the barometer at the time having the height of thirty inches. It
will be found that so long as the water is kept boiling, the
column of mercury in the tube will remain stationary. Let the
point corresponding with the top of the column be marked on the
tube, or on the scale attached to it. This is called the _boiling
point_. Let the space on the scale between the freezing and
boiling points be now divided into 180 equal parts: each of these
parts is called a _degree_. Let the same divisions be continued
upon the scale below the freezing point, until thirty-two
divisions be taken; let the lowest division be then marked 0, and
let the successive divisions upwards from that be numbered 1, 2,
3, &c. In like manner, let the same divisions be continued above
the boiling point, as far as the tube will admit.
It is evident that, under these circumstances, the freezing point
will be marked by 32, and the boiling point by 212. It is usual to
express the degrees of a thermometer in the same manner as the
degrees of a circle, by placing a small ° above the number. Thus
the freezing point is expressed by 32°, and the boiling point by
212°.
The reason the degrees were commenced at 32° below the freezing
point was, because, when the thermometer was invented, that
temperature was supposed to be the lowest degree of cold possible,
being that of a certain mixture of [Pg100] snow and salt. This,
however, has since been found to be an error, very much lower
temperatures being obtained by various physical expedients.
The temperature of a body is, then, that elevation to which the
thermometer would rise when immersed in that body. Thus, if in
plunging the thermometer in water we found the mercury to rise or
fall to the division marked 100, we should then say, the
temperature of the water was 100°.
Let us suppose a spirit lamp, or other regular source of heat,
applied to a bath of mercury, so as to maintain the mercury at a
fixed temperature of 200°, and let another vessel, containing a
quantity of ice at a temperature of 20° be immersed in the
mercury. Let a thermometer be placed in the mercury, and another
in the ice. The following effects will then ensue. The thermometer
immersed in the ice will be observed gradually to rise from 20°
upwards, until it indicates the temperature of 32°. It will then
become stationary, and the ice which had hitherto remained in a
solid state will begin to melt and be converted into water. This
process of liquefaction will continue for a considerable time,
during which the thermometer immersed in the ice will constantly
be maintained at 32°. At the moment, however, when the last
portion of ice is liquefied, the thermometer will begin again to
rise. The coincidence of this ascent of the thermometer with the
completion of the liquefaction of the ice, may be very easily
observed, because the ice being lighter, bulk for bulk, than
water, will float on the surface, and so long as a particle of it
remains unmelted it will be distinctly seen.
Now it cannot be doubted that, during the whole of this process,
the mercury, supposed to be maintained at 200°, constantly imparts
heat to the ice; yet, from the moment the liquefaction begins,
until it is completed, no increased temperature is exhibited by
the thermometer immersed in the melting ice. If during this part
of the process no heat were received by the ice from the mercury,
the consequence would be, that the application of the lamp would
cause the temperature of the mercury to rise above 200°, which may
be easily demonstrated by withdrawing the vessel of ice from the
mercurial bath during the process of liquefaction. The moment
[Pg101] it is withdrawn, the thermometer immersed in the mercury,
instead of remaining fixed at 200°, will begin to rise, although
the action of the lamp remains the same as before; from which it
is evident that the heat which now causes the mercury to rise
above 200° was before received by the melting ice.
The heat which thus enters ice in the process of liquefaction, and
which is not indicated by the thermometer, is for this reason
called _latent heat_. It will be perceived that this phrase is the
name of a fact, and not of an hypothesis. That heat really enters
the water, and is contained in it, has been established by the
experiments; and to declare that it is present there, is to
declare an established fact. To call it by the name _latent_ heat,
is to declare another established fact, viz., that it is not
sensible to the thermometer.
These facts show us that heat is capable of existing in bodies in
two distinct states, in one of which it is sensible to the
thermometer, and in the other not. Heat which is sensible to the
thermometer is called, for distinction, _sensible_ or _free heat_.
It may be here observed, that heat which is sensible to the
thermometer is also perceptible by the senses, and heat not
sensible to the thermometer is not perceptible by the senses.
Thus, ice at 32° and water at 32° _feel_ equally cold, and yet we
have seen that the latter contains considerably more heat than the
former.
Dr. Black, who first noticed the remarkable fact to which we have
now alluded, inferred that ice is converted into water by
communicating to it a certain quantity or dose of heat, which
enters into combination with it in a manner analogous to that
which takes place when bodies combine chemically. The heat, thus
combined with the solid ice, loses its property of affecting the
senses or the thermometer, and the effects therefore bear a
resemblance to those cases of chemical combination in which the
constituent elements change their sensible properties when they
form the compound.
The fact that the thermometer immersed in the ice remains stationary
only as long as the process of liquefaction is going on, shows that
this absorption of heat is necessarily connected with that process,
and that, were it not for the conversion of [Pg102] the solid ice
into liquid water, the heat which is so received would be sensible,
and would cause the thermometer immersed in the ice to rise. Before
the time of Black it was supposed that the slightest addition of
heat would cause solid ice to be converted into water, and that the
thermometer would immediately pass from the freezing temperature to
higher degrees. The experiments above described, however, show the
falsehood of such a supposition. If, while the mercurial bath, in
which the ice is immersed, is maintained at the temperature of 200°,
the length of time necessary to complete the liquefaction of the ice
be observed, it would be found that that time is about twenty-eight
times the length of time which it would take to raise the liquid
water from 32° to 37°; and if it be assumed that the same quantity
of heat is imparted to the ice, during the process of liquefaction,
during each minute, as is imparted to the water, during each minute,
in rising from 32° to 37°, it will follow, that to liquefy the ice
requires twenty-eight times as much heat as is necessary to raise
the water from 32° to 37°. It appears, therefore, that, instead of a
small quantity of heat being necessary to melt the ice, a very
considerable portion is absorbed in that process.
Having ascertained the remarkable fact, that heat is absorbed in a
large quantity in the conversion of ice into water, without
rendering the body so absorbing it warmer, let us now inquire what
the exact quantity of heat so absorbed is. We have already stated
that, if the quantity communicated in equal times be the same, the
heat necessary to liquefy a given weight of ice would be
twenty-eight times as much as would be necessary to raise the same
weight of water from 32° to 37°; or, if the heat necessary to
raise water through every 5° be the same, that quantity of heat
would be sufficient to raise water from 32° to 172°: and hence we
infer, that as much heat is absorbed in the liquefaction of a
given quantity of ice as would raise the same quantity of water
through 140 degrees of the thermometric scale.
(52.) Let us now examine the analogous effects produced by the
continued application of heat to water in the liquid state.
Let a small quantity of water be placed in a glass flask of
considerable size, and then closed so as to prevent the escape
[Pg103] of any vapour. Let this vessel be now placed over the
flame of a spirit lamp, so as to cause the water it contains to
boil. For a considerable time the water will be observed to boil,
and apparently to diminish in quantity, until at length all the
water disappears, and the vessel is apparently empty. If the
vessel be now removed from the lamp, and suspended in a cool
atmosphere, the whole of the interior of its surface will
presently appear to be covered with a dewy moisture; and at length
a quantity of water will collect in the bottom of it, equal to
that which had been in it at the commencement of the process. That
no water has at any period of the experiment escaped from it, may
be easily determined, by performing the experiment with the glass
flask suspended from the arm of a balance, counterpoised by a
sufficient weight suspended from the other arm. The equilibrium
will be preserved throughout, and the vessel will be found to have
the same weight, when to all appearance it is empty, as when it
contains the liquid water. It is evident, therefore, that the
water exists in the vessel in every stage of the process, but that
it becomes invisible when the process of boiling has continued for
a certain length of time; and it may be shown that it will
continue to be invisible, provided the flask be exposed to a
temperature considerably elevated. Thus, for example, if it be
suspended in a vessel of boiling water, the water which it
contains will continue to be invisible; but the moment it is
withdrawn from the boiling water, and exposed to the cold air, the
water will again become visible, as above mentioned, forming a dew
on the inner surface, and finally collecting in the bottom, as in
the commencement of the experiment.
In fact, the liquid has, by the process of boiling, been converted
into _vapour_, or _steam_, which is a body similar in its leading
properties to common air, and, like it, is invisible. It will
hereafter appear that it likewise possesses the property of
elasticity, and other mechanical qualities enjoyed by gases in
general.
(53.) Again, let an open vessel be filled with water at 60°, and
placed in a mercurial bath, which is maintained, by a fire or lamp
applied to it, at the temperature of 230°. Place a thermometer in
the water, and it will be observed gradually to [Pg104] rise as
the temperature of the water is increased by the heat which it
receives from the mercury in which it is immersed. The water will
steadily rise in this manner until it attains the temperature of
212°; but here the thermometer immersed in it will become
stationary. At the same time the water contained in the vessel
will become agitated, and its surface will present the same
appearance as if bubbles of air were rising from the bottom, and
issuing at the top. A cloudy vapour will be given off in large
quantities from its surface. This process is called _ebullition_
or _boiling_. If it be continued for any considerable time, the
quantity of water in the vessel will be sensibly diminished; and
at length every particle of it will disappear, and the vessel will
remain empty. During the whole of this process, the thermometer
immersed in the water will remain stationary at 212°.
Now, it will be asked, what has become of the water? It cannot be
imagined that it has been annihilated. We shall be able to answer
this by adopting means to prevent the escape of any particle of
matter from the vessel containing the water, into the atmosphere
or elsewhere. Let us suppose that the top of the vessel containing
the water is closed, with the exception of a neck communicating
with a tube, and let that tube be carried into another close
vessel removed from the cistern of heated mercury, and plunged in
another cistern of cold water. Such an apparatus is represented in
_fig._ 15.
[Illustration: _Fig._ 15.]
A is a cistern of heated mercury, in which the glass vessel B,
containing water, is immersed. From the top of the vessel B
proceeds a glass tube C, inclining downwards, and entering a glass
vessel D, which is immersed in a cistern E of cold water. If the
process already described be continued until the water by constant
ebullition has disappeared, as already mentioned, [Pg105] from
the vessel B, it will be found that a quantity of water will be
collected in the vessel D; and if this water be weighed, it will
be found to have exactly the same weight as the water had which
was originally placed in the vessel B. It is, therefore, quite
apparent that the water has passed by the process of boiling from
the one vessel to the other; but, in its passage, it was not
perceptible by the sight. The tube C and the upper part of the
vessel B, had the same appearance, exactly, as if they had been
filled with atmospheric air. That they are not merely filled with
atmospheric air may, however, be easily proved. When the process
of boiling first commences, it will be found that the tube C is
cold, and the inner surface dry. When the process of ebullition
has continued a short time, the tube C will become gradually
heated, and the inner surface of it covered with moisture. After a
time, however, this moisture disappears, and the tube attains the
temperature 212°. In this state it continues until the whole of
the water is discharged from the vessel B to the vessel D.
(54.) These effects are easily explained. The water in the vessel
B is incapable of receiving any higher temperature than 212°,
consistently with its retaining the liquid form. Small portions,
therefore, are constantly converted into steam by the heat
received from the surrounding mercury, and bubbles of steam are
formed on the bottom and sides of the vessel B. These bubbles,
being very much lighter, bulk for bulk, than water, rise rapidly
through the water, just in the same manner as bubbles of air
would, and produce that peculiar agitation at its surface which
has been taken as the external indication of boiling. They escape
from the surface, and collect in the upper part of the vessel. The
steam thus collected, when it first enters the tube C, is cooled
below the temperature of 212° by the surface of the tube; and
consequently, being incapable of remaining in the state of vapour
at any lower temperature than 212°, it is reconverted into water,
and forms the dewy moisture which is observed in the commencement
of the process on the interior of the tube C. At length, however,
the whole of the tube C is heated to the temperature of 212°, and
the moisture which was previously collected upon its inner
[Pg106] surface is again converted into steam. As the quantity of
steam evolved from the water in B increases, it drives before it
the steam previously collected in the tube C, and forces it into
the vessel B. Here it encounters the inner surface of this vessel,
which is kept constantly cold by being surrounded with the cold
water in which it is immersed; and the vapour, being thus
immediately reduced below the temperature of 212°, is reconverted
into water. At first it collects in a dew on the surface of the
vessel D; but as this accumulates, it drops into the bottom of the
vessel, and forms a more considerable quantity. As the quantity of
water is observed to be gradually diminished in the vessel B, the
quantity will be found to be gradually increased in the vessel D;
and if the operation be suspended at any stage of the process, and
the water in the two vessels weighed, it will be found that the
weight of the water in D is exactly equal to the weight which the
water in B has lost.
(55.) The demonstration is, therefore, perfect, that the gradual
diminution of the boiling water in the vessel B is produced by the
conversion of that water into steam by the heat. In the process
first described, when the top of the vessel B was supposed to be
open, this steam made its escape into the air, where it was first
dispersed, and subsequently cooled in separate particles, and was
deposited in minute globules of moisture on the ground and on
surrounding objects.
(56.) In reviewing this process, we are struck by the fact, that
the continued application of heat to the vessel B is incapable of
raising the temperature of the water contained in it above 212°.
This presents an obvious analogy to the process of liquefaction,
and leads to inquiries of a similar nature, which are attended
with a like result. We must either infer, that the water, having
arrived at 212°, received no more heat from the mercury; or that
such heat, if received, is incapable of affecting the thermometer;
or, finally, that the steam which passes off carries this heat
with it. That the water receive heat from the mercury, will be
proved by the fact, that, if the vessel B be removed from the
mercury, other things remaining as before, the temperature of the
mercury will rapidly rise, and if the fire be continued, it will
even boil; but so long as the [Pg107] vessel B remains immersed,
it prevents the mercury from increasing in temperature. It
therefore receives that heat which would otherwise raise the
temperature of the quicksilver.
[Illustration: _Fig._ 16.]
If a thermometer be immersed in the steam which collects in the
upper part of the vessel B, it will show the same temperature (of
212°) as the water from which it is raised. The heat, therefore,
received from the mercury, is clearly not imparted in a sensible
form to the steam, which has the same temperature in the form of
steam as it had in the form of water. What has been already
explained respecting liquefaction would lead us, by analogy, to
suspect that the heat imparted by the mercury to the water has
become latent in the steam, and is instrumental to the conversion
of water into steam, in the same manner as heat has been shown to
be instrumental to the conversion of ice into water. As the fact
was in that case detected by mixing ice with water, so we shall,
in the present instance, try it by a like test, viz. by mixing
water with steam. Let about five ounces and a half of water, at
the temperature of 32°, be placed in a vessel A (_fig._ 16.), and
let another vessel B, in which water is kept constantly boiling at
the temperature of 212°, communicate with A by a pipe C proceeding
from the top, so that the steam may be conducted from B, and
escape from the mouth of the pipe at some depth below the surface
of the water in A. As the steam issues from the pipe, it will be
immediately reconverted into water by the cold water which it
encounters; and, by continuing this process, the water in A will
be gradually heated by the steam combined with it and received
through the pipe C. If this process be continued until the water
in A is raised to the temperature of 212°, it will boil. Let it
then be weighed, and it will be found to weigh six ounces and a
half: from whence we infer, that one ounce of water has been
received from the vessel B in the form of steam, and has been
reconverted into water by the inferior temperature of the water in
A. Now, this ounce of water received in the form of steam into the
vessel A had, when in that form, the temperature of 212°. It is
now [Pg108] converted into the liquid form, and still retains the
same temperature of 212°; but it has caused the five ounces and a
half of water with which it has been mixed, to rise from the
temperature of 32° to the temperature of 212°,—and this, _without
losing any temperature itself_. It follows, therefore, that, in
returning to the liquid state, it has parted with as much heat as
is capable of raising five times and a half its own weight of
water from 32° to 212°. This heat was combined with the steam,
though not sensible to the thermometer; and was, therefore,
_latent_. Had it been sensible in the water in B, it would have
caused the water to have risen through a number of thermometric
degrees, amounting to five times and a half the excess of 212°
above 32°; that is, through five times and a half 180°; for it has
caused five times and a half its own weight of water to receive an
equal increase of temperature. But five times and a half 180° is
990°, or, to use round numbers (for minute accuracy is not here
our object), 1000°. It follows, therefore, that an ounce of water,
in passing from the liquid state at 212° to the state of steam at
212°, receives as much heat as would be sufficient to raise it
through 1000 thermometric degrees, if that heat, instead of
becoming latent, had been sensible.
(57.) In order to derive all the knowledge from these experiments
which they are capable of imparting, it will be necessary to
examine very carefully how water comports itself under a variety
of different circumstances.
If water be boiled in an open vessel, with a thermometer immersed,
on different days, it will be observed that the fixed temperature
which it assumes in boiling will be subject to a variation within
certain small limits. Thus, at one time, it will be found to boil
at the temperature of 210°; while, at others, the thermometer
immersed in it will rise to 213°; and, on different occasions, it
will fix itself at different points within these limits. It will
also be found, if the same experiment be performed at the same
time in distant places, that the boiling points will be subject to
a like variation. Now, it is natural to inquire what cause
produces this variation; and we shall be led to the discovery of
the cause, by examining what other physical effects undergo a
simultaneous change. [Pg109]
If we observe the height of the barometer at the time of making
each experiment, we shall find a very remarkable correspondence
between it and the boiling temperature. Invariably, whenever the
barometer stands at the same height, the boiling temperature will
be the same. Thus, if the barometer stands at 30 inches, the
boiling temperature will be 212°. If the barometer fall to 29-1/2
inches, the thermometer stands at a small fraction above 211°. If
the barometer rise to 30-1/2 inches, the boiling temperature rises
to nearly 213°. The variation in the boiling temperature is, then,
accompanied by a variation in the pressure of the atmosphere
indicated by the barometer; and it is constantly found that the
boiling point will remain unchanged, so long as the atmospheric
pressure remains unchanged, and that every increase in the one
causes a corresponding increase in the other.
(58.) From these facts it must be inferred, that the pressure
excited on the surface of the water has a tendency to resist its
ebullition, and to make it necessary, before it can boil, that it
should receive a higher temperature; and, on the contrary, that
every diminution of pressure on the surface of the water will give
an increased facility to the process of ebullition, or will cause
that process to take place at a lower temperature. As these facts
are of the utmost importance in the theory of heat, it may be
useful to verify them by direct experiment.
If the variable pressure excited on the surface of the water by
the atmosphere be the cause of the change in the boiling
temperature, it must happen, that any change of pressure produced
by artificial means on the surface of the water must likewise
change the boiling point, according to the same law. Thus, if a
pressure considerably greater than the atmospheric pressure be
excited on a liquid, the boiling point may be expected to rise
considerably above 212°; and, on the other hand, if the surface of
the water be relieved from the pressure of the atmosphere, and be
submitted to a considerably diminished pressure, the water would
boil below 212°.
[Illustration: _Fig._ 17.]