might answer this purpose better. The experiment, however, as here
described, is difficult, and requires a very nice apparatus. But
if, instead of phials or tubes, two watch-glasses be used, water
may be frozen almost instantly in the same manner. The two glasses
are placed over one another, with a few drops of water interposed
between them, and the uppermost glass is filled with ether. After
working the pump for a minute or two, the glasses are found to
adhere strongly together, and a thin layer of ice is seen between
them.]
CAROLINE.
It is indeed wonderful to see water freeze in contact with a boiling
fluid!
EMILY.
I am at a loss to conceive how the ether can pass to the state of vapour
without an addition of caloric. Does it not contain more caloric in a
state of vapour, than in a state of liquidity?
MRS. B.
It certainly does; for though it is the pressure of the atmosphere which
condenses it into a liquid, it is by forcing out the caloric that
belongs to it when in an aëriform state.
EMILY.
You have, therefore, two difficulties to explain, Mrs. B. --First, from
whence the ether obtains the caloric necessary to convert it into vapour
when it is relieved from the pressure of the atmosphere; and, secondly,
what is the reason that the water, in which the bottle of ether stands,
is frozen?
CAROLINE.
Now, I think, I can answer both these questions. The ether obtains the
addition of caloric required, from the water in the glass; and the loss
of caloric, which the latter sustains, is the occasion of its freezing.
MRS. B.
You are perfectly right; and if you look at the thermometer which I have
placed in the water, whilst I am working the pump, you will see that
every time bubbles of vapour are produced, the mercury descends; which
proves that the heat of the water diminishes in proportion as the ether
boils.
EMILY.
This I understand now very well; but if the water freezes in consequence
of yielding its caloric to the ether, the equilibrium of heat must, in
this case, be totally destroyed. Yet you have told us, that the exchange
of caloric between two bodies of equal temperature, was always equal;
how, then, is it that the water, which was originally of the same
temperature as the ether, gives out caloric to it, till the water is
frozen, and the ether made to boil?
MRS. B.
I suspected that you would make these objections; and, in order to
remove them, I enclosed two thermometers in the air-pump; one which
stands in the glass of water, the other in the phial of ether; and you
may see that the equilibrium of temperature is not destroyed; for as the
thermometer descends in the water, that in the ether sinks in the same
manner; so that both thermometers indicate the same temperature, though
one of them is in a boiling, the other in a freezing liquid.
EMILY.
The ether, then, becomes colder as it boils? This is so contrary to
common experience, that I confess it astonishes me exceedingly.
CAROLINE.
It is, indeed, a most extraordinary circumstance. But pray, how do you
account for it?
MRS. B.
I cannot satisfy your curiosity at present; for before we can attempt to
explain this apparent paradox, it is necessary to become acquainted with
the subject of LATENT HEAT: and that, I think, we must defer till our
next interview.
CAROLINE.
I believe, Mrs. B., that you are glad to put off the explanation; for it
must be a very difficult point to account for.
MRS. B.
I hope, however, that I shall do it to your complete satisfaction.
EMILY.
But before we part, give me leave to ask you one question. Would not
water, as well as ether, boil with less heat, if deprived of the
pressure of the atmosphere?
MRS. B.
Undoubtedly. You must always recollect that there are two forces to
overcome, in order to make a liquid boil or evaporate; the attraction of
aggregation, and the weight of the atmosphere. On the summit of a high
mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat
is required to make water boil, than in the plain, where the weight of
the atmosphere is greater.* Indeed if the weight of the atmosphere be
entirely removed by means of a good air-pump, and if water be placed in
the exhausted receiver, it will evaporate so fast, however cold it
maybe, as to give it the appearance of boiling from the surface. But
without the assistance of the air-pump, I can show you a very pretty
experiment, which proves the effect of the pressure of the atmosphere in
this respect.
Observe, that this Florence flask is about half full of water, and the
upper half of invisible vapour, the water being in the act of boiling.
--I take it from the lamp, and cork it carefully--the water, you see,
immediately ceases boiling. --I shall now dip the flask into a bason of
cold water.†
[Footnote *: On the top of Mount Blanc, water boiled when heated
only to 187 degrees, instead of 212 degrees.]
[Footnote †: The same effect may be produced by wrapping a cold
wet linen cloth round the upper part of the flask. In order to
show how much the water cools whilst it is boiling, a thermometer,
graduated on the tube itself, may be introduced into the bottle
through the cork.]
CAROLINE.
But look, Mrs. B., the hot water begins to boil again, although the cold
water must rob it more and more of its caloric! What can be the reason
of that?
MRS. B.
Let us examine its temperature. You see the thermometer immersed in it
remains stationary at 180 degrees, which is about 30 degrees below the
boiling point. When I took the flask from the lamp, I observed to you
that the upper part of it was filled with vapour; this being compelled
to yield its caloric to the cold water, was again condensed into water--
What, then, filled the upper part of the flask?
EMILY.
Nothing; for it was too well corked for the air to gain admittance, and
therefore the upper part of the flask must be a vacuum.
MRS. B.
The water below, therefore, no longer sustains the pressure of the
atmosphere, and will consequently boil at a much lower temperature.
Thus, you see, though it had lost many degrees of heat, it began boiling
again the instant the vacuum was formed above it. The boiling has now
ceased, the temperature of the water being still farther reduced; if it
had been ether, instead of water, it would have continued boiling much
longer, for ether boils, under the usual atmospheric pressure, at a
temperature as low as 100 degrees; and in a vacuum it boils at almost
any temperature; but water being a more dense fluid, requires a more
considerable quantity of caloric to make it evaporate quickly, even when
the pressure of the atmosphere is removed.
EMILY.
What proportion of vapour can the atmosphere contain in a state of
solution?
MRS. B.
I do not know whether it has been exactly ascertained by experiment; but
at any rate this proportion must vary, both according to the temperature
and the weight of the atmosphere; for the lower the temperature, and the
greater the pressure, the smaller must be the proportion of vapour that
the atmosphere can contain.
To conclude the subject of free caloric, I should mention _Ignition_, by
which is meant that emission of light which is produced in bodies at a
very high temperature, and which is the effect of accumulated caloric.
EMILY.
You mean, I suppose, that light which is produced by a burning body?
MRS. B.
No: ignition is quite independent of combustion. Clay, chalk, and indeed
all incombustible substances, may be made red hot. When a body burns,
the light emitted is the effect of a chemical change which takes place,
whilst ignition is the effect of caloric alone, and no other change than
that of temperature is produced in the ignited body.
All solid bodies, and most liquids, are susceptible of ignition, or, in
other words, of being heated so as to become luminous; and it is
remarkable that this takes place pretty nearly at the same temperature
in all bodies, that is, at about 800 degrees of Fahrenheit’s scale.
EMILY.
But how can liquids attain so high a temperature, without being
converted into vapour?
MRS. B.
By means of confinement and pressure. Water confined in a strong iron
vessel (called Papin’s digester) can have its temperature raised to
upwards of 400 degrees. Sir James Hall has made some very curious
experiments on the effects of heat assisted by pressure; by means of
strong gun-barrels, he succeeded in melting a variety of substances
which were considered as infusible: and it is not unlikely that, by
similar methods, water itself might be heated to redness.
EMILY.
I am surprised at that: for I thought that the force of steam was such
as to destroy almost all mechanical resistance.
MRS. B.
The expansive force of steam is prodigious; but in order to subject
water to such high temperatures, it is prevented by confinement from
being converted into steam, and the expansion of heated water is
comparatively trifling. --But we have dwelt so long on the subject of
free caloric, that we must reserve the other modifications of that agent
to our next meeting, when we shall endeavour to proceed more rapidly.
CONVERSATION IV.
ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND LATENT HEAT.
MRS. B.
We are now to examine the other modifications of caloric.
CAROLINE.
I am very curious to know of what nature they can be; for I have no
notion of any kind of heat that is not perceptible to the senses.
MRS. B.
In order to enable you to understand them, it will be necessary to enter
into some previous explanations.
It has been discovered by modern chemists, that bodies of a different
nature, heated to the same temperature, do not contain the same quantity
of caloric.
CAROLINE.
How could that be ascertained? Have you not told us that it is
impossible to discover the absolute quantity of caloric which bodies
contain?
MRS. B.
True; but at the same time I said that we were enabled to form a
judgment of the proportions which bodies bore to each other in this
respect. Thus it is found that, in order to raise the temperature of
different bodies the same number of degrees, different quantities of
caloric are required for each of them. If, for instance, you place a
pound of lead, a pound of chalk, and a pound of milk, in a hot oven,
they will be gradually heated to the temperature of the oven; but the
lead will attain it first, the chalk next, and the milk last.
CAROLINE.
That is a natural consequence of their different bulks; the lead being
the smallest body, will be heated soonest, and the milk, which is the
largest, will require the longest time.
MRS. B.
That explanation will not do, for if the lead be the least in bulk, it
offers also the least surface to the caloric, the quantity of heat
therefore which can enter into it in the same space of time is
proportionally smaller.
EMILY.
Why, then, do not the three bodies attain the temperature of the oven at
the same time?
MRS. B.
It is supposed to be on account of the different capacity of these
bodies for caloric.
CAROLINE.
What do you mean by the capacity of a body for caloric?
MRS. B.
I mean a certain disposition of bodies to require more or less caloric
for raising their temperature to any degree of heat. Perhaps the fact
may be thus explained:
Let us put as many marbles into this glass as it will contain, and pour
some sand over them--observe how the sand penetrates and lodges between
them. We shall now fill another glass with pebbles of various forms--you
see that they arrange themselves in a more compact manner than the
marbles, which, being globular, can touch each other by a single point
only. The pebbles, therefore, will not admit so much sand between them;
and consequently one of these glasses will necessarily contain more sand
than the other, though both of them be equally full.
CAROLINE.
This I understand perfectly. The marbles and the pebbles represent two
bodies of different kinds, and the sand the caloric contained in them;
it appears very plain, from this comparison, that one body may admit of
more caloric between its particles than another.
MRS. B.
You can no longer be surprised, therefore, that bodies of a different
capacity for caloric should require different proportions of that fluid
to raise their temperatures equally.
EMILY.
But I do not conceive why the body that contains the most caloric should
not be of the highest temperature; that is to say, feel hot in
proportion to the quantity of caloric it contains?
MRS. B.
The caloric that is employed in filling the capacity of a body, is not
free caloric; but is imprisoned as it were in the body, and is therefore
imperceptible: for we can feel only the caloric which the body parts
with, and not that which it retains.
CAROLINE.
It appears to me very extraordinary that heat should be confined in a
body in such a manner as to be imperceptible.
MRS. B.
If you lay your hand on a hot body, you feel only the caloric which
leaves it, and enters your hand; for it is impossible that you should be
sensible of that which remains in the body. The thermometer, in the same
manner, is affected only by the free caloric which a body transmits to
it, and not at all by that which it does not part with.
CAROLINE.
I begin to understand it: but I confess that the idea of insensible heat
is so new and strange to me, that it requires some time to render it
familiar.
MRS. B.
Call it insensible caloric, and the difficulty will appear much less
formidable. It is indeed a sort of contradiction to call it heat, when
it is so situated as to be incapable of producing that sensation. Yet
this modification of caloric is commonly called SPECIFIC HEAT.
CAROLINE.
But it certainly would have been more correct to have called it
_specific caloric_.
EMILY.
I do not understand how the term _specific_ applies to this modification
of caloric?
MRS. B.
It expresses the relative quantity of caloric which different _species_
of bodies of the same weight and temperature are capable of containing.
This modification is also frequently called _heat of capacity_, a term
perhaps preferable, as it explains better its own meaning.
You now understand, I suppose, why the milk and chalk required a longer
portion of time than the lead to raise their temperature to that of the
oven?
EMILY.
Yes: the milk and chalk having a greater capacity for caloric than the
lead, a greater proportion of that fluid became insensible in those
bodies: and the more slowly, therefore, their temperature was raised.
CAROLINE.
But might not this difference proceed from the different conducting
powers of heat in these three bodies, since that which is the best
conductor must necessarily attain the temperature of the oven first?
MRS. B.
Very well observed, Caroline. This objection would be insurmountable, if
we could not, by reversing the experiment, prove that the milk, the
chalk, and the lead, actually absorbed different quantities of caloric,
and we know that if the different time they took in heating, proceeded
merely from their different conducting powers, they would each have
acquired an equal quantity of caloric.
CAROLINE.
Certainly. But how can you reverse this experiment?
MRS. B.
It may be done by cooling the several bodies to the same degree in an
apparatus adapted to receive and measure the caloric which they give
out. Thus, if you plunge them into three equal quantities of water, each
at the same temperature, you will be able to judge of the relative
quantity of caloric which the three bodies contained, by that, which, in
cooling, they communicated to their respective portions of water: for
the same quantity of caloric which they each absorbed to raise their
temperature, will abandon them in lowering it; and on examining the
three vessels of water, you will find the one in which you immersed the
lead to be the least heated; that which held the chalk will be the next;
and that which contained the milk will be heated the most of all. The
celebrated Lavoisier has invented a machine to estimate, upon this
principle, the specific heat of bodies in a more perfect manner; but I
cannot explain it to you, till you are acquainted with the next
modification of caloric.
EMILY.
The more dense a body is, I suppose, the less is its capacity for
caloric?
MRS. B.
This is not always the case with bodies of different nature; iron, for
instance, contains more specific heat than tin, though it is more dense.
This seems to show that specific heat does hot merely depend upon the
interstices between the particles; but, probably, also upon some
peculiar constitution of the bodies which we do not comprehend.
EMILY.
But, Mrs. B., it would appear to me more proper to compare bodies by
_measure_, rather than by _weight_, in order to estimate their specific
heat. Why, for instance, should we not compare _pints_ of milk, of
chalk, and of lead, rather than _pounds_ of those substances; for equal
weights may be composed of very different quantities?
MRS. B.
You are mistaken, my dear; equal weight must contain equal quantities of
matter; and when we wish to know what is the relative quantity of
caloric, which substances of various kinds are capable of containing
under the same temperature, we must compare equal weights, and not equal
bulks of those substances. Bodies of the same weight may undoubtedly be
of very different dimensions; but that does not change their real
quantity of matter. A pound of feathers does not contain one atom more
than a pound of lead.
CAROLINE.
I have another difficulty to propose. It appears to me, that if the
temperature of the three bodies in the oven did not rise equally, they
would never reach the same degree; the lead would always keep its
advantage over the chalk and milk, and would perhaps be boiling before
the others had attained the temperature of the oven. I think you might
as well say that, in the course of time, you and I should be of the same
age?
MRS. B.
Your comparison is not correct, Caroline. As soon as the lead reached
the temperature of the oven, it would remain stationary; for it would
then give out as much heat as it would receive. You should recollect
that the exchange of radiating heat, between two bodies of equal
temperature, is equal: it would be impossible, therefore, for the lead
to accumulate heat after having attained the temperature of the oven;
and that of the chalk and milk therefore would ultimately arrive at the
same standard. Now I fear that this will not hold good with respect to
our ages, and that, as long as I live, I shall never cease to keep my
advantage over you.
EMILY.
I think that I have found a comparison for specific heat, which is very
applicable. Suppose that two men of equal weight and bulk, but who
required different quantities of food to satisfy their appetites, sit
down to dinner, both equally hungry; the one would consume a much
greater quantity of provisions than the other, in order to be equally
satisfied.
MRS. B.
Yes, that is very fair; for the quantity of food necessary to satisfy
their respective appetites, varies in the same manner as the quantity of
caloric requisite to raise equally the temperature of different bodies.
EMILY.
The thermometer, then, affords no indication of the specific heat of
bodies?
MRS. B.
None at all: no more than satiety is a test of the quantity of food
eaten. The thermometer, as I have repeatedly said, can be affected only
by free caloric, which alone raises the temperature of bodies.
But there is another mode of proving the existence of specific heat,
which affords a very satisfactory illustration of that modification.
This, however, I did not enlarge upon before, as I thought it might
appear to you rather complicated. --If you mix two fluids of different
temperatures, let us say the one at 50 degrees, and the other at 100
degrees, of what temperature do you suppose the mixture will be?
CAROLINE.
It will be no doubt the medium between the two, that is to say, 75
degrees.
MRS. B.
That will be the case if the two bodies happen to have the same capacity
for caloric; but if not, a different result will be obtained. Thus, for
instance, if you mix together a pound of mercury, heated at 50 degrees,
and a pound of water heated at 100 degrees, the temperature of the
mixture, instead of being 75 degrees, will be 80 degrees; so that the
water will have lost only 12 degrees, whilst the mercury will have
gained 38 degrees; from which you will conclude that the capacity of
mercury for heat is less than that of water.
CAROLINE.
I wonder that mercury should have so little specific heat. Did we not
see it was a much better conductor of heat than water?
MRS. B.
And it is precisely on that account that its specific heat is less. For
since the conductive power of bodies depends, as we have observed
before, on their readiness to receive heat and part with it, it is
natural to expect that those bodies which are the worst conductors
should absorb the most caloric before they are disposed to part with it
to other bodies. But let us now proceed to LATENT HEAT.
CAROLINE.
And pray what kind of heat is that?
MRS. B.
It is another modification of combined caloric, which is so analogous to
specific heat, that most chemists make no distinction between them; but
Mr. Pictet, in his Essay on Fire, has so clearly discriminated them,
that I am induced to adopt his view of the subject. We therefore call
_latent heat_ that portion of insensible caloric which is employed in
changing the state of bodies; that is to say, in converting solids into
liquids, or liquids; into vapour. When a body changes its state from
solid to liquid, or from liquid to vapour, its expansion occasions a
sudden and considerable increase of capacity for heat, in consequence of
which it immediately absorbs a quantity of caloric, which becomes fixed
in the body which it has transformed; and, as it is perfectly concealed
from our senses, it has obtained the name of _latent_ heat.
CAROLINE.
I think it would be much more correct to call this modification latent
caloric instead of latent heat, since it does not excite the sensation
of heat.
MRS. B.
This modification of heat was discovered and named by Dr. Black long
before the French chemists introduced the term caloric, and we must not
presume to alter it, as it is still used by much better chemists than
ourselves. And, besides, you are not to suppose that the nature of heat
is altered by being variously modified: for if latent heat and specific
heat do not excite the same sensations as free caloric, it is owing to
their being in a state of confinement, which prevents them from acting
upon our organs; and consequently, as soon as they are extricated from
the body in which they are imprisoned, they return to their state of
free caloric.
EMILY.
But I do not yet clearly see in what respect latent heat differs from
specific heat; for they are both of them imprisoned and concealed in
bodies.
MRS. B.
Specific heat is that which is employed in filling the capacity of a
body for caloric, in the state in which this body actually exists; while
latent heat is that which is employed only in effecting a change of
state, that is, in converting bodies from a solid to a liquid, or from a
liquid to an aëriform state. But I think that, in a general point of
view, both these modifications might be comprehended under the name of
_heat of capacity_, as in both cases the caloric is equally engaged in
filling the capacities of bodies.
I shall now show you an experiment, which I hope will give you a clear
idea of what is understood by latent heat.
The snow which you see in this phial has been cooled by certain chemical
means (which I cannot well explain to you at present), to 5 or 6 degrees
below the freezing point, as you will find indicated by the thermometer
which is placed in it. We shall expose it to the heat of a lamp, and you
will see the thermometer gradually rise, till it reaches the freezing
point----
EMILY.
But there it stops, Mrs. B., and yet the lamp burns just as well as
before. Why is not its heat communicated to the thermometer?
CAROLINE.
And the snow begins to melt, therefore it must be rising above the
freezing point?
MRS. B.
The heat no longer affects the thermometer, because it is wholly
employed in converting the ice into water. As the ice melts, the caloric
becomes _latent_ in the new-formed liquid, and therefore cannot raise
its temperature; and the thermometer will consequently remain
stationary, till the whole of the ice be melted.
CAROLINE.
Now it is all melted, and the thermometer begins to rise again.
MRS. B.