caloric no longer becomes latent; and therefore the heat which the water
now receives raises its temperature, as you find the thermometer
indicates.
EMILY.
But I do not think that the thermometer rises so quickly in the water as
it did in the ice, previous to its beginning to melt, though the lamp
burns equally well?
MRS. B.
That is owing to the different specific heat of ice and water. The
capacity of water for caloric being greater than that of ice, more heat
is required to raise its temperature, and therefore the thermometer
rises slower in the water than in the ice.
EMILY.
True; you said that a solid body always increased its capacity for heat
by becoming fluid; and this is an instance of it.
MRS. B.
Yes, and the latent heat is that which is absorbed in consequence of the
greater capacity which the water has for heat, in comparison to ice.
I must now tell you a curious calculation founded on that consideration.
I have before observed to you that though the thermometer shows us the
comparative warmth of bodies, and enables us to determine the same point
at different times and places, it gives us no idea of the absolute
quantity of heat in any body. We cannot tell how low it ought to fall by
the privation of all heat, but an attempt has been made to infer it in
the following manner. It has been found by experiment, that the capacity
of water for heat, when compared with that of ice, is as 10 to 9, so
that, at the same temperature, ice contains one tenth of caloric less
than water. By experiment also it is observed, that in order to melt
ice, there must be added to it as much heat, as would, if it did not
melt it, raise its temperature 140 degrees. This quantity of heat is
therefore absorbed when the ice, by being converted into water, is made
to contain one-ninth more caloric than it did before. Therefore 140
degrees is a ninth part of the heat contained in ice at 30 degrees; and
the point of zero, or the absolute privation of heat, must consequently
be 1260 degrees below 32 degrees.
This mode of investigating so curious a question is ingenious, but its
correctness is not yet established by similar calculations for other
bodies. The points of absolute cold, indicated by this method in various
bodies, are very remote from each other; it is however possible, that
this may arise from some imperfection in the experiments.
CAROLINE.
It is indeed very ingenious--but we must now attend to our present
experiment. The water begins to boil, and the thermometer is again
stationary.
MRS. B.
Well, Caroline, it is your turn to explain the phenomenon.
CAROLINE.
It is wonderfully curious! The caloric is now busy in changing the water
into steam, in which it hides itself, and becomes insensible. This is
another example of latent heat, producing a change of form. At first it
converted a solid body into a liquid, and now it turns the liquid into
vapour!
MRS. B.
You see, my dear, how easily you have become acquainted with these
modifications of insensible heat, which at first appeared so
unintelligible. If, now, we were to reverse these changes, and condense
the vapour into water, and the water into ice, the latent heat would
re-appear entirely, in the form of free caloric.
EMILY.
Pray do let us see the effect of latent heat returning to its free
state.
MRS. B.
For the purpose of showing this, we need simply conduct the vapour
through this tube into this vessel of cold water, where it will part
with its latent heat and return to its liquid form.
EMILY.
How rapidly the steam heats the water!
MRS. B.
That is because it does not merely impart its free caloric to the water,
but likewise its latent heat. This method of heating liquids, has been
turned to advantage, in several economical establishments. The
steam-kitchens, which are getting into such general use, are upon the
same principle. The steam is conveyed through a pipe in a similar
manner, into the several vessels which contain the provisions to be
dressed, where it communicates to them its latent caloric, and returns
to the state of water. Count Rumford makes great use of this principle
in many of his fire-places: his grand maxim is to avoid all unnecessary
waste of caloric, for which purpose he confines the heat in such a
manner, that not a particle of it shall unnecessarily escape; and while
he economises the free caloric, he takes care also to turn the latent
heat to advantage. It is thus that he is enabled to produce a degree of
heat superior to that which is obtained in common fire-places, though he
employs less fuel.
EMILY.
When the advantages of such contrivances are so clear and plain,
I cannot understand why they are not universally used.
MRS. B.
A long time is always required before innovations, however useful, can
be reconciled with the prejudices of the vulgar.
EMILY.
What a pity it is that there should be a prejudice against new
inventions; how much more rapidly the world would improve, if such
useful discoveries were immediately and universally adopted!
MRS. B.
I believe, my dear, that there are as many novelties attempted to be
introduced, the adoption of which would be prejudicial to society, as
there are of those which would be beneficial to it. The well-informed,
though by no means exempt from error, have an unquestionable advantage
over the illiterate, in judging what is likely or not to prove
serviceable; and therefore we find the former more ready to adopt such
discoveries as promise to be really advantageous, than the latter, who
having no other test of the value of a novelty but time and experience,
at first oppose its introduction. The well-informed, however, are
frequently disappointed in their most sanguine expectations, and the
prejudices of the vulgar, though they often retard the progress of
knowledge, yet sometimes, it must be admitted, prevent the propagation
of error. --But we are deviating from our subject.
We have converted steam into water, and are now to change water into
ice, in order to render the latent heat sensible, as it escapes from the
water on its becoming solid. For this purpose we must produce a degree
of cold that will make water freeze.
CAROLINE.
That must be very difficult to accomplish in this warm room.
MRS. B.
Not so much as you think. There are certain chemical mixtures which
produce a rapid change from the solid to the fluid state, or the
reverse, in the substances combined, in consequence of which change
latent heat is either extricated or absorbed.
EMILY.
I do not quite understand you.
MRS. B.
This snow and salt, which you see me mix together, are melting rapidly;
heat, therefore, must be absorbed by the mixture, and cold produced.
CAROLINE.
It feels even colder than ice, and yet the snow is melted. This is very
extraordinary.
MRS. B.
The cause of the intense cold of the mixture is to be attributed to the
change from a solid to a fluid state. The union of the snow and salt
produces a new arrangement of their particles, in consequence of which
they become liquid; and the quantity of caloric, required to effect this
change, is seized upon by the mixture wherever it can be obtained. This
eagerness of the mixture for caloric, during its liquefaction, is such,
that it converts part of its own free caloric into latent heat, and it
is thus that its temperature is lowered.
EMILY.
Whatever you put in this mixture, therefore, would freeze?
MRS. B.
Yes; at least any fluid that is susceptible of freezing at that
temperature. I have prepared this mixture of salt and snow for the
purpose of freezing the water from which you are desirous of seeing the
latent heat escape. I have put a thermometer in the glass of water that
is to be frozen, in order that you may see how it cools.
CAROLINE.
The thermometer descends, but the heat which the water is now losing, is
its _free_, not its _latent_ heat.
MRS. B.
Certainly; it does not part with its latent heat till it changes its
state and is converted into ice.
EMILY.
But here is a very extraordinary circumstance! The thermometer is fallen
below the freezing point, and yet the water is not frozen.
MRS. B.
That is always the case previous to the freezing of water when it is in
a state of rest. Now it begins to congeal, and you may observe that the
thermometer again rises to the freezing point.
CAROLINE.
It appears to me very strange that the thermometer should rise the very
moment that the water freezes; for it seems to imply that the water was
colder before it froze than when in the act of freezing.
MRS. B.
It is so; and after our long dissertation on this circumstance, I did
not think it would appear so surprising to you. Reflect a little, and I
think you will discover the reason of it.
CAROLINE.
It must be, no doubt, the extrications of latent heat, at the instant
the water freezes, that raises the temperature.
MRS. B.
Certainly; and if you now examine the thermometer, you will find that
its rise was but temporary, and lasted only during the disengagement of
the latent heat--now that all the water is frozen it falls again, and
will continue to fall till the ice and mixture are of an equal
temperature.
EMILY.
And can you show us any experiments in which liquids, by being mixed,
become solid, and disengage latent heat?
MRS. B.
I could show you several; but you are not yet sufficiently advanced to
understand them well. I shall, however, try one, which will afford you a
striking instance of the fact. The fluid which you see in this phial
consists of a quantity of a certain salt called _muriat of lime_,
dissolved in water. Now, if I pour into it a few drops of this other
fluid, called _sulphuric acid_, the whole, or very nearly the whole,
will be instantaneously converted into a solid mass.
EMILY.
How white it turns! I feel the latent heat escaping, for the bottle is
warm, and the fluid is changed to a solid white substance like chalk!
CAROLINE.
This is, indeed, the most curious experiment we have seen yet. But pray
what is that white vapour that ascends from the mixture?
MRS. B.
You are not yet enough of a chemist to understand that. --But take care,
Caroline, do not approach too near it, for it has a very pungent smell.
I shall show you another instance similar to that of the water, which
you observed to become warmer as it froze. I have in this phial a
solution of a salt called sulphat of soda or Glauber’s salt, made very
strong, and corked up when it was hot, and kept without agitation till
it became cold, as you may feel the phial is. Now when I take out the
cork and let the air fall upon it, (for being closed when boiling, there
was a vacuum in the upper part) observe that the salt will suddenly
crystallize. . . .
CAROLINE.
Surprising! how beautifully the needles of salt have shot through the
whole phial!
MRS. B.
Yes, it is very striking--but pray do not forget the object of the
experiment. Feel how warm the phial has become by the conversion of part
of the liquid into a solid.
EMILY.
Quite warm I declare! this is a most curious experiment of the
disengagement of latent heat.
MRS. B.
The slakeing of lime is another remarkable instance of the extrication
of latent heat. Have you never observed how quick-lime smokes when water
is poured upon it, and how much heat it produces?
CAROLINE.
Yes; but I do not understand what change of state takes place in the
lime that occasions its giving out latent heat; for the quick-lime,
which is solid, is (if I recollect right) reduced to powder, by this
operation, and is, therefore, rather expanded than condensed.
MRS. B.
It is from the water, not the lime, that the latent heat is set free.
The water incorporates with, and becomes solid in the lime; in
consequence of which, the heat, which kept it in a liquid state, is
disengaged, and escapes in a sensible form.
CAROLINE.
I always thought that the heat originated in the lime. It seems very
strange that water, and cold water too, should contain so much heat.
EMILY.
After this extrication of caloric, the water must exist in a state of
ice in the lime, since it parts with the heat which kept it liquid.
MRS. B.
It cannot properly be called ice, since ice implies a degree of cold, at
least equal to the freezing point. Yet as water, in combining with lime,
gives out more heat than in freezing, it must be in a state of still
greater solidity in the lime, than it is in the form of ice; and you may
have observed that it does not moisten or liquefy the lime in the
smallest degree.
EMILY.
But, Mrs. B., the smoke that rises is white; if it was only pure caloric
which escaped, we might feel, but could not see it.
MRS. B.
This white vapour is formed by some of the particles of lime, in a state
of fine dust, which are carried off by the caloric.
EMILY.
In all changes of state, then, a body either absorbs or disengages
latent heat?
MRS. B.
You cannot exactly say _absorbs latent heat_, as the heat becomes latent
only on being confined in the body; but you may say, generally, that
bodies, in passing from a solid to a liquid form, or from the liquid
state to that of vapour, absorb heat; and that when the reverse takes
place, heat is disengaged.*
[Footnote *: This rule, if not universal, admits of very few
exceptions.]
EMILY.
We can now, I think, account for the ether boiling, and the water
freezing in vacuo, at the same temperature.†
[Footnote †: See page 102.]
MRS. B.
Let me hear how you explain it.
EMILY.
The latent heat, which the water gave out in freezing, was immediately
absorbed by the ether, during its conversion into vapour; and therefore,
from a latent state in one liquid, it passed into a latent state in the
other.
MRS. B.
But this only partly accounts for the result of the experiment; it
remains to be explained why the temperature of the ether, while in a
state of ebullition, is brought down to the freezing temperature of the
water. --It is because the ether, during its evaporation, reduces its
own temperature, in the same proportion as that of the water, by
converting its free caloric into latent heat: so that, though one liquid
boils, and the other freezes, their temperatures remain in a state of
equilibrium.
EMILY.
But why does not water, as well as ether, reduce its own temperature by
evaporating?
MRS. B.
The fact is that it does, though much less rapidly than ether. Thus, for
instance, you may often have observed, in the heat of summer, how much
any particular spot may be cooled by watering, though the water used for
that purpose be as warm as the air itself. Indeed so much cold may be
produced by the mere evaporation of water, that the inhabitants of
India, by availing themselves of the most favourable circumstances for
this process which their warm climate can afford, namely, the cool of
the night, and situations most exposed to the night breeze, succeed in
causing water to freeze, though the temperature of the air be as high as
60 degrees. The water is put into shallow earthen trays, so as to expose
an extensive surface to the process of evaporation, and in the morning,
the water is found covered with a thin cake of ice, which is collected
in sufficient quantity to be used for purposes of luxury.
CAROLINE.
How delicious it must be to drink liquids so cold in those tropical
climates! But, Mrs. B., could we not try that experiment?
MRS. B.
If we were in the country, I have no doubt but that we should be able to
freeze water, by the same means, and under similar circumstances. But we
can do it immediately, upon a small scale, in this very room, in which
the thermometer stands at 70 degrees. For this purpose we need only
place some water in a little cup under the receiver of the air-pump
(PLATE V. fig. 1.), and exhaust the air from it. What will be the
consequence, Caroline?
[Illustration: Plate V. Vol. I. page 138.
Fig. 1.
The air-pump & receiver for Mr. Leslie’s experiment.
C a saucer with sulphuric Acid.
B a glass or earthen cup containing Water.
D a stand for the cup with its legs made of Glass.
A a Thermometer.
Fig. 2. Dr. Wollaston’s Cryophorus.
Fig. 5. Dr. Marcet’s mode of using the Cryophorus.
Fig. 3. & 4. the different parts of Fig. 5. seen separate.]
CAROLINE.
Of course the water will evaporate more quickly, since there will no
longer be any atmospheric pressure on its surface: but will this be
sufficient to make the water freeze?
MRS. B.
Probably not, because the vapour will not be carried off fast enough;
but this will be accomplished without difficulty if we introduce into
the receiver (fig. 1.), in a saucer, or other large shallow vessel, some
strong sulphuric acid, a substance which has a great attraction for
water, whether in the form of vapour, or in the liquid state. This
attraction is such that the acid will instantly absorb the moisture as
it rises from the water, so as to make room for the formation of fresh
vapour; this will of course hasten the process, and the cold produced
from the rapid evaporation of the water, will, in a few minutes, be
sufficient to freeze its surface.* We shall now exhaust the air from the
receiver.
[Footnote *: This experiment was first devised by Mr. Leslie, and
has since been modified in a variety of forms.]
EMILY.
Thousands of small bubbles already rise through the water from the
internal surface of the cup; what is the reason of this?
MRS. B.
These are bubbles of air which were partly attached to the vessel, and
partly diffused in the water itself; and they expand and rise in
consequence of the atmospheric pressure being removed.
CAROLINE.
See, Mrs. B.; the thermometer in the cup is sinking fast; it has already
descended to 40 degrees!
EMILY.
The water seems now and then violently agitated on the surface, as if it
was boiling; and yet the thermometer is descending fast!
MRS. B.
You may call it _boiling_, if you please, for this appearance is, as
well as boiling, owing to the rapid formation of vapour; but here, as
you have just observed, it takes place from the surface, for it is only
when heat is applied to the bottom of the vessel that the vapour is
formed there. --Now crystals of ice are actually shooting all over the
surface of the water.
CAROLINE.
How beautiful it is! The surface is now entirely frozen--but the
thermometer remains at 32 degrees.
MRS. B.
And so it will, conformably with our doctrine of latent heat, until the
whole of the water is frozen; but it will then again begin to descend
lower and lower, in consequence of the evaporation which goes on from
the surface of the ice.
EMILY.
This is a most interesting experiment; but it would be still more
striking if no sulphuric acid were required.
MRS. B.
I will show you a freezing instrument, contrived by Dr. Wollaston, upon
the same principle as Mr. Leslie’s experiment, by which water may be
frozen by its own evaporation alone, without the assistance of sulphuric
acid.
This tube, which, as you see (PLATE V. fig. 2.), is terminated at each
extremity by a bulb, one of which is half full of water, is internally
perfectly exhausted of air; the consequence of this is, that the water
in the bulb is always much disposed to evaporate. This evaporation,
however, does not proceed sufficiently fast to freeze the water; but if
the empty ball be cooled by some artificial means, so as to condense
quickly the vapour which rises from the water, the process may be thus
so much promoted as to cause the water to freeze in the other ball. Dr.
Wollaston has called this instrument _Cryophorus_.
CAROLINE.
So that cold seems to perform here the same part which the sulphuric
acid acted in Mr. Leslie’s experiment?
MRS. B.
Exactly so; but let us try the experiment.
EMILY.
How will you cool the instrument? You have neither ice nor snow.
MRS. B.
True: but we have other means of effecting this.* You recollect what an
intense cold can be produced by the evaporation of ether in an exhausted
receiver. We shall inclose the bulb in this little bag of fine flannel
(fig. 3.), then soke it in ether, and introduce it into the receiver of
the air-pump. (Fig. 5.) For this purpose we shall find it more
convenient to use a cryophorus of this shape (fig. 4.), as its elongated
bulb passes easily through a brass plate which closes the top of the
receiver. If we now exhaust the receiver quickly, you will see, in less
than a minute, the water freeze in the other bulb, out of the receiver.
[Footnote *: This mode of making the experiment was proposed, and
the particulars detailed, by Dr. Marcet, in the 34th vol. of
Nicholson’s Journal, page 119.]
EMILY.
The bulb already looks quite dim, and small drops of water are
condensing on its surface.
CAROLINE.
And now crystals of ice shoot all over the water. This is, indeed,
a very curious experiment!
MRS. B.
You will see, some other day, that, by a similar method, even
quicksilver may be frozen. --But we cannot at present indulge in any
further digression.
Having advanced so far on the subject of heat, I may now give you an
account of the calorimeter, an instrument invented by Lavoisier, upon
the principles just explained, for the purpose of estimating the
specific heat of bodies. It consists of a vessel, the inner surface of
which is lined with ice, so as to form a sort of hollow globe of ice, in
the midst of which the body, whose specific heat is to be ascertained,
is placed. The ice absorbs caloric from this body, till it has brought
it down to the freezing point; this caloric converts into water a
certain portion of the ice which runs out through an aperture at the
bottom of the machine; and the quantity of ice changed to water is a
test of the quantity of caloric which the body has given out in
descending from a certain temperature to the freezing point.
CAROLINE.
In this apparatus, I suppose, the milk, chalk, and lead, would melt
different quantities of ice, in proportion to their different capacities
for caloric?
MRS. B.
Certainly: and thence we are able to ascertain, with precision, their
respective capacities for heat. But the calorimeter affords us no more
idea of the absolute quantity of heat contained in a body, than the
thermometer; for though by means of it we extricate both the free and
combined caloric, yet we extricate them only to a certain degree, which
is the freezing point; and we know not how much they contain of either
below that point.
EMILY.
According to the theory of latent heat, it appears to me that the
weather should be warm when it freezes, and cold in a thaw: for latent
heat is liberated from every substance that it freezes, and such a large
supply of heat must warm the atmosphere; whilst, during a thaw, that
very quantity of free heat must be taken from the atmosphere, and return
to a latent state in the bodies which it thaws.
MRS. B.
Your observation is very natural; but consider that in a frost the
atmosphere is so much colder than the earth, that all the caloric which
it takes from the freezing bodies is insufficient to raise its
temperature above the freezing point; otherwise the frost must cease.
But if the quantity of latent heat extricated does not destroy the
frost, it serves to moderate the suddenness of the change of temperature
of the atmosphere, at the commencement both of frost, and of a thaw. In
the first instance, its extrication diminishes the severity of the cold;
and, in the latter, its absorption moderates the warmth occasioned by a
thaw: it even sometimes produces a discernible chill, at the breaking up
of a frost.
CAROLINE.
But what are the general causes that produce those sudden changes in the
weather, especially from hot to cold, which we often experience?
MRS. B.
This question would lead us into meteorological discussions, to which I
am by no means competent. One circumstance, however, we can easily
understand. When the air has passed over cold countries, it will
probably arrive here at a temperature much below our own, and then it
must absorb heat from every object it meets with, which will produce a
general fall of temperature.
CAROLINE.
But pray, now that we know so much of the effects of heat, will you
inform us whether it is really a distinct body, or, as I have heard,
a peculiar kind of motion produced in bodies?
MRS. B.
As I before told you, there is yet much uncertainty as to the nature of
these subtle agents. But I am inclined to consider heat not as mere
motion, but as a separate substance. Late experiments too appear to make
it a compound body, consisting of the two electricities, and in our next
conversation I shall inform you of the principal facts on which that
opinion is founded.
CONVERSATION V.
ON THE CHEMICAL AGENCIES OF ELECTRICITY.
MRS. B.