bodies which radiate, or part with their own caloric freely, should not
have the power of transmitting with equal facility that which they
receive from other bodies.
EMILY.
Yet no body can be said to possess caloric of its own, if all caloric is
originally derived from the sun.
MRS. B.
When I speak of a body radiating its own caloric, I mean that which it
has absorbed and incorporated either immediately from the sun’s rays, or
through the medium of any other substance.
CAROLINE.
It seems natural enough that the power of absorption should be in
opposition to that of reflection, for the more caloric a body receives,
the less it will reject.
EMILY.
And equally so that the power of radiation should correspond with that
of absorption. It is, in fact, cause and effect; for a body cannot
radiate heat without having previously absorbed it; just as a spring
that is well fed flows abundantly.
MRS. B.
Fluids are in general very bad radiators of caloric; and air neither
radiates nor absorbs caloric in any sensible degree.
We have not yet concluded our observations on free caloric. But I shall
defer, till our next meeting, what I have further to say on this
subject. I believe it will afford us ample conversation for another
interview.
CONVERSATION III.
CONTINUATION OF THE SUBJECT.
MRS. B.
In our last conversation, we began to examine the tendency of caloric to
restore an equilibrium of temperature. This property, when once well
understood, affords the explanation of a great variety of facts which
appeared formerly unaccountable. You must observe, in the first place,
that the effect of this tendency is gradually to bring all bodies that
are in contact to the same temperature. Thus, the fire which burns in
the grate, communicates its heat from one object to another, till every
part of the room has an equal proportion of it.
EMILY.
And yet this book is not so cold as the table on which it lies, though
both are at an equal distance from the fire, and actually in contact
with each other, so that, according to your theory, they should be
exactly of the same temperature.
CAROLINE.
And the hearth, which is much nearer the fire than the carpet, is
certainly the colder of the two.
MRS. B.
If you ascertain the temperature of these several bodies by a
thermometer (which is a much more accurate test than your feeling), you
will find that it is exactly the same.
CAROLINE.
But if they are of the same temperature, why should the one feel colder
than the other?
MRS. B.
The hearth and the table feel colder than the carpet or the book,
because the latter are not such good _conductors of heat_ as the former.
Caloric finds a more easy passage through marble and wood, than through
leather and worsted; the two former will therefore absorb heat more
rapidly from your hand, and consequently give it a stronger sensation of
cold than the two latter, although they are all of them really of the
same temperature.
CAROLINE.
So, then, the sensation I feel on touching a cold body, is in proportion
to the rapidity with which my hand yields its heat to that body?
MRS. B.
Precisely; and, if you lay your hand successively on every object in the
room, you will discover which are good, and which are bad conductors of
heat, by the different degrees of cold you feel. But, in order to
ascertain this point, it is necessary that the several substances should
be of the same temperature, which will not be the case with those that
are very near the fire, or those that are exposed to a current of cold
air from a window or door.
EMILY.
But what is the reason that some bodies are better conductors of heat
than others?
MRS. B.
This is a point not well ascertained. It has been conjectured that a
certain union or adherence takes place between the caloric and the
particles of the body through which it passes. If this adherence be
strong, the body detains the heat, and parts with it slowly and
reluctantly; if slight, it propagates it freely and rapidly. The
conducting power of a body is therefore, inversely, as its tendency to
unite with caloric.
EMILY.
That is to say, that the best conductors are those that have the least
affinity for caloric.
MRS. B.
Yes; but the term affinity is objectionable in this case, because, as
that word is used to express a chemical attraction (which can be
destroyed only by decomposition), it cannot be applicable to the slight
and transient union that takes place between free caloric and the bodies
through which it passes; an union which is so weak, that it constantly
yields to the tendency which caloric has to an equilibrium. Now you
clearly understand, that the passage of caloric, through bodies that are
good conductors, is much more rapid than through those that are bad
conductors, and that the former both give and receive it more quickly,
and therefore, in a given time, more abundantly, than bad conductors,
which makes them feel either hotter or colder, though they may be, in
fact, both of the same temperature.
CAROLINE.
Yes, I understand it now; the table, and the book lying upon it, being
really of the same temperature, would each receive, in the same space of
time, the same quantity of heat from my hand, were their conducting
powers equal; but as the table is the best conductor of the two, it will
absorb the heat from my hand more rapidly, and consequently produce a
stronger sensation of cold than the book.
MRS. B.
Very well, my dear; and observe, likewise, that if you were to heat the
table and the book an equal number of degrees above the temperature of
your body, the table, which before felt the colder, would now feel the
hotter of the two; for, as in the first case it took the heat most
rapidly from your hand, so it will now impart heat most rapidly to it.
Thus the marble table, which seems to us colder than the mahogany one,
will prove the hotter of the two to the ice; for, if it takes heat more
rapidly from our hands, which are warmer, it will give out heat more
rapidly to the ice, which is colder. Do you understand the reason of
these apparently opposite effects?
EMILY.
Perfectly. A body which is a good conductor of caloric, affords it a
free passage; so that it penetrates through that body more rapidly than
through one which is a bad conductor; and consequently, if it is colder
than your hand, you lose more caloric, and if it is hotter, you gain
more than with a bad conductor of the same temperature.
MRS. B.
But you must observe that this is the case only when the conductors are
either hotter or colder than your hand; for, if you heat different
conductors to the temperature of your body, they will all feel equally
warm, since the exchange of caloric between bodies of the same
temperature is equal. Now, can you tell me why flannel clothing, which
is a very bad conductor of heat, prevents our feeling cold?
CAROLINE.
It prevents the cold from penetrating . . . . . . . .
MRS. B.
But you forget that cold is only a negative quality.
CAROLINE.
True; it only prevents the heat of our bodies from escaping so rapidly
as it would otherwise do.
MRS. B.
Now you have explained it right; the flannel rather keeps in the heat,
than keeps out the cold. Were the atmosphere of a higher temperature
than our bodies, it would be equally efficacious in keeping their
temperature at the same degree, as it would prevent the free access of
the external heat, by the difficulty with which it conducts it.
EMILY.
This, I think, is very clear. Heat, whether external or internal, cannot
easily penetrate flannel; therefore in cold weather it keeps us warm;
and if the weather was hotter than our bodies, it would keep us cool.
MRS. B.
The most dense bodies are, generally speaking, the best conductors of
heat; probably because the denser the body the greater are the number of
points or particles that come in contact with caloric. At the common
temperature of the atmosphere a piece of metal will feel much colder
than a piece of wood, and the latter than a piece of woollen cloth; this
again will feel colder than flannel; and down, which is one of the
lightest, is at the same time one of the warmest bodies.
CAROLINE.
This is, I suppose, the reason that the plumage of birds preserves them
so effectually from the influence of cold in winter?
MRS. B.
Yes; but though feathers in general are an excellent preservative
against cold, down is a kind of plumage peculiar to aquatic birds, and
covers their chest, which is the part most exposed to the water; for
though the surface of the water is not of a lower temperature than the
atmosphere, yet, as it is a better conductor of heat, it feels much
colder, consequently the chest of the bird requires a warmer covering
than any other part of its body. Besides, the breasts of aquatic birds
are exposed to cold not only from the temperature of the water, but also
from the velocity with which the breast of the bird strikes against it;
and likewise from the rapid evaporation occasioned in that part by the
air against which it strikes, after it has been moistened by dipping
from time to time into the water.
If you hold a finger of one hand motionless in a glass of water, and at
the same time move a finger of the other hand swiftly through water of
the same temperature, a different sensation will be soon perceived in
the different fingers.
Most animal substances, especially those which Providence has assigned
as a covering for animals, such as fur, wool, hair, skin, &c. are bad
conductors of heat, and are, on that account, such excellent
preservatives against the inclemency of winter, that our warmest apparel
is made of these materials.
EMILY.
Wood is, I dare say, not so good a conductor as metal, and it is for
that reason, no doubt, that silver teapots have always wooden handles.
MRS. B.
Yes; and it is the facility with which metals conduct caloric that made
you suppose that a silver pot radiated more caloric than an earthen one.
The silver pot is in fact hotter to the hand when in contact with it;
but it is because its conducting power more than counterbalances its
deficiency in regard to radiation.
We have observed that the most dense bodies are in general the best
conductors; and metals, you know, are of that class. Porous bodies, such
as the earths and wood, are worse conductors, chiefly, I believe, on
account of their pores being filled with air; for air is a remarkably
bad conductor.
CAROLINE.
It is a very fortunate circumstance that air should be a bad conductor,
as it tends to preserve the heat of the body when exposed to cold
weather.
MRS. B.
It is one of the many benevolent dispensations of Providence, in order
to soften the inclemency of the seasons, and to render almost all
climates habitable to man.
In fluids of different densities, the power of conducting heat varies no
less remarkably; if you dip your hand into this vessel full of mercury,
you will scarcely conceive that its temperature is not lower than that
of the atmosphere.
CAROLINE.
Indeed I know not how to believe it, it feels so extremely cold. --But
we may easily ascertain its true temperature by the thermometer. --It is
really not colder than the air;--the apparent difference then is
produced merely by the difference of the conducting power in mercury and
in air.
MRS. B.
Yes; hence you may judge how little the sense of feeling is to be relied
on as a test of the temperature of bodies, and how necessary a
thermometer is for that purpose.
It has indeed been doubted whether fluids have the power of conducting
caloric in the same manner as solid bodies. Count Rumford, a very few
years since, attempted to prove, by a variety of experiments, that
fluids, when at rest, were not at all endowed with this property.
CAROLINE.
How is that possible, since they are capable of imparting cold or heat
to us; for if they did not conduct heat, they would neither take it
from, nor give it to us?
MRS. B.
Count Rumford did not mean to say that fluids would not communicate
their heat to solid bodies; but only that heat does not pervade fluids,
that is to say, is not transmitted from one particle of a fluid to
another, in the same manner as in solid bodies.
EMILY.
But when you heat a vessel of water over the fire, if the particles of
water do not communicate heat to each other, how does the water become
hot throughout?
MRS. B.
By constant agitation. Water, as you have seen, expands by heat in the
same manner as solid bodies; the heated particles of water, therefore,
at the bottom of the vessel, become specifically lighter than the rest
of the liquid, and consequently ascend to the surface, where, parting
with some of their heat to the colder atmosphere, they are condensed,
and give way to a fresh succession of heated particles ascending from
the bottom, which having thrown off their heat at the surface, are in
their turn displaced. Thus every particle is successively heated at the
bottom, and cooled at the surface of the liquid; but as the fire
communicates heat more rapidly than the atmosphere cools the succession
of surfaces, the whole of the liquid in time becomes heated.
CAROLINE.
This accounts most ingeniously for the propagation of heat upwards. But
suppose you were to heat the upper surface of a liquid, the particles
being specifically lighter than those below, could not descend: how
therefore would the heat be communicated downwards?
MRS. B.
If there were no agitation to force the heated surface downwards, Count
Rumford assures us that the heat would not descend. In proof of this he
succeeded in making the upper surface of a vessel of water boil and
evaporate, while a cake of ice remained frozen at the bottom.
CAROLINE.
That is very extraordinary indeed!
MRS. B.
It appears so, because we are not accustomed to heat liquids by their
upper surface; but you will understand this theory better if I show you
the internal motion that takes place in liquids when they experience a
change of temperature. The motion of the liquid itself is indeed
invisible from the extreme minuteness of its particles; but if you mix
with it any coloured dust, or powder, of nearly the same specific
gravity as the liquid, you may judge of the internal motion of the
latter by that of the coloured dust it contains. --Do you see the small
pieces of amber moving about in the liquid contained in this phial?
CAROLINE.
Yes, perfectly.
MRS. B.
We shall now immerse the phial in a glass of hot water, and the motion
of the liquid will be shown, by that which it communicates to the amber.
EMILY.
I see two currents, the one rising along the sides of the phial, the
other descending in the centre: but I do not understand the reason of
this.
MRS. B.
The hot water communicates its caloric, through the medium of the phial,
to the particles of the fluid nearest to the glass; these dilate and
ascend laterally to the surface, where, in parting with their heat, they
are condensed, and in descending, form the central current.
CAROLINE.
This is indeed a very clear and satisfactory experiment; but how much
slower the currents now move than they did at first?
MRS. B.
It is because the circulation of particles has nearly produced an
equilibrium of temperature between the liquid in the glass and that in
the phial.
CAROLINE.
But these communicate laterally, and I thought that heat in liquids
could be propagated only upwards.
MRS. B.
You do not take notice that the heat is imparted from one liquid to the
other, through the medium of the phial itself, the external surface of
which receives the heat from the water in the glass, whilst its internal
surface transmits it to the liquid it contains. Now take the phial out
of the hot water, and observe the effect of its cooling.
EMILY.
The currents are reversed; the external current now descends, and the
internal one rises. --I guess the reason of this change:-- the phial
being in contact with cold air instead of hot water, the external
particles are cooled instead of being heated; they therefore descend and
force up the central particles, which, being warmer, are consequently
lighter.
MRS. B.
It is just so. Count Rumford hence infers that no alteration of
temperature can take place in a fluid, without an internal motion of its
particles, and as this motion is produced only by the comparative levity
of the heated particles, heat cannot be propagated downwards.
But though I believe that Count Rumford’s theory as to heat being
incapable of pervading fluids is not strictly correct, yet there is, no
doubt, much truth in his observation, that the communication is
materially promoted by a motion of the parts; and this accounts for the
cold that is found to prevail at the bottom of the lakes in Switzerland,
which are fed by rivers issuing from the snowy Alps. The water of these
rivers being colder, and therefore more dense than that of the lakes,
subsides to the bottom, where it cannot be affected by the warmer
temperature of the surface; the motion of the waves may communicate this
temperature to some little depth, but it can descend no further than the
agitation extends.
EMILY.
But when the atmosphere is colder than the lake, the colder surface of
the water will descend, for the very reason that the warmer will not.
MRS. B.
Certainly: and it is on this account that neither a lake, nor any body
of water whatever, can be frozen until every particle of the water has
risen to the surface to give off its caloric to the colder atmosphere;
therefore the deeper a body of water is, the longer will be the time it
requires to be frozen.
EMILY.
But if the temperature of the whole body of water be brought down to the
freezing point, why is only the surface frozen?
MRS. B.
The temperature of the whole body is lowered, but not to the freezing
point. The diminution of heat, as you know, produces a contraction in
the bulk of fluids, as well as of solids. This effect, however, does not
take place in water below the temperature of 40 degrees, which is 8
degrees above the freezing point. At that temperature, therefore, the
internal motion, occasioned by the increased specific gravity of the
condensed particles, ceases; for when the water at the surface no longer
condenses, it will no longer descend, and leave a fresh surface exposed
to the atmosphere: this surface alone, therefore, will be further
exposed to its severity, and will soon be brought down to the freezing
point, when it becomes ice, which being a bad conductor of heat,
preserves the water beneath a long time from being affected by the
external cold.
CAROLINE.
And the sea does not freeze, I suppose, because its depth is so great,
that a frost never lasts long enough to bring down the temperature of
such a great body of water to 40 degrees?
MRS. B.
That is one reason why the sea, as a large mass of water, does not
freeze. But, independently of this, salt water does not freeze till it
is cooled much below 32 degrees, and with respect to the law of
condensation, salt water is an exception, as it condenses even many
degrees below the freezing point. When the caloric of fresh water,
therefore, is imprisoned by the ice on its surface, the ocean still
continues throwing off heat into the atmosphere, which is a most signal
dispensation of Providence to moderate the intensity of the cold in
winter.
CAROLINE.
This theory of the non-conducting power of liquids, does not, I suppose,
hold good with respect to air, otherwise the atmosphere would not be
heated by the rays of the sun passing through it?
MRS. B.
Nor is it heated in that way. The pure atmosphere is a perfectly
transparent medium, which neither radiates, absorbs, nor conducts
caloric, but transmits the rays of the sun to us without in any way
diminishing their intensity. The air is therefore not more heated, by
the sun’s rays passing through it, than diamond, glass, water, or any
other transparent medium.
CAROLINE.
That is very extraordinary! Are glass windows not heated then by the sun
shining on them?
MRS. B.
No; not if the glass be perfectly transparent. A most convincing proof
that glass transmits the rays of the sun without being heated by them is
afforded by the burning lens, which by converging the rays to a focus
will set combustible bodies on fire, without its own temperature being
raised.
EMILY.
Yet, Mrs. B., if I hold a piece of glass near the fire it is almost
immediately warmed by it; the glass therefore must retain some of the
caloric radiated by the fire? Is it that the solar rays alone pass
freely through glass without paying tribute? It seems unaccountable that
the radiation of a common fire should have power to do what the sun’s
rays cannot accomplish.
MRS. B.
It is not because the rays from the fire have more power, but rather
because they have less, that they heat glass and other transparent
bodies. It is true, however, that as you approach the source of heat the
rays being nearer each other, the heat is more condensed, and can
produce effects of which the solar rays, from the great distance of
their source, are incapable. Thus we should find it impossible to roast
a joint of meat by the sun’s rays, though it is so easily done by
culinary heat. Yet caloric emanated from burning bodies, which is
commonly called _culinary heat_, has neither the intensity nor the
velocity of solar rays. All caloric, we have said, is supposed to
proceed originally from the sun; but after having been incorporated with
terrestrial bodies, and again given out by them, though its nature is
not essentially altered, it retains neither the intensity nor the
velocity with which it first emanated from that luminary; it has
therefore not the power of passing through transparent mediums, such as
glass and water, without being partially retained by those bodies.
EMILY.
I recollect that in the experiment on the reflection of heat, the glass
skreen which you interposed between the burning taper and the mirror,
arrested the rays of caloric, and suffered only those of light to pass
through it.
CAROLINE.
Glass windows, then, though they cannot be heated by the sun shining on
them, may be heated internally by a fire in the room? But, Mrs. B.,
since the atmosphere is not warmed by the solar rays passing through it,
how does it obtain heat; for all the fires that are burning on the
surface of the earth would contribute very little towards warming it?
EMILY.
The radiation of heat is not confined to burning bodies: for all bodies,
you know, have that property; therefore, not only every thing upon the
surface of the earth, but the earth itself, must radiate heat; and this
terrestrial caloric, not having, I suppose, sufficient power to traverse
the atmosphere, communicates heat to it.
MRS. B.
Your inference is extremely well drawn, Emily; but the foundation on
which it rests is not sound; for the fact is, that terrestrial or
culinary heat, though it cannot pass through the denser transparent
mediums, such as glass or water, without loss, traverses the atmosphere
completely: so that all the heat which the earth radiates, unless it
meet with clouds or any foreign body to intercept its passage, passes
into the distant regions of the universe.
CAROLINE.
What a pity that so much heat should be wasted!
MRS. B.
Before you are tempted to object to any law of nature, reflect whether
it may not prove to be one of the numberless dispensations of Providence
for our good. If all the heat which the earth has received from the sun,
since the creation had been accumulated in it, its temperature by this
time would, no doubt, have been more elevated than any human being could
have borne.
CAROLINE.
I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at
such a high temperature, might have scorched our feet, we should always
have had a cool refreshing air to breathe, since the radiation of the
earth does not heat the atmosphere.
EMILY.
The cool air would have afforded but very insufficient refreshment,
whilst our bodies were exposed to the burning radiation of the earth.
MRS. B.