temperature of any particular body, or of the medium in which it is
immersed; it serves only to point out the _difference_ of temperature
between the two bulbs, when placed under different circumstances. For
this reason it has been called _differential_ thermometer. You will see
by-and-bye to what particular purposes this instrument applies.
EMILY.
But do common thermometers indicate the exact quantity of caloric
contained either in the atmosphere, or in any body with which they are
in contact?
MRS. B.
No: first, because there are other modifications of caloric which do not
affect the thermometer; and, secondly, because the temperature of a
body, as indicated by the thermometer, is only relative. When, for
instance, the thermometer remains stationary at the freezing point, we
know that the atmosphere (or medium in which it is placed, whatever it
may be) is as cold as freezing water; and when it stands at the boiling
point, we know that this medium is as hot as boiling water; but we do
not know the positive quantity of heat contained either in freezing or
boiling water, any more than we know the real extremes of heat and cold;
and consequently we cannot determine that of the body in which the
thermometer is placed.
CAROLINE.
I do not quite understand this explanation.
MRS. B.
Let us compare a thermometer to a well, in which the water rises to
different heights, according as it is more or less supplied by the
spring which feeds it: if the depth of the well is unfathomable, it must
be impossible to know the absolute quantity of water it contains; yet we
can with the greatest accuracy measure the number of feet the water has
risen or fallen in the well at any time, and consequently know the
precise quantity of its increase or diminution, without having the least
knowledge of the whole quantity of water it contains.
CAROLINE.
Now I comprehend it very well; nothing appears to me to explain a thing
so clearly as a comparison.
EMILY.
But will thermometers bear any degree of heat?
MRS. B.
No; for if the temperature were much above the highest degree marked on
the scale of the thermometer, the mercury would burst the tube in an
attempt to ascend. And at any rate, no thermometer can be applied to
temperatures higher than the boiling point of the liquid used in its
construction, for the steam, on the liquid beginning to boil, would
burst the tube. In furnaces, or whenever any very high temperature is to
be measured, a pyrometer, invented by Wedgwood, is used for that
purpose. It is made of a certain composition of baked clay, which has
the peculiar property of contracting by heat, so that the degree of
contraction of this substance indicates the temperature to which it has
been exposed.
EMILY.
But is it possible for a body to contract by heat? I thought that heat
dilated all bodies whatever.
MRS. B.
This is not an exception to the rule. You must recollect that the bulk
of the clay is not compared, whilst hot, with that which it has when
cold; but it is from the change which the clay has undergone by _having
been_ heated that the indications of this instrument are derived. This
change consists in a beginning fusion which tends to unite the particles
of clay more closely, thus rendering it less pervious or spongy.
Clay is to be considered as a spongy body, having many interstices or
pores, from its having contained water when soft. These interstices are
by heat lessened, and would by extreme heat be entirely obliterated.
CAROLINE.
And how do you ascertain the degrees of contraction of Wedgwood’s
pyrometer?
MRS. B.
The dimensions of a piece of clay are measured by a scale graduated on
the side of a tapered groove, formed in a brass ruler; the more the clay
is contracted by the heat, the further it will descend into the narrow
part of the tube.
Before we quit the subject of expansion, I must observe to you that, as
liquids expand more readily than solids, so elastic fluids, whether air
or vapour, are the most expansible of all bodies.
It may appear extraordinary that all elastic fluids whatever, undergo
the same degree of expansion from equal augmentations of temperature.
EMILY.
I suppose, then, that all elastic fluids are of the same density?
MRS. B.
Very far from it; they vary in density, more than either liquids or
solids. The uniformity of their expansibility, which at first may appear
singular, is, however, readily accounted for. For if the different
susceptibilities of expansion of bodies arise from their various degrees
of attraction of cohesion, no such difference can be expected in elastic
fluids, since in these the attraction of cohesion does not exist, their
particles being on the contrary possessed of an elastic or repulsive
power; they will therefore all be equally expanded by equal degrees of
caloric.
EMILY.
True; as there is no power opposed to the expansive force of caloric in
elastic bodies, its effect must be the same in all of them.
MRS. B.
Let us now proceed to examine the other properties of free caloric.
Free caloric always tends to diffuse itself equally, that is to say,
when two bodies are of different temperatures, the warmer gradually
parts with its heat to the colder, till they are both brought to the
same temperature. Thus, when a thermometer is applied to a hot body, it
receives caloric; when to a cold one, it communicates part of its own
caloric, and this communication continues until the thermometer and the
body arrive at the same temperature.
EMILY.
Cold, then, is nothing but a negative quality, simply implying the
absence of heat.
MRS. B.
Not the total absence, but a diminution of heat; for we know of no body
in which some caloric may not be discovered.
CAROLINE.
But when I lay my hand on this marble table I feel it _positively_ cold,
and cannot conceive that there is any caloric in it.
MRS. B.
The cold you experience consists in the loss of caloric that your hand
sustains in an attempt to bring its temperature to an equilibrium with
the marble. If you lay a piece of ice upon it, you will find that the
contrary effect will take place; the ice will be melted by the heat
which it abstracts from the marble.
CAROLINE.
Is it not in this case the air of the room, which being warmer than the
marble, melts the ice?
MRS. B.
The air certainly acts on the surface which is exposed to it, but the
table melts that part with which it is in contact.
CAROLINE.
But why does caloric tend to an equilibrium? It cannot be on the same
principle as other fluids, since it has no weight?
MRS. B.
Very true, Caroline, that is an excellent objection. You might also,
with some propriety, object to the term _equilibrium_ being applied to a
body that is without weight; but I know of no expression that would
explain my meaning so well. You must consider it, however, in a
figurative rather than a literal sense; its strict meaning is an _equal
diffusion_. We cannot, indeed, well say by what power it diffuses itself
equally, though it is not surprising that it should go from the parts
which have the most to those which have the least. This subject is best
explained by a theory suggested by Professor Prevost of Geneva, which is
now, I believe, generally adopted.
According to this theory, caloric is composed of particles perfectly
separate from each other, every one of which moves with a rapid velocity
in a certain direction. These directions vary as much as imagination can
conceive, the result of which is, that there are rays or lines of these
particles moving with immense velocity in every possible direction.
Caloric is thus universally diffused, so that when any portion of space
happens to be in the neighbourhood of another, which contains more
caloric, the colder portion receives a quantity of calorific rays from
the latter, sufficient to restore an equilibrium of temperature. This
radiation does not only take place in free space, but extends also to
bodies of every kind. Thus you may suppose all bodies whatever
constantly radiating caloric: those that are of the same temperature
give out and absorb equal quantities, so that no variation of
temperature is produced in them; but when one body contains more free
caloric than another, the exchange is always in favour of the colder
body, until an equilibrium is effected; this you found to be the case
when the marble table cooled your hand, and again when it melted the
ice.
CAROLINE.
This reciprocal radiation surprises me extremely; I thought, from what
you first said, that the hotter bodies alone emitted rays of caloric
which were absorbed by the colder; for it seems unnatural that a hot
body should receive any caloric from a cold one, even though it should
return a greater quantity.
MRS. B.
It may at first appear so, but it is no more extraordinary than that a
candle should send forth rays of light to the sun, which, you know, must
necessarily happen.
CAROLINE.
Well, Mrs. B--, I believe that I must give up the point. But I wish I
could _see_ these rays of caloric; I should then have greater faith in
them.
MRS. B.
Will you give no credit to any sense but that of sight? You may feel the
rays of caloric which you receive from any body of a temperature higher
than your own; the loss of the caloric you part with in return, it is
true, is not perceptible; for as you gain more than you lose, instead of
suffering a diminution, you are really making an acquisition of caloric.
It is, therefore, only when you are parting with it to a body of a lower
temperature, that you are sensible of the sensation of cold, because you
then sustain an absolute loss of caloric.
EMILY.
And in this case we cannot be sensible of the small quantity of heat we
receive in exchange from the colder body, because it serves only to
diminish the loss.
MRS. B.
Very well, indeed, Emily. Professor Pictet, of Geneva, has made some
very interesting experiments, which prove not only that caloric radiates
from all bodies whatever, but that these rays may be reflected,
according to the laws of optics, in the same manner as light. I shall
repeat these experiments before you, having procured mirrors fit for the
purpose; and it will afford us an opportunity of using the differential
thermometer, which is particularly well adapted for these experiments.
--I place an iron bullet, (PLATE III. Fig. 1.) about two inches in
diameter, and heated to a degree not sufficient to render it luminous,
in the focus of this large metallic concave mirror. The rays of heat
which fall on this mirror are reflected, agreeably to the property of
concave mirrors, in a parallel direction, so as to fall on a similar
mirror, which, you see, is placed opposite to the first, at the distance
of about ten feet; thence the rays converge to the focus of the second
mirror, in which I place one of the bulbs of this thermometer. Now,
observe in what manner it is affected by the caloric which is reflected
on it from the heated bullet. --The air is dilated in the bulb which we
placed in the focus of the mirror, and the liquor rises considerably in
the opposite leg.
[Illustration: Plate III. Vol. I. p. 54
Mr. Pictet’s Apparatus for the Reflection of Heat.
Fig. 1.
A.A. & B.B Concave mirrors fixed on stands.
C Heated Bullet placed in the focus of the mirror A.
D Thermometer, with its bulb placed in the focus of the mirror B.
1.2.3.4 Rays of Caloric radiating from the bullet & falling on the
mirror A.
5.6.7.8 The same rays reflected from the mirror A to the mirror B.
9.10.11.12 The same rays reflected by the mirror B to the
Thermometer.]
EMILY.
But would not the same effect take place, if the rays of caloric from
the heated bullet fell directly on the thermometer, without the
assistance of the mirrors?
MRS. B.
The effect would in that case be so trifling, at the distance at which
the bullet and the thermometer are from each other, that it would be
almost imperceptible. The mirrors, you know, greatly increase the
effect, by collecting a large quantity of rays into a focus; place your
hand in the focus of the mirror, and you will find it much hotter there
than when you remove it nearer to the bullet.
EMILY.
That is very true; it appears extremely singular to feel the heat
diminish in approaching the body from which it proceeds.
CAROLINE.
And the mirror which produces so much heat, by converging the rays, is
itself quite cold.
MRS. B.
The same number of rays that are dispersed over the surface of the
mirror are collected by it into the focus; but, if you consider how
large a surface the mirror presents to the rays, and, consequently, how
much they are diffused in comparison to what they are at the focus,
which is little more than a point, I think you can no longer wonder that
the focus should be so much hotter than the mirror.
The principal use of the mirrors in this experiment is, to prove that
the calorific emanation is reflected in the same manner as light.
CAROLINE.
And the result, I think, is very conclusive.
MRS. B.
The experiment may be repeated with a wax taper instead of the bullet,
with a view of separating the light from the caloric. For this purpose a
transparent plate of glass must be interposed between the mirrors; for
light, you know, passes with great facility through glass, whilst the
transmission of caloric is almost wholly impeded by it. We shall find,
however, in this experiment, that some few of the calorific rays pass
through the glass together with the light, as the thermometer rises a
little; but, as soon as the glass is removed, and a free passage left to
the caloric, it will rise considerably higher.
EMILY.
This experiment, as well as that of Dr. Herschell’s, proves that light
and heat may be separated; for in the latter experiment the separation
was not perfect, any more than in that of Mr. Pictet.
CAROLINE.
I should like to repeat this experiment, with the difference of
substituting a cold body instead of the hot one, to see whether cold
would not be reflected as well as heat.
MRS. B.
That experiment was proposed to Mr. Pictet by an incredulous philosopher
like yourself, and he immediately tried it by substituting a piece of
ice in the place of the heated bullet.
CAROLINE.
Well, Mrs. B., and what was the result?
MRS. B.
That we shall see; I have procured some ice for the purpose.
EMILY.
The thermometer falls considerably!
CAROLINE.
And does not that prove that cold is not merely a _negative_ quality,
implying simply an inferior degree of heat? The cold must be _positive_,
since it is capable of reflection.
MRS. B.
So it at first appeared to Mr. Pictet; but upon a little consideration
he found that it afforded only an additional proof of the reflection of
heat: this I shall endeavour to explain to you.
According to Mr. Prevost’s theory, we suppose that all bodies whatever
radiate caloric; the thermometer used in these experiments therefore
emits calorific rays in the same manner as any other substance. When its
temperature is in equilibrium with that of the surrounding bodies, it
receives as much caloric as it parts with, and no change of temperature
is produced. But when we introduce a body of a lower temperature, such
as a piece of ice, which parts with less caloric than it receives, the
consequence is, that its temperature is raised, whilst that of the
surrounding bodies is proportionally lowered.
EMILY.
If, for instance, I was to bring a large piece of ice into this room,
the ice would in time be melted, by absorbing caloric from the general
radiation which is going on throughout the room; and as it would
contribute very little caloric in return for what is absorbed, the room
would necessarily be cooled by it.
MRS. B.
Just so; and as in consequence of the mirrors, a more considerable
exchange of rays takes place between the ice and the thermometer, than
between these and any of the surrounding bodies, the temperature of the
thermometer must be more lowered than that of any other adjacent object.
CAROLINE.
I confess I do not perfectly understand your explanation.
MRS. B.
This experiment is exactly similar to that made with the heated bullet:
for, if we consider the thermometer as the hot body (which it certainly
is in comparison to the ice), you may then easily understand that it is
by the loss of the calorific rays which the thermometer sends to the
ice, and not by any cold rays received from it, that the fall of the
mercury is occasioned: for the ice, far from emitting rays of cold,
sends forth rays of caloric, which diminish the loss sustained by the
thermometer.
Let us say, for instance, that the radiation of the thermometer towards
the ice is equal to 20, and that of the ice towards the thermometer to
10: the exchange in favour of the ice is as 20 is to 10, or the
thermometer absolutely loses 10, whilst the ice gains 10.
CAROLINE.
But if the ice actually sends rays of caloric to the thermometer, must
not the latter fall still lower when the ice is removed?
MRS. B.
No; for the space that the ice occupied, admits rays from all the
surrounding bodies to pass through it; and those being of the same
temperature as the thermometer, will not affect it, because as much heat
now returns to the thermometer as radiates from it.
CAROLINE.
I must confess that you have explained this in so satisfactory a manner,
that I cannot help being convinced now that cold has no real claim to
the rank of a positive being.
MRS. B.
Before I conclude the subject of radiation I must observe to you that
different bodies, (or rather surfaces,) possess the power of radiating
caloric in very different degrees.
Some very curious experiments have been made by Mr. Leslie on this
subject, and it was for this purpose that he invented the differential
thermometer; with its assistance he ascertained that black surfaces
radiate most, glass next, and polished surfaces the least of all.
EMILY.
Supposing these surfaces, of course, to be all of the same temperature.
MRS. B.
Undoubtedly. I will now show you the very simple and ingenious
apparatus, by means of which he made these experiments. This cubical tin
vessel or canister, has each of its sides externally covered with
different materials; the one is simply blackened; the next is covered
with white paper; the third with a pane of glass, and in the fourth the
polished tin surface remains uncovered. We shall fill this vessel with
hot water, so that there can be no doubt but that all its sides will be
of the same temperature. Now let us place it in the focus of one of the
mirrors, making each of its sides front it in succession. We shall begin
with the black surface.
CAROLINE.
It makes the thermometer which is in the focus of the other mirror rise
considerably. Let us turn the paper surface towards the mirror. The
thermometer falls a little, therefore of course this side cannot emit or
radiate so much caloric as the blackened side.
EMILY.
This is very surprising; for the sides are exactly of the same size, and
must be of the same temperature. But let us try the glass surface.
MRS. B.
The thermometer continues falling, and with the plain surface it falls
still lower; these two surfaces therefore radiate less and less.
CAROLINE.
I think I have found out the reason of this.
MRS. B.
I should be very happy to hear it, for it has not yet (to my knowledge)
been accounted for.
CAROLINE.
The water within the vessel gradually cools, and the thermometer in
consequence gradually falls.
MRS. B.
It is true that the water cools, but certainly in much less proportion
than the thermometer descends, as you will perceive if you now change
the tin surface for the black one.
CAROLINE.
I was mistaken certainly, for the thermometer rises again now that the
black surface fronts the mirror.
MRS. B.
And yet the water in the vessel is still cooling, Caroline.
EMILY.
I am surprised that the tin surface should radiate the least caloric,
for a metallic vessel filled with hot water, a silver teapot, for
instance, feels much hotter to the hand than one of black earthen ware.
MRS. B.
That is owing to the different power which various bodies possess for
_conducting_ caloric, a property which we shall presently examine. Thus,
although a metallic vessel feels warmer to the hand, a vessel of this
kind is known to preserve the heat of the liquid within, better than one
of any other materials; it is for this reason that silver teapots make
better tea than those of earthen ware.
EMILY.
According to these experiments, light-coloured dresses, in cold weather,
should keep us warmer than black clothes, since the latter radiate so
much more than the former.
MRS. B.
And that is actually the case.
EMILY.
This property, of different surfaces to radiate in different degrees,
appears to me to be at variance with the equilibrium of caloric; since
it would imply that those bodies which radiate most, must ultimately
become coldest.
Suppose that we were to vary this experiment, by using two metallic
vessels full of boiling water, the one blackened, the other not; would
not the black one cool the first?
CAROLINE.
True; but when they were both brought down to the temperature of the
room, the interchange of caloric between the canisters and the other
bodies of the room being then equal, their temperatures would remain the
same.
EMILY.
I do not see why that should be the case; for if different surfaces of
the same temperature radiate in different degrees when heated, why
should they not continue to do so when cooled down to the temperature of
the room?
MRS. B.
You have started a difficulty, Emily, which certainly requires
explanation. It is found by experiment that the power of absorption
corresponds with and is proportional to that of radiation; so that under
equal temperatures, bodies compensate for the greater loss they sustain
in consequence of their greater radiation by their greater absorption;
so that if you were to make your experiment in an atmosphere heated like
the canisters, to the temperature of boiling water, though it is true
that the canisters would radiate in different degrees, no change of
temperature would be produced in them, because they would each absorb
caloric in proportion to their respective radiation.
EMILY.
But would not the canisters of boiling water also absorb caloric in
different degrees in a room of the common temperature?
MRS. B.
Undoubtedly they would. But the various bodies in the room would not, at
a lower temperature, furnish either of the canisters with a sufficiency
of caloric to compensate for the loss they undergo; for, suppose the
black canister to absorb 400 rays of caloric, whilst the metallic one
absorbed only 200; yet if the former radiate 800, whilst the latter
radiates only 400, the black canister will be the first cooled down to
the temperature of the room. But from the moment the equilibrium of
temperature has taken place, the black canister, both receiving and
giving out 400 rays, and the metallic one 200, no change of temperature
will take place.
EMILY.
I now understand it extremely well. But what becomes of the surplus of
calorific rays, which good radiators emit and bad radiators refuse to
receive; they must wander about in search of a resting-place?
MRS. B.
They really do so; for they are rejected and sent back, or, in other
words, _reflected_ by the bodies which are bad radiators of caloric; and
they are thus transmitted to other bodies which happen to lie in their
way, by which they are either absorbed or again reflected, according as
the property of reflection, or that of absorption, predominates in these
bodies.
CAROLINE.
I do not well understand the difference between radiating and reflecting
caloric, for the caloric that is reflected from a body proceeds from it
in straight lines, and may surely be said to radiate from it?
MRS. B.
It is true that there at first appears to be a great analogy between
_radiation_ and _reflection_, as they equally convey the idea of the
transmission of caloric.
But if you consider a little, you will perceive that when a body
_radiates_ caloric, the heat which it emits not only proceeds from, but
has its origin in the body itself. Whilst when a body _reflects_
caloric, it parts with none of its own caloric, but only reflects that
which it receives from other bodies.
EMILY.
Of this difference we have very striking examples before us, in the tin
vessel of water, and the concave mirrors; the first radiates its own
heat, the latter reflect the heat which they receive from other bodies.
CAROLINE.