bodies for each other? It appears to me more extraordinary or unnatural,
if I may use the expression, than the attraction of cohesion, which
unites particles of a similar nature.
MRS. B.
Chemical attraction may, like that of cohesion or gravitation, be one of
the powers inherent in matter which, in our present state of knowledge,
admits of no other satisfactory explanation than an immediate reference
to a divine cause. Sir H. Davy, however, whose important discoveries
have opened such improved views in chemistry, has suggested an
hypothesis which may throw great light upon that science. He supposes
that there are two kinds of electricity, with one or other of which all
bodies are united. These we distinguish by the names of _positive_ and
_negative_ electricity; those bodies are disposed to combine, which
possess opposite electricities, as they are brought together by the
attraction which these electricities have for each other. But, whether
this hypothesis be altogether founded on truth or not, it is impossible
to question the great influence of electricity in chemical combinations.
EMILY.
So, that we must suppose that the two electricities always attract each
other, and thus compel the bodies in which they exist to combine?
CAROLINE.
And may not this be also the cause of the attraction of cohesion?
MRS. B.
No, for in particles of the same nature the same electricities must
prevail, and it is only the different or opposite electric fluids that
attract each other.
CAROLINE.
These electricities seem to me to be a kind of chemical spirit, which
animates the particles of bodies, and draws them together.
EMILY.
If it is known, then, with which of the electricities bodies are united,
it can be inferred which will, and which will not, combine together?
MRS. B.
Certainly. --I should not omit to mention, that some doubts have been
entertained whether electricity be really a material agent, or whether
it might not be a power inherent in bodies, similar to, or, perhaps
identical with, attraction.
EMILY.
But what then would be the electric spark which is visible, and must
therefore be really material?
MRS. B.
What we call the electric spark, may, Sir H. Davy says, be merely the
heat and light, or fire produced by the chemical combinations with which
these phenomena are always connected. We will not, however, enter more
fully on this important subject at present, but reserve the principal
facts which relate to it to a future conversation.
Before we part, however, I must recommend you to fix in your memory the
names of the simple bodies, against our next interview.
CONVERSATION II.
ON LIGHT AND HEAT OR CALORIC.
CAROLINE.
We have learned by heart the names of all the simple bodies which you
have enumerated, and we are now ready to enter on the examination of
each of them successively. You will begin, I suppose, with LIGHT?
MRS. B.
Respecting the nature of light we have little more than conjectures. It
is considered by most philosophers as a real substance, immediately
emanating from the sun, and from all luminous bodies, from which it is
projected in right lines with prodigious velocity. Light, however, being
imponderable, it cannot be confined and examined by itself; and
therefore it is to the effects it produces on other bodies, rather than
to its immediate nature, that we must direct our attention.
The connection between light and heat is very obvious; indeed, it is
such, that it is extremely difficult to examine the one independently of
the other.
EMILY.
But, is it possible to separate light from heat; I thought they were
only different degrees of the same thing, fire?
MRS. B.
I told you that fire was not now considered as a simple element. Whether
light and heat be altogether different agents, or not, I cannot pretend
to decide; but, in many cases, light may be separated from heat. The
first discovery of this was made by a celebrated Swedish chemist,
Scheele. Another very striking illustration of the separation of heat
and light was long after pointed out by Dr. Herschell. This philosopher
discovered that these two agents were emitted in the rays of the sun,
and that heat was less refrangible than light; for, in separating the
different coloured rays of light by a prism (as we did some time ago),
he found that the greatest heat was beyond the spectrum, at a little
distance from the red rays, which, you may recollect, are the least
refrangible.
EMILY.
I should like to try that experiment.
MRS. B.
It is by no means an easy one: the heat of a ray of light, refracted by
a prism, is so small, that it requires a very delicate thermometer to
distinguish the difference of the degree of heat within and without the
spectrum. For in this experiment the heat is not totally separated from
the light, each coloured ray retaining a certain portion of it, though
the greatest part is not sufficiently refracted to fall within the
spectrum.
EMILY.
I suppose, then, that those coloured rays which are the least
refrangible, retain the greatest quantity of heat?
MRS. B.
They do so.
EMILY.
Though I no longer doubt that light and heat can be separated, Dr.
Herschell’s experiment does not appear to me to afford sufficient proof
that they are essentially different; for light, which you call a simple
body, may likewise be divided into the various coloured rays.
MRS. B.
No doubt there must be some difference in the various coloured rays.
Even their chemical powers are different. The blue rays, for instance,
have the greatest effect in separating oxygen from bodies, as was found
by Scheele; and there exist also, as Dr. Wollaston has shown, rays more
refrangible than the blue, which produce the same chemical effect, and,
what is very remarkable, are invisible.
EMILY.
Do you think it possible that heat may be merely a modification of
light?
MRS. B.
That is a supposition which, in the present state of natural philosophy,
can neither be positively affirmed nor denied. Let us, therefore,
instead of discussing theoretical points, be contented with examining
what is known respecting the chemical effects of light.
Light is capable of entering into a kind of transitory union with
certain substances, and this is what has been called phosphorescence.
Bodies that are possessed of this property, after being exposed to the
sun’s rays, appear luminous in the dark. The shells of fish, the bones
of land animals, marble, limestone, and a variety of combinations of
earths, are more or less powerfully phosphorescent.
CAROLINE.
I remember being much surprised last summer with the phosphorescent
appearance of some pieces of rotten wood, which had just been dug out of
the ground; they shone so bright that I at first supposed them to be
glow-worms.
EMILY.
And is not the light of a glow-worm of a phosphorescent nature?
MRS. B.
It is a very remarkable instance of phosphorescence in living animals;
this property, however, is not exclusively possessed by the glow-worm.
The insect called the lanthorn-fly, which is peculiar to warm climates,
emits light as it flies, producing in the dark a remarkably sparkling
appearance. But it is more common to see animal matter in a dead state
possessed of a phosphorescent quality; sea fish is often eminently so.
EMILY.
I have heard that the sea has sometimes had the appearance of being
illuminated, and that the light is supposed to proceed from the spawn of
fishes floating on its surface.
MRS. B.
This light is probably owing to that or some other animal matter. Sea
water has been observed to become luminous from the substance of a fresh
herring having been immersed in it; and certain insects, of the Medusa
kind, are known to produce similar effects.
But the strongest phosphorescence is produced by chemical compositions
prepared for the purpose, the most common of which consists of oyster
shells and sulphur, and is known by the name of Canton’s Phosphorus.
EMILY.
I am rather surprised, Mrs. B., that you should have said so much of the
light emitted by phosphorescent bodies without taking any notice of that
which is produced by burning bodies.
MRS. B.
The light emitted by the latter is so intimately connected with the
chemical history of combustion, that I must defer all explanation of it
till we come to the examination of that process, which is one of the
most interesting in chemical science.
Light is an agent capable of producing various chemical changes. It is
essential to the welfare both of the animal and vegetable kingdoms; for
men and plants grow pale and sickly if deprived of its salutary
influence. It is likewise remarkable for its property of destroying
colour, which renders it of great consequence in the process of
bleaching.
EMILY.
Is it not singular that light, which in studying optics we were taught
to consider as the source and origin of colours, should have also the
power of destroying them?
CAROLINE.
It is a fact, however, that we every day experience; you know how it
fades the colours of linens and silks.
EMILY.
Certainly. And I recollect that endive is made to grow white instead of
green, by being covered up so as to exclude the light. But by what means
does light produce these effects?
MRS. B.
This I cannot attempt to explain to you until you have obtained a
further knowledge of chemistry. As the chemical properties of light can
be accounted for only in their reference to compound bodies, it would be
useless to detain you any longer on this subject; we may therefore pass
on to the examination of heat, or caloric, with which we are somewhat
better acquainted.
HEAT and LIGHT may be always distinguished by the different sensations
they produce, _Light_ affects the sense of sight; _Caloric_ that of
feeling; the one produces _Vision_, the other the sensation of _Heat_.
Caloric is found to exist in a variety of forms or modifications, and I
think it will be best to consider it under the two following heads, viz.
1. FREE OR RADIANT CALORIC.
2. COMBINED CALORIC.
The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE;
it comprehends all heat which is perceptible to the senses, and affects
the thermometer.
EMILY.
You mean such as the heat of the sun, of fire, of candles, of stoves; in
short, of every thing that burns?
MRS. B.
And likewise of things that do not burn, as, for instance, the warmth of
the body; in a word, all heat that is _sensible_, whatever may be its
degree, or the source from which it is derived.
CAROLINE.
What then are the other modifications of caloric? It must be a strange
kind of heat that cannot be perceived by our senses.
MRS. B.
None of the modifications of caloric should properly be called _heat_;
for heat, strictly speaking, is the sensation produced by caloric, on
animated bodies; this word, therefore, in the accurate language of
science, should be confined to express the sensation. But custom has
adapted it likewise to inanimate matter, and we say _the heat of an
oven_, _the heat of the sun_, without any reference to the sensation
which they are capable of exciting.
It was in order to avoid the confusion which arose from thus confounding
the cause and effect, that modern chemists adopted the new word
_caloric_, to denote the principle which produces heat; yet they do not
always, in compliance with their own language, limit the word _heat_ to
the expression of the sensation, since they still frequently employ it
in reference to the other modifications of caloric which are quite
independent of sensation.
CAROLINE.
But you have not yet explained to us what these other modifications of
caloric are.
MRS. B.
Because you are not acquainted with the properties of free caloric, and
you know that we have agreed to proceed with regularity.
One of the most remarkable properties of free caloric is its power of
_dilating_ bodies. This fluid is so extremely subtle, that it enters and
pervades all bodies whatever, forces itself between their particles, and
not only separates them, but frequently drives them asunder to a
considerable distance from each other. It is thus that caloric dilates
or expands a body so as to make it occupy a greater space than it did
before.
EMILY.
The effect it has on bodies, therefore, is directly contrary to that of
the attraction of cohesion; the one draws the particles together, the
other drives them asunder.
MRS. B.
Precisely. There is a continual struggle between the attraction of
aggregation, and the expansive power of caloric; and from the action of
these two opposite forces, result all the various forms of matter, or
degrees of consistence, from the solid, to the liquid and aëriform
state. And accordingly we find that most bodies are capable of passing
from one of these forms to the other, merely in consequence of their
receiving different quantities of caloric.
CAROLINE.
That is very curious; but I think I understand the reason of it. If a
great quantity of caloric is added to a solid body, it introduces itself
between the particles in such a manner as to overcome, in a considerable
degree, the attraction of cohesion; and the body, from a solid, is then
converted into a fluid.
MRS. B.
This is the case whenever a body is fused or melted; but if you add
caloric to a liquid, can you tell me what is the consequence?
CAROLINE.
The caloric forces itself in greater abundance between the particles of
the fluid, and drives them to such a distance from each other, that
their attraction of aggregation is wholly destroyed: the liquid is then
transformed into vapour.
MRS. B.
Very well; and this is precisely the case with boiling water, when it is
converted into steam or vapour, and with all bodies that assume an
aëriform state.
EMILY.
I do not well understand the word aëriform?
MRS. B.
Any elastic fluid whatever, whether it be merely vapour or permanent
air, is called aëriform.
But each of these various states, solid, liquid, and aëriform, admit of
many different degrees of density, or consistence, still arising
(chiefly at least) from the different quantities of caloric the bodies
contain. Solids are of various degrees of density, from that of gold, to
that of a thin jelly. Liquids, from the consistence of melted glue, or
melted metals, to that of ether, which is the lightest of all liquids.
The different elastic fluids (with which you are not yet acquainted) are
susceptible of no less variety in their degrees of density.
EMILY.
But does not every individual body also admit of different degrees of
consistence, without changing its state?
MRS. B.
Undoubtedly; and this I can immediately show you by a very simple
experiment. This piece of iron now exactly fits the frame, or ring, made
to receive it; but if heated red hot, it will no longer do so, for its
dimensions will be so much increased by the caloric that has penetrated
into it, that it will be much too large for the frame.
The iron is now red hot; by applying it to the frame, we shall see how
much it is dilated.
EMILY.
Considerably so indeed! I knew that heat had this effect on bodies, but
I did not imagine that it could be made so conspicuous.
MRS. B.
By means of this instrument (called a Pyrometer) we may estimate, in the
most exact manner, the various dilatations of any solid body by heat.
The body we are now going to submit to trial is this small iron bar;
I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by
lighting the three lamps beneath it: when the bar expands, it increases
in length as well as thickness; and, as one end communicates with this
wheel-work, whilst the other end is fixed and immoveable, no sooner does
it begin to dilate than it presses against the wheel-work, and sets in
motion the index, which points out the degrees of dilatation on the
dial-plate.
[Illustration: Plate I. Vol. I. p. 38.
Fig. 1. Pyrometer.
A.A Bar of Metal.
1.2.3 Lamps burning.
B.B Wheel work.
C Index.
Fig. 2
A.A Glass tubes with bulbs.
B.B Glasses of water in which they are immersed.]
EMILY.
This is, indeed, a very curious instrument; but I do not understand the
use of the wheels: would it not be more simple, and answer the purpose
equally well, if the bar, in dilating, pressed against the index, and
put it in motion without the intervention of the wheels?
MRS. B.
The use of the wheels is merely to multiply the motion, and therefore
render the effect of the caloric more obvious; for if the index moved no
more than the bar increased in length, its motion would scarcely be
perceptible; but by means of the wheels it moves in a much greater
proportion, which therefore renders the variations far more conspicuous.
By submitting different bodies to the test of the pyrometer, it is found
that they are far from dilating in the same proportion. Different metals
expand in different degrees, and other kinds of solid bodies vary still
more in this respect. But this different susceptibility of dilatation is
still more remarkable in fluids than in solid bodies, as I shall show
you. I have here two glass tubes, terminated at one end by large bulbs.
We shall fill the bulbs, the one with spirit of wine, the other with
water. I have coloured both liquids, in order that the effect may be
more conspicuous. The spirit of wine, you see, dilates by the warmth of
my hand as I hold the bulb.
EMILY.
It certainly does, for I see it is rising into the tube. But water, it
seems, is not so easily affected by heat; for scarcely any change is
produced on it by the warmth of the hand.
MRS. B.
True; we shall now plunge the bulbs into hot water, (PLATE I. Fig. 2.)
and you will see both liquids rise in the tubes; but the spirit of wine
will ascend highest.
CAROLINE.
How rapidly it expands! Now it has nearly reached the top of the tube,
though the water has hardly begun to rise.
EMILY.
The water now begins to dilate. Are not these glass tubes, with liquids
rising within them, very like thermometers?
MRS. B.
A thermometer is constructed exactly on the same principle, and these
tubes require only a scale to answer the purpose of thermometers: but
they would be rather awkward in their dimensions. The tubes and bulbs of
thermometers, though of various sizes, are in general much smaller than
these; the tube too is hermetically closed, and the air excluded from
it. The fluid most generally used in thermometers is mercury, commonly
called quicksilver, the dilatations and contractions of which correspond
more exactly to the additions, and subtractions, of caloric, than those
of any other fluid.
CAROLINE.
Yet I have often seen coloured spirit of wine used in thermometers.
MRS. B.
The expansions and contractions of that liquid are not quite so uniform
as those of mercury; but in cases in which it is not requisite to
ascertain the temperature with great precision, spirit of wine will
answer the purpose equally well, and indeed in some respects better, as
the expansion of the latter is greater, and therefore more conspicuous.
This fluid is used likewise in situations and experiments in which
mercury would be frozen; for mercury becomes a solid body, like a piece
of lead or any other metal, at a certain degree of cold: but no degree
of cold has ever been known to freeze spirit of wine.
A thermometer, therefore, consists of a tube with a bulb, such as you
see here, containing a fluid whose degrees of dilatation and contraction
are indicated by a scale to which the tube is fixed. The degree which
indicates the boiling point, simply means that, when the fluid is
sufficiently dilated to rise to this point, the heat is such that water
exposed to the same temperature will boil. When, on the other hand, the
fluid is so much condensed as to sink to the freezing point, we know
that water will freeze at that temperature. The extreme points of the
scales are not the same in all thermometers, nor are the degrees always
divided in the same manner. In different countries philosophers have
chosen to adopt different scales and divisions. The two thermometers
most used are those of Fahrenheit, and of Reaumur; the first is
generally preferred by the English, the latter by the French.
EMILY.
The variety of scale must be very inconvenient, and I should think
liable to occasion confusion, when French and English experiments are
compared.
MRS. B.
The inconvenience is but very trifling, because the different gradations
of the scales do not affect the principle upon which thermometers are
constructed. When we know, for instance, that Fahrenheit’s scale is
divided into 212 degrees, in which 32° corresponds with the freezing
point, and 212° with the point of boiling water: and that Reaumur’s is
divided only into 80 degrees, in which 0° denotes the freezing point,
and 80° that of boiling water, it is easy to compare the two scales
together, and reduce the one into the other. But, for greater
convenience, thermometers are sometimes constructed with both these
scales, one on either side of the tube; so that the correspondence of
the different degrees of the two scales is thus instantly seen. Here
is one of these scales, (PLATE II. Fig. 1.) by which you can at
once perceive that each degree of Reaumur’s corresponds to 2¼ of
Fahrenheit’s division. But I believe the French have, of late, given
the preference to what they call the centigrade scale, in which the
space between the freezing and the boiling point is divided into 100
degrees.
[Illustration: Plate II. Vol. I. p. 42.
Fig. 1. Thermometer.
Fahrenheit’s Scale.
Reaumur’s Scale.
Boiling point of Water
Freezing point of Water
Fig. 2. Differential Thermometer.]
CAROLINE.
That seems to me the most reasonable division, and I cannot guess
why the freezing point is called 32°, or what advantage is derived
from it.
MRS. B.
There really is no advantage in it; and it originated in a mistaken
opinion of the instrument-maker, Fahrenheit, who first constructed these
thermometers. He mixed snow and salt together, and produced by that
means a degree of cold which he concluded was the greatest possible, and
therefore made his scale begin from that point. Between that and boiling
water he made 212 degrees, and the freezing point was found to be at
32°.
EMILY.
Are spirit of wine, and mercury, the only liquids used in the
construction of thermometers?
MRS. B.
I believe they are the only liquids now in use, though some others, such
as linseed oil, would make tolerable thermometers: but for experiments
in which a very quick and delicate test of the changes of temperature is
required, air is the fluid sometimes employed. The bulb of air
thermometers is filled with common air only, and its expansion and
contraction are indicated by a small drop of any coloured liquor, which
is suspended within the tube, and moves up and down, according as the
air within the bulb and tube expands or contracts. But in general, air
thermometers, however sensible to changes of temperature, are by no
means accurate in their indications.
I can, however, show you an air thermometer of a very peculiar
construction, which is remarkably well adapted for some chemical
experiments, as it is equally delicate and accurate in its indications.
CAROLINE.
It looks like a double thermometer reversed, the tube being bent, and
having a large bulb at each of its extremities. (PLATE II. Fig. 2.)
EMILY.
Why do you call it an air thermometer; the tube contains a coloured
liquid?
MRS. B.
But observe that the bulbs are filled with air, the liquid being
confined to a portion of the tube, and answering only the purpose of
showing, by its motion in the tube, the comparative dilatation or
contraction of the air within the bulbs, which afford an indication of
their relative temperature. Thus if you heat the bulb A, by the warmth
of your hand, the fluid will rise towards the bulb B, and the contrary
will happen if you reverse the experiment.
But if, on the contrary, both tubes are of the same temperature, as is
the case now, the coloured liquid, suffering an equal pressure on each
side, no change of level takes place.
CAROLINE.
This instrument appears, indeed, uncommonly delicate. The fluid is set
in motion by the mere approach of my hand.
MRS. B.