass, if
allowed to continue a sufficient length of time submerged in the
mixture, will necessarily acquire its temperature, and the thermometer
immersed in it will stand at zero. Let the ice be now withdrawn from
the mixture, still keeping the thermometer immersed in it, and let it
be exposed to the atmosphere at the ordinary temperature, say 60°. At
first the thermometer will be observed gradually and continuously to
rise until it attain the elevation of 32°; it will then become
stationary, and the ice will begin to melt: the thermometer will
continue standing at 32° until the ice shall be completely liquefied.
The liquid ice and the thermometer being contained in the same vessel,
it will be found, when the liquefaction is completed, that the
thermometer will again begin to rise, and will continue to rise until
it attain the temperature of the atmosphere, viz. 60°. Hitherto the
ice or water has received a supply of heat from the surrounding air;
but now an equilibrium of temperature having been established, no
further supply of heat can be received; and if we would investigate
the further effects of increased heat, it will be necessary to expose
the liquid to fire, or some other source of heat. But previous to
this, let us observe the time which the thermometer remains stationary
during the liquefaction of the ice: if noted by a chronometer, it
would be found to be a hundred and forty times the time during which
the water in the liquid state was elevated one degree; the inference
from which is, that in order to convert the solid ice into liquid
water, it was necessary to receive from the surrounding atmosphere one
hundred and forty times as much heat as would elevate the liquid water
one degree in temperature; or, in other words, that to liquefy a
given weight of ice requires as much heat as would raise the same
weight of water 140° in temperature: or from 32° to 172°.
The latent heat of water acquired in liquefaction is therefore 140°.
(14.) Let us now suppose that, a spirit lamp being applied to the
water already raised to 60°, the effects of a further supply of heat
be observed: the thermometer will continue to rise until it attain the
elevation of 212°, the barometer being supposed to stand at 30 inches.
The thermometer having attained this elevation will cease to rise; the
water will therefore cease to become hotter, and at the same time
bubbles of steam will be observed to be formed at the bottom of the
vessel containing the water, near the flame of the spirit lamp. These
bubbles will rise through the water, and escape at the surface,
exhibiting the phenomena of ebullition, and the water will undergo the
process of _boiling_.
During this process, the thermometer will constantly be maintained at
the same elevation of 212°; but if the time be noted, it will be found
that the water will be altogether evaporated, if the same source of
heat be continued to be applied to it six and a half times as long as
was necessary to raise it from the freezing to the boiling-point.
Thus, if the application of the lamp to water at 32°, be capable of
raising that water to 212° in one hour, the same lamp will require to
be applied to the boiling water for six hours and a half, in order to
convert the whole of it into steam. Now if the steam into which it is
thus converted were carefully preserved in a receiver, maintained at
the temperature of 212°, this steam would be found to have that
temperature, and not a greater one; but it would be found to fill a
space about 1700 times greater than the space it occupied in the
liquid state, and it would possess an elastic force equal to the
pressure of the atmosphere under which it was boiled; that is to say,
it would press the sides of the vessel which contained it with a
pressure equivalent to that of a column of mercury of 30 inches in
height; or what is the same thing, at the rate of about 15lb. on every
square inch of surface.
(15.) As the quantity of heat expended in raising the water from 32°
to 212°, is 180°; and as the quantity of heat necessary to convert the
same water into steam is six and a half times this quantity, it
follows that the quantity of heat requisite for converting a given
weight of water into steam, will be found by multiplying 180° by
5-1/2. The product of these numbers being 990°, it follows, that, to
convert a given weight of water at 212° into steam of the same
temperature, under the pressure of the atmosphere, when the barometer
stands at 30 inches, requires as much heat as would be necessary to
raise the same water 990° higher in temperature. The heat, not being
sensible to the thermometer, is latent heat; and accordingly it may be
stated, that the latent heat, necessary to convert water into steam
under this pressure is, in round numbers, 1000°.
(16.) All the effects of heat which we have just described may be
satisfactorily accounted for, by supposing that the principle of heat
imparts to the constituent atoms of bodies a force, by virtue of which
they acquire a tendency to repel each other. But in conjunction with
this, it is necessary to notice another force, which is known to exist
in nature: there is observable among the corpuscles of bodies a force,
in virtue of which they have a tendency to cohere, and collect
themselves together in solid concrete masses: this force is called the
attraction of cohesion. These two forces–the natural cohesion of the
particles, and the repulsive energy introduced by heat–are directly
opposed to one another, and the state of the body will be decided by
the predominance of the one or the other, or their mutual equilibrium.
If the natural cohesion of the constituent particles of the body
considerably predominate over the repulsive energy introduced by the
heat, then the cohesion will take effect; the particles of the body
will coalesce, the mass will become rigid and solid, and the
particles will hold together in one invariable mass, so that they
cannot drop asunder by the mere effect of their weight. In such cases,
a more or less considerable force must be applied, in order to break
the body, or to tear its parts asunder. Such is the quality which
characterises the state, which in mechanics is called the state of
solidity.
If the repulsive energy introduced by the application of heat be
equal, or nearly equal, to the natural cohesion with which the
particles of the body are endued, then the predominance of the
cohesive force may be insufficient to resist the tendency which the
particles may have to drop asunder by their weight. In such a case,
the constituent particles of the body cannot cohere in a solid mass,
but will separate by their weight, fall asunder, and drop into the
various corners, and adapt themselves to the shape of any vessel in
which the body may be contained. In fact, the body will take the
liquid form. In this state, however, it does not follow that the
cohesive principle will be altogether inoperative: it may, and does,
in some cases, exist in a perceptible degree, though insufficient to
resist the separate gravitation of the particles. The tendency which
particles of liquids have, in some cases, to collect in globules,
plainly indicates the predominance of the cohesive principle: drops of
water collected upon the window pane; drops of rain condensed in the
atmosphere; the tear which trickles on the cheek; drops of mercury,
which glide over any flat surface, and which it is difficult to
subdivide or scatter into smaller parts; are all obvious indications
of the predominance of the cohesive principle in liquids.
By the due application of heat, even this small degree of cohesion may
be conquered, and a preponderance of the opposite principle of
repulsion may be created. But another physical influence here
interposes its aid, and conspires with cohesion in resisting the
transmission of the body from the liquid to the vaporous state: this
force is no other than the pressure of the atmosphere, already
explained. This pressure has an obvious tendency to restrain the
particles of the liquid, to press them together, and to resist their
separation. The repulsive principle of the heat introduced must
therefore not only neutralize the cohesion, but must also impart to
the atoms of the liquid a sufficient elasticity or repulsive energy to
enable them to fly asunder, and assume the vaporous form in spite of
this atmospheric resistance.
Now it is clear, that if this atmospheric resistance be subject to any
variation in its intensity, from causes whether natural or artificial,
the repulsive energy necessary to be introduced by the heat, will vary
proportionally: if the atmospheric pressure be diminished, then less
heat will be necessary to vaporize the liquid. If, on the other hand,
this pressure be increased, a greater quantity of heat will be
required to impart the necessary elasticity.
(17.) From this reasoning we must expect that any cause, whether
natural or artificial, which diminishes the atmospheric pressure upon
the surface of a liquid, will cause that liquid to boil at a lower
temperature: and on the other hand, any cause which may increase the
atmospheric pressure upon the liquid, will render it necessary to
raise it to a higher temperature before it can boil.
These inferences we accordingly find supported by experience. Under a
pressure of 15lb. on the square inch, _i. e._ when the barometer is at
30 inches, water boils at the temperature of 212° of the common
thermometer. But if water at a lower temperature, suppose 180°, be
placed under the receiver of an air-pump, and, by the process of
exhaustion the atmospheric pressure be removed, or very much
diminished, the water will boil, although its temperature still remain
at 180°, as may be indicated by a thermometer placed in it.
On the other hand, if a thermometer be inserted air-tight in the lid
of a close digester containing water with common atmospheric air above
it, when the vessel is heated the air acquires an increased
elasticity; and being confined by the cover, presses, with increased
force, on the surface of the water. By observing the thermometer while
the vessel is exposed to the action of heat, it will be seen to rise
considerably above 212°, suppose to 230°, and would continue so to
rise until the strength of the vessel could no longer resist the
pressure within it.
The temperature at which water boils is commonly said to be 212°,
which is called _the boiling-point_ of the thermometer; but, strictly
speaking, this is only true when the barometer stands at 30 inches. If
it be lower, water will boil at a lower temperature, because the
atmospheric pressure is less; and if it be higher, as at 31, water
will not boil until it receives a higher temperature, the pressure
being greater.
According as the cohesive forces of the particles of liquids are more
or less active, they boil at greater or less temperatures. In general
the lighter liquids, such as _alcohol_ and _ether_, boil at lower
temperatures. These fluids, in fact, would boil by merely removing the
atmospheric pressure, as may be proved by placing them under the
receiver of an air-pump, and withdrawing the air. From this we may
conclude that these and similar substances would never exist in the
liquid state at all, but for the atmospheric pressure.
(18.) The elasticity of vapour raised from a boiling liquid, is equal
to the pressure under which it is produced. Thus, steam raised from
water at 212°, and, therefore, under a pressure of 15lb. on the square
inch, is endued with an elastic force which would exert a pressure on
the sides of any vessel which confines it, also equal to 15lb. on the
square inch. Since an increased pressure infers an increased
temperature in boiling, it follows, that where steam of a higher
pressure than the atmosphere is required, it is necessary that the
water should be boiled at a higher temperature.
(19.) We have already stated that there is a certain point at which
the temperature of a liquid will cease to rise, and that all the heat
communicated to it beyond this is consumed in the formation of vapour.
It has been ascertained, that when water boils at 212°, under a
pressure equal to 30 inches of mercury, a cubic inch of water forms a
cubic foot[4] of steam, the elastic force of which is equal to the
atmospheric pressure, and the temperature of which is 212°. Since
there are 1728 cubic inches in a cubic foot, it follows, that when
water at this temperature passes from the liquid to the vaporous
state, it is dilated into 1728 times its bulk.
[Footnote 4: The terms _cubic inch_ and _cubic foot_ are easily
explained. A common die, used in games of chance, has the figure which
is called a cube. It is a solid having twelve straight edges equal to
one another. It has six sides, each of which is square, and which are
also equal to one another. If its edges be each one inch in length, it
is called a _cubic inch_, if one foot, a _cubic foot_, if one yard, a
_cubic yard_, &c. This figure is represented in perspective, in fig.
6.]
(20.) We have seen that about 1000 degrees of heat must be
communicated to any given quantity of water at 212°, in order to
convert it into steam of the same temperature, and possessing a
pressure amounting to about 15 pounds on the square inch, and that
such steam will occupy above 1700 times the bulk of the water from
which it was raised. Now we might anticipate, that by abstracting the
heat thus employed in converting the liquid into vapour, a series of
changes would be produced exactly the reverse of those already
described; and such is found to be actually the case. Let us suppose a
vessel, the capacity of which is 1728 cubic inches, to be filled with
steam, of the temperature of 212°, and exerting a pressure of 15
pounds on the square inch; let 5-1/2 cubic inches of water, at the
temperature of 32°, be injected into this vessel, immediately the
steam will impart the heat, which it has absorbed in the process of
vaporisation to the water thus injected, and will itself resume the
liquid form. It will shrink into its primitive dimensions of one cubic
inch, and the heat which it will dismiss will be sufficient to raise
the 5-1/2 cubic inches of injected water to the temperature of 212°.
The contents of the vessel will thus be 6-1/2 cubic inches of water at
the temperature of 212°. One of these cubic inches is in fact the
steam which previously filled the vessel reconverted into water, the
other 5-1/2 are the injected water which has been raised from the
temperature of 32° to 212° by the heat which has been dismissed by the
steam in resuming the liquid state. It will be observed that in this
transmission no temperature is lost, since the cubic inch of water
into which the steam is converted has the same temperature as the
steam had before the cold water was injected.
These consequences are in perfect accordance with the results already
obtained from observing the time necessary to convert a given quantity
of water into steam by the application of heat. From the present
result it follows, that in the reduction of a given quantity of steam
to water it parts with as much heat as is sufficient to raise 5-1/2
cubic inches from 32° to 212°, that is, 5-1/2 times 180° or 990°.
(21.) There is an effect produced in this process to which it is
material that we should attend. The steam which filled the space of
1728 cubic inches shrinks when reconverted into water into the
dimensions of 1 cubic inch. It therefore leaves 1727 cubic inches of
the vessel it contains unoccupied. By this property steam is rendered
instrumental in the formation of a vacuum.
By allowing steam to circulate through a vessel, the air may be
expelled from it, and its place filled by steam. If the vessel be then
closed and cooled the steam will be reduced to water, and, falling in
drops on the bottom and sides of the vessel, the space which it filled
will become a _vacuum_. This may be easily established by experiment.
Let a long glass tube be provided with a hollow ball at one end, and
having the other end open.[5] Let a small quantity of spirits be
poured in at the open end, and placing the glass ball over the flame
of a lamp, let the spirits be boiled. After some time the steam will
be observed to issue copiously from the open end of the tube which is
presented upwards. When this takes place, let the tube be inverted,
and its open end plunged in a basin of cold water. The heat being thus
removed, the cool air will reconvert the steam in the tube into
liquid, and a vacuum will be produced, into which the pressure of the
atmosphere on the surface of the water in the basin will force the
water through the tube, and it will rush up with considerable force,
and fill the glass ball.
[Footnote 5: A common glass flask with a long neck will answer the
purpose.]
In this experiment it is better to use spirits than water, because
they boil at a lower heat, and expose the glass to less liability to
break, and also the tube may more easily be handled.
