HEAT—WHAT IT IS—THEORY OF HEAT—THE THERMOMETER—EXPANSION BY
HEAT—EBULLITION AND DISTILLATION—LATENT HEAT—SPECIFIC HEAT.
What is Heat?—We will consider this question, and endeavour to explain
it before we speak of its effects on water and other matter.
Heat is now believed to be the effects of the rapid motion of all
the particles of a body. It is quite certain that a heated body is
no heavier than the same body before it was made “hot,” so the heat
could not have gone into it, nor does the “heat” leave it when it has
become what we call “cold,” which is a relative term. Heat is therefore
believed to be a vibratory motion, or the effects of very rapid motion
of matter.
The Science of Heat, as we may term it, is only in its infancy, or
certainly has scarcely come of age. Formerly heat was considered a
chemical agent, and was termed caloric, but now heat is found to
be motion, which affects our nerves of feeling and sight; and, as
Professor Stewart tells us, “a heated body gives a series of blows to
the medium around it; and although these blows do not affect the ear,
they affect the eye, and give us a sense of light.”
Although it is only within a comparatively few years that heat has been
really looked upon as other than matter, many ancient philosophers
regarded it as merely a _quality_ of matter. They thought it the active
principle of the universe. Epicurus declared that heat was an effluxion
of minute spherical particles possessing rapid motion, and Lucretius
maintained that the sun’s light and heat are the result of motion of
primary particles. Fire was worshipped as the active agent of the
universe, and Prometheus was fabled to have stolen fire from heaven to
vivify mankind. The views of the ancients were more or less adopted in
the Middle Ages; but John Locke recognized the theory of heat being a
motion of matter. He says: “What in our sensation is _heat_, in the
object is nothing but _motion_.”
Gradually two theories arose concerning heat;—one, the Material
theory—the theory of Caloric or Phlogiston; the other, the Kinetic
theory. Before the beginning of the present century the former theory
was generally accepted, and the development of heat was accounted for
by asserting that the friction or percussion altered the capacity
for heat of the substances acted upon. The heat was squeezed out by
the hammer, and the amount of heat in the world was regarded as a
certain quantity, which passed from one body to another, and that some
substances contained, or could “store away,” more of the material
called heat than other substances. Heat was the material of fire—the
principle of it, or _materia ignis_; and by these theories Heat,
or Caloric, was gradually adopted as a separate material agent—an
invisible and subtle matter producing certain phenomena when liberated.
So the two theories concerning heat arose at the end of the last
century. One, as we have said, is known as the Material, the other as
the Kinetic theory. The latter is the theory of motion, so called from
the Greek _kinesis_ (motion), or sometimes known as the Dynamic theory
of heat, from _dunamis_ (force); or again as Thermo-dynamics.
But any possibility of producing a new supply of heat was denied by the
materialists. They knew that some bodies possessed a greater capacity
for heat than others; but Count Rumford, at Munich, in 1797, astonished
an audience by making water boil without any fire! He had observed the
great extent to which a cannon became heated while being bored in the
gun factory, and influenced by those who maintained the material theory
of heat, paid great attention to the evolution of heat. He accordingly
endeavoured to produce heat by friction, and by means of horse power
he caused a steel borer to work upon a cylinder of metal. The shavings
were permitted to drop into a pan of water at 60° Fahrenheit. In an
hour after the commencement of the operation the temperature of the
water had risen to 107°: in another half-hour the heat of it was up
to 142°: and in two hours had measured 170°. Upon this he says: “It
is hardly necessary to add that anything which any insulated body or
system of bodies can continue to furnish without limitation cannot
possibly be a material substance, and it appears to me to be extremely
difficult, if not quite impossible, to form any distinct idea of
anything capable of being excited and communicated in these experiments
except by _motion_.”
A few years later Sir Humphrey Davy made his conclusive experiments,
and the Material theory of heat received its death-blow.
Sir Humphrey Davy—referring to the fact that water at a freezing
temperature has “more heat in it” (as it was believed) than ice at the
same temperature—said: “If I, by friction, liquify ice, a substance
will be produced which contains a far greater absolute amount of heat
than ice. In this case it cannot reasonably be affirmed that I merely
render _sensible_ heat which had been previously _insensible_ in the
frozen mass. Liquification will conclusively prove the _generation_ of
heat.
This reasoning could not be doubted. Sir Humphrey Davy made the
experiment. He rubbed together two pieces of ice in the air, and in
a vacuum surrounded by a freezing mixture. The ice became liquified,
and so the generation of heat by “mechanical means” was proved. Its
immateriality was demonstrated, but the Material theory was not even
then abandoned by its adherents.
So things continued, until in 1842-3, Doctor Julius Meyer, of
Heilbronn, and Doctor Joule, of Manchester, separately, and by
different means, arrived at the conclusion that a certain definite
amount of mechanical work corresponds to a certain definite amount
of Heat, and _vice versâ_. Thus was a great support afforded to
the Dynamic theory. This fact Doctor Joule communicated to the
_Philosophical Magazine_ in 1843, and the conclusions he came to were—
1. “That the quantity of heat produced by the friction of bodies,
whether solid or liquid, is always in proportion to the force expended;
2. “That the quantity of heat capable of increasing the temperature of
a pound of water (weighed _in vacuo_ and taken at between 55° and 60°
Fahr.) by 1° Fahr., requires for its evolution the expenditure of a
mechanical force represented by the fall of 772 lbs. through the space
of one foot.”
[Illustration: Fig. 76.—Melting a piece of tin on a card.]
This is the “mechanical equivalent of heat.” The first paper written
by Mr. Joule demonstrated that the temperature of water rises when
forced through narrow tubes; and to heat it one degree, the force of
770 foot pounds was necessary, which means that the 1 lb. of water
falling 770 feet, got hotter by one degree when it reached the earth.
He subsequently arrived at the more exact conclusions quoted above.
So heat is now known to be a series of vibrations, or vibratory
motions, as sound vibrations, which we cannot hear nor see, but the
effects of which are known to us as light and heat.
In considering heat we must put aside the idea of warmth and cold, for
they are only different degrees of heat, not the absence of it.
The study of heat can be briefly undertaken without any complicated
apparatus. If we desire a proof of the great conducting power of
metals, let us place a fine piece of muslin tightly stretched over
a lump of polished metal. On the muslin we put a burning ember, and
excite combustion by blowing on it; the muslin is not burned in the
least, the heat being entirely absorbed by the metal, which draws
it through the material into itself. Fig. 76 represents a similar
experiment: it consists of melting some tin on a playing card, held
over the flame of a spirit lamp. The metal becomes completely melted
without the card being burnt. It is through a similar effect that
metals appear cold to us when we take them in our hands; by their
conductibility they remove the heat from our hands, and give us the
peculiar impression which we do not experience when in contact with
substances that are bad conductors, such as wood, woollen materials,
etc.
[Illustration: Fig. 77.—Boiling water in a paper case.]
Fig. 77 shows the method of boiling water in paper. We make a small
paper box, such as those made by school-boys, and suspend it by four
threads to a piece of wood held horizontally at a suitable height. We
fill this improvised vessel with water, and place it over the flame of
a spirit lamp. The paper is not burnt, because the water absorbs all
the heat into itself. After a few minutes the water begins to boil,
sending forth clouds of steam, but the paper remains intact. It is well
to perform this operation over a plate, in case of accident, as the
water may be spilt. We may also make use of an egg-shell as a little
vessel in which to heat the water, by resting it on a wire ring over
the flame of the spirit lamp.
[Illustration: Fig. 78.—Experiment on the regelation of ice.]
Fig. 78 shows the arrangement of a very remarkable experiment, but
little known, on the refreezing of ice. A block of ice is placed on the
edge of two iron chairs, and is encircled by a piece of wire, to which
is suspended the weight of say five pounds. The wire penetrates slowly,
and in about an hour’s time has passed completely through the lump
of ice, and the weight, with the piece of wire, falls to the ground.
What happens then to the block of ice?—You imagine, doubtless, that
it is cut in two. No such thing; it is intact, and in a single lump
as it was previous to the experiment. In proportion as the wire was
sunk through the mass, the slit has been closed again by refreezing.
Ice or snow during the winter may serve for a number of experiments
relating to heat. If we wish to demonstrate the influence of colours
on radiation, we take two pieces of cloth of the same size,—one white,
and the other black,—and place them both on the snow, if possible,
when there is a gleam of sunlight. In a short time it will be found
that the snow underneath the black cloth has melted to a much greater
extent than that beneath the white cloth, because black absorbs heat
more than white, which, on the contrary, has a tendency to reflect it.
We perceive very plainly the difference in temperature by touching the
two cloths. The white cloth feels cold in comparison with the black
cloth.
It is hardly necessary to point out experiments on the expansion of
bodies. They can be performed in a number of different ways; by placing
water in a narrow-necked bottle, and warming it over the fire, we can
ascertain the expansion of liquids under the influence of heat. We may
in this way construct a complete thermometer.
We may now consider the _Sources of Heat_, or causes of its
development, which are various, and in many cases apparent. The first
great source is the Sun, and it has been calculated that the heat
received by the earth in one year is sufficient to melt an envelope
of ice surrounding it one hundred and five feet thick. Of course the
heat at the surface of the sun is enormously greater than this, about
one-half being absorbed in the atmosphere before it reaches us at all.
In fact, it is impossible to give you an idea of the enormous heat
given out by the sun to the earth (which is a _very_ small fraction
indeed of the whole), stars, and planets, all of which give out heat.
We know that heat is stored in the earth, and that it is in a very
active condition we can perceive from the hot springs, lava, and flame
which are continually erupting from the earth in various places. These
sources of heat are beyond our control.
But apart from the extra- and intra-terrestrial sources of heat there
are mechanical causes for its generation upon our globe, such as
friction, percussion, or compression. The savage or the woodman can
procure heat and fire by rubbing a pointed stick in a grooved log. The
wooden “breaks” of a locomotive are often set on fire by friction of
the wheels, so they require grease, and the wheels on the rails will
develop heat and sparks. Our matches, and many other common instances
of the generation of heat (and fire) by friction, will occur to every
reader. Water may be heated by shaking it in a bottle, taking care to
wrap something round it to keep the warmth of the hand from the glass.
By percussion, such as hammering a nail or piece of iron, the solid
bar may be made “red-hot”; and when cannon are bored at Woolwich the
shavings of steel are too hot to hold even if soap-and-water has been
playing upon the boring-machine.
The production of heat by chemical action is termed _combustion_, and
this is the means by which all artificial heat for our daily wants is
supplied. We can also produce heat by electricity. A familiar and not
always pleasant instance of this is seen in the flash of lightning
which will fuse metals, and experiment may do the same upon a smaller
scale. These are, in brief, the Sources of Heat, and we may speak of
its effects.
We may take it for granted that no matter from what source heat is
derived, it exhibits the same phenomena in its relation to objects. One
of the most usual of these phenomena is expansion. Let us take water,
and see the effect of heat upon it.
We know that a certain weight of water under the same conditions has
always the same volume; and although the attributes of the liquid
vary under different circumstances, under the _same_ conditions its
properties are exactly the same. Now, water expands very much when
under the influence of heat, like all liquids; solids and gases also
expand upon the application of heat.
We can easily establish these statements. A metallic ring when heated
is larger than when cool. A small quantity of air in a bladder when
heated will fill the bladder, and water will boil over the vessel, or
expand into steam, and perhaps burst the boiler. So expansion is the
tendency of what we term heat.
We make use of this quality of heat in the thermometer, by which we
can measure the temperature not only of liquids or solids, but of
the atmosphere. The reading of the thermometer varies in different
countries, for the degrees are differently marked, but the construction
of the instrument is the same. It is called thermometer from two
Greek words signifying the measure of heat. It is a notable fact that
Castelli, writing in 1638, says to Ferdinand Cæsarina: “I remembered
an experiment which Signor Galileo had shown me more than thirty-five
years ago. He took a glass bottle about the size of a hen’s egg, the
neck of which was two palms long, and as narrow as a straw. Having
well heated the bulb in his hands, he placed its mouth in a vessel
containing water, and withdrawing the heat of his hand from the bulb,
the water instantly rose in the neck more than a palm above the level
of the water in the vessel.”
Here, then, we have an air-thermometer, but as it was affected by the
_pressure_ as well as the temperature of the atmosphere, it could not
be relied upon as a “measurer of heat.” Until Torricelli propounded
the principle of the barometer, this “weather-glass” of Galileo was
used, for the philosopher divided the stem into divisions, and the
air-thermometer served the purpose of our modern instruments.
The actual inventor of the thermometer is not known. It has been
attributed to Galileo, to Drebbel, and to Robert Fludd. There is little
doubt, however, that Galileo and Drebbel were both acquainted with it,
but whether either claimed the honour of the invention, whether they
discovered it independently, or together, we cannot say. Sanctorio, of
Padua, and Drebbel have also been credited with the invention. We may
add that the spirit thermometer was invented in 1655-1656. It was a
rough form of our present thermometer, and roughly graduated. But it
was hermetically closed to the air, and a great improvement on the old
“weather-glass. Edmond Halley introduced mercury as the liquid for the
instrument in 1680. Otto von Guerike first suggested the freezing point
of water as the lowest limit, and Renaldini, in 1694, proposed that the
boiling and freezing points of water should be the limit of the scale.
Let us now explain the construction and varied markings of the three
kinds of thermometers in use. By noting the differences between the
scales every reader will be able to read the records from foreign
countries noted upon the Centigrade and Réaumur instruments, which are
all based upon the theory that heat expands liquids.
[We used to hear the expression, “Heat expands, and cold contracts,”
but we trust that all our readers have now learnt that there is no
such thing as _cold_. It is only a negative term. We feel things cold
because they extract some warmth from our fingers, not because the
substances have no heat.]
Thermometers are made of very fine bore glass tubes. One end has a
bowl, or bulb, the other is at first open. By heating the bowl the air
in the tube is driven away by the open end, which is quickly dipped in
a bowl of mercury. The mercury will then occupy a certain space in the
tube; and if it be heated till the liquid boils, all the air will be
driven out by the mercurial vapour. By once again dipping the tube in
the quicksilver the glass will be filled. Then, before it cools, close
the open end of the tube, and the thermometer is so far made. Having
now caught our thermometer we must proceed to mark it, which is an easy
process. By plunging the mercury into pounded melting ice we can get
the freezing point, and boiling water will give us the boiling point.
The intermediate scale can be then indicated.
If mercury and glass expanded equally there would be no rise in the
latter. Extreme delicacy of the thermometer can be arrived at by using
a very fine tube, particularly if it be also flat.
The freezing point in Fahrenheit’s scale is 32°; in the Centigrade
it is 0°, and the boiling point 100°. This was the scale adopted
by Celsius, a Swede, and is much used. Réaumur called the freezing
point 0°, and the boiling point 80°. There is another scale, almost
obsolete,—that of Delisle, who called boiling point zero, and freezing
point 150°.
There is no difficulty in converting degrees on one scale into degrees
on the other. Fahrenheit made his zero at the greatest cold he could
get; viz., snow and salt. The freezing point of water is 32° above his
zero. Therefore 212-32 gives 180° the difference between the freezing
and boiling points of water. So 180° Fahr. corresponds to 100° Cent.,
and to 80° Réaumur, reckoning from freezing point.
[Illustration: Fig. 79.—Thermometer.]
The following tables will explain the differences:—
TABLE I.
1° Fahr. = 0·55° Cent., or 0·44° Réaumur.
1° Cent. = ·80° Réaumur, or 1·80° Fahr.
1° Réaumur = 1·25° Cent., or 2·25° Fahr.
TABLE II.
|---------------------------+-------+-------+---------|
| | Fahr. | Cent. | Réaumur.|
|---------------------------+-------+-------+---------|
| Boiling point | 212 | 100 | 80 |
| | 194 | 90 | 72 |
| | 176 | 80 | 64 |
| | 158 | 70 | 56 |
| | 140 | 60 | 48 |
| | 122 | 50 | 40 |
| | 104 | 40 | 32 |
| | 86 | 30 | 24 |
| | 68 | 20 | 16 |
| | 50 | 10 | 8 |
| Freezing point of water | 32 | 0 | 0 |
| | 14 | -10 | -8 |
| | -4 | -20 | -16 |
| Freezing point of Mercury | -40 | -40 | -32 |
|---------------------------+-------+-------+---------|
Alcohol is used in thermometers in very cold districts, as it does not
freeze even at a temperature of -132° Fahr.
We have now explained the way in which we can measure heat by the
expansion of mercury in a tube. We can also find out that solids and
gases expand also. Engineers always make allowances for the effects
of winter and summer weather when building bridges; in summer the
bridge gets longer, and unless due provision were made it would become
strained and weakened. So there are compensating girders, and the
structure remains safe.
The effects of expansion by heat are very great and very destructive at
times. Instances of boilers bursting will occur to every reader. It is
very important to be able to ascertain the extent to which solid bodies
will expand. Such calculations have been made, and are in daily use.
We can crack a tumbler by pouring hot water into it, or by placing it
on the “hob.” A few minutes’ consideration will assure us that the
lower particles of the glass expanded before the rest, and cracked our
tumbler. A gradual heating, particularly if the glass be thin, will
ensure safety. Thick glass will crack sooner than thin.
Again, many people at railway stations have asked us, “Why don’t they
join the rails together on this line?” We reply that if every length
of rail were tightly fixed against its neighbour, the whole railway
would be displaced. The iron expands and joins up close in hot weather.
In wet weather, also, the wooden pegs and the sleepers swell with
moisture, and get tightened up. Everyone knows how much more smoothly
a train travels in warm, wet weather. This is due to the expansion of
the iron and the swelling of the sleepers and pegs in the “chairs.”
A railway 400 miles long expands 338 yards in summer,—that is the
difference in length between the laid railroad in summer and in winter.
This can be proved. Iron expands 0·001235 of its length for every
180° Fahr. Divided by 180 it gives us the expansion for 1°, which is
0·00000686, taking the difference of winter and summer at 70° Fahr.
Multiply these together, and the result (0·00048620 of its length)
by the number of yards in 400 miles, and we find our answer 338
yards. Expansion acts in solids and most liquids by the destruction
of cohesion between the particles. Gases, however, having much less
cohesion amid the particles, will expand far more under a given heat
than either solids or liquids, and liquids expand more than solids for
the same reason, and more rapidly at a high temperature than at a low
one.
We have spoken of expansion. We may give an instance in which the
subsequent contraction of heated metal is useful. Walls sometimes
get out of the perpendicular, and require pulling together. No force
which can be conveniently applied would accomplish this so well as the
cooling force due to the potential energy of iron. Rods are passed
through the walls and braced up by nuts. The rods are then heated, and
as they cool they contract and pull the walls with them.
When glass is suddenly cooled, the inner skin, as it were, presses with
great force against the cooled surface, but as it is quite tight no
explosion can follow. But break the tail, or scratch it with a diamond,
and the strain is taken off. The glass drop crumbles with the effect
of the explosion, as in the cases of Prince Rupert’s drops, and the
Bologna flasks; the continuity is broken, and pulverization results.
But a very curious exception to the general laws of expansion is
noticed in the case of nearly freezing water. We know water expands
by heat, at first gradually, and then to an enormous extent in steam.
But when cooling water, instead of getting more and more contracted,
only contracts down to 39·2° Fahr., it then begins to expand, and at
the moment it freezes into ice it expands very much—about one-twelfth
of its volume, but according to Professor Huxley it weighs exactly the
same, and the steam produced from that given quantity of water will
weigh just exactly what the water and the ice produced by it weigh
individually. At 39·2° Fahr. water is at its maximum density, or in
other words, a vessel of a certain size will hold more water when it
is at 39° Fahr. than at any other time. Whether the water be heated or
cooled at this temperature, it _expands_ to the boiling or freezing
point when it becomes steam or ice, as the case may be.
Water, when heated, is lighter than cold water. You can prove this in
filling a bath from two taps of hot and cold water at the same time.
The cold falls to the bottom, and if you do not stir up the water when
mixed you will have a hot surface and a cold foundation. The heat
increases the volume of water, it becomes lighter, and comes uppermost.
Steam and Water and Ice are all the same things under different
conditions, although to the eye they are so different. They are alike
inasmuch as a given weight of water will weigh as much when converted
into ice or developed into steam. The half ounce of water will weigh
half an ounce as ice or as steam, but the volume or bulk will vary
greatly, as will be understood when we state that one cubic inch of
water will produce 1,700 cubic inches of steam, and 1-1/11 cubic inch
of ice; but at the same time each will yield, when decomposed, just the
same amount of oxygen and hydrogen.
Let us now consider the _Effects of Heat upon Water_. We have all seen
the vapour that hangs above a locomotive engine. We call it “steam.” It
is not pure steam, for steam is really invisible. The visible vapour is
steam on its way to become water again. On a very hot dry day we cannot
distinguish the vapour at all.
The first effect of heat upon water is to expand it; and as the heat
is applied we know that the water continues to expand and bubble up;
and at last, when the temperature is as high as 212°, we say water
“boils”—that is, at that heat it begins to pass away in vapour, and you
will find that the temperature of the steam is the same as the boiling
water. While undergoing this transformation, the water increases in
volume to 1,700 times its original bulk, although it will weigh the
same as the water. So steam has 1,700 less specific gravity than water.
It is perhaps scarcely necessary to remind our readers that water,
when heated, assumes tremendous force. Air likewise expands with great
violence, and the vessels containing either steam or air frequently
burst, with destructive effects. Solid bodies also expand when heated,
and the most useful and accurate observations have been made, so that
the temperatures at which solid bodies expand are now exactly known.
Air also expands by heat.
While speaking of Expansion by Heat, we may remark that a rapid
movement is imparted to the air by Heat. In any ordinary room the air
below is cool, while if we mount a ladder to hang up a picture, for
instance, we shall find the air quite hot near the ceiling. This is
quite in keeping with the effects of heat upon water. The hot particles
rise to the top in a vessel, and thus a motion is conveyed to the
water. So in our rooms. The heated air rushes up the chimney and causes
a draught, and this produces motion, as we have seen by fig. 39, in
which the cardboard spiral was set in motion by heated air. A balloon
will ascend, because it is filled with heated air or gas; and we all
have seen the paper balloons which will ascend if a sponge containing
spirit of wine be set on fire underneath them.
Winds are also only currents of air produced by unequal temperature in
different places. The heated air ascends, and the colder fluid rushes
in sometimes with great velocity to fill the space. “Land” and “sea”
breezes are constant; the cool air blows in from the sea during the
day, and as the land cools more rapidly at night, the breeze passes
out again. When we touch upon _Meteorology_, we will have more to say
respecting Air Currents and the various Atmospheric Phenomena.
We know that water can be made to boil by heat, but it is not perhaps
generally known that it will apparently boil by _cold_, and the
experiment may thus be made:—A flask half-full of water is maintained
at ebullition for some minutes. It is removed from the source of heat,
corked, inverted, and placed in one of the rings of a retort stand. If
cold water is poured on the upturned bottom of the flask, the fluid
will start into violent ebullition. The upper portion of the flask is
filled with steam, which maintains a certain pressure on the water.
By cooling the upper portion of the flask some of this is condensed,
and the pressure reduced. The temperature at which water boils varies
with the pressure. When it is reduced, water boils at a lower heat. By
pouring the cold water over the flask we condense the steam so that the
water is hot enough to boil at the reduced pressure. To assert that
water boils by the application of cold is a chemical sophism.
_Ebullition_ and _Evaporation_ may be now considered, and these are the
two principal modes by which liquids assume the gaseous condition. The
difference is, when water boils we term it ebullition (from the Latin
_ebullio_, I boil); evaporation means vapour given out by water not
boiling (from _evaporo_, I disperse in vapour).
There are two operations based upon the properties which bodies possess
of assuming the form of vapour under the influence of heat, which
are called _Distillation_ and _Sublimation_. These we will consider
presently.
Ebullition then means a bubbling up or boiling; and when water is
heated in an open vessel two forces oppose its conversion into vapour;
viz., its own cohesive force and atmospheric pressure. At length, at
212° Fahr., the particles of water have gained by heat a force greater
than the opposing forces; bubbles of vapour rise up from the bottom and
go off in vapour. This is _ebullition_, and at that point the tension
of the vapour is equal to the pressure of the atmosphere, for if not,
the bubbles would not form. All this time of boiling, notwithstanding
any increase of heat, the thermometer will not rise above 212° (Fahr.),
for all the heat is employed in turning the water to steam.
We have said the ebullition takes place at 212° Fahr. (or 100° C.),
but that is only at a certain level. If we ascend 600 feet high we
shall find that water will boil at a less temperature; and on the top
of a mountain (say Mont Blanc) water will boil at 185° Fahr.; so at
an elevation of three miles water boils at a temperature less by 27°
Fahr. An increase of pressure similarly will raise the boiling point
of water. The heights of mountains are often ascertained by noticing
the boiling point of water on their summits, the general rule being a
fall of one degree for every 530 feet elevation at medium altitudes. We
append a few instances taken at random:—
+--------------------+------------------+-------------+---------------+
| Place. |Height above level| Barometer | Boiling point |
| | of the sea—Feet. | mean height.|of water, Fahr.|
+--------------------+------------------+-------------+---------------+
| Quito | 9,541 | 20·75 | 194·2 |
| Mexico | 7,471 | 22·52 | 198·1 |
| St. Gothard | 6,808 | 23·07 | 199·2 |
| Garonne (Pyrenees) | 4,738 | 24·96 | 203·0 |
| Geneva | 1,221 | 28·54 | 209·5 |
| Paris (1st floor) | 213 | 29·69 | 211·5 |
| Sea level | 0 | 30·00 | 212·0 |
+--------------------+------------------+-------------+---------------+
[The difference for a degree depends upon the height, varying between
510 and 590 feet, according to the elevation reached. The approximate
height of a mountain can be found by multiplying 530 by the number
of degrees between the boiling point and 212°. In some very elevated
regions travellers have even failed to boil potatoes.]
The boiling point of liquid may be altered by mixing some substance
with it; and although such a substance as sawdust would not alter the
boiling point of water, yet if the foreign matter be dissolved in the
liquid it will alter the boiling point. Even the air dissolved in
liquids alters their boiling point, and water freed from air will not
boil till it is raised to a temperature much higher than 212° Fahr.
Water will boil at a higher temperature in a glass vessel than in
metal, because there is a greater attraction between water and glass.
We said above that an increase of pressure will raise the boiling point
of water. Under the pressure of one atmosphere—that is, when there
is a pressure of 15 lbs. on the square inch—water boils at 212°. But
under a pressure of two atmospheres, the boiling point rises to 234°,
and of four atmospheres, 294°. So we see by increasing the pressure
the water may be almost indefinitely heated, and it will not boil. We
can understand that in a very deep vessel the layer of water at the
bottom has to sustain the pressure of the water in addition to the
weight of the atmosphere above it. The pressure of thirty-four feet of
water is equal to the atmospheric pressure of 15 lbs. on the square
inch, and thus at such a distance water must be heated to 234° before
it will boil. Professor Bunsen founded his Theory of the Geysers upon
this fact, for he maintained that water falling into the earth lost
much air, and required with the super-incumbent pressure a very high
temperature to boil it. When it did boil it generated steam so suddenly
that it exploded upwards, throwing up vapour and the water with it, as
water poured into a very hot basin will do.
_Evaporation_ may now be considered, and is distinguished from
Ebullition by the production of vapour on the _surface_ of liquids, the
latter term signifying the formation of vapour in _the body_ of the
liquid. Evaporation takes place at all temperatures, and from every
liquid surface exposed to the air. We know what we call a “drying
wind.” The air in fresh layers continually passing over the wet ground,
takes up the moisture; like the east wind, for instance, which has
great capabilities of that nature. Damp air can only take up a certain
quantity, and when it contains as much water as corresponds to the
temperature it can take no more, and is “saturated with moisture”;
then evaporation ceases. Heat is a great cause of evaporation, and
the greater the surface the more rapid the process, and in a vacuum
more readily than in atmospheric air. Evaporation is resorted to
very commonly to produce coolness; for instance, the universal fan,
by increasing evaporation from a heated skin, generates a feeling
of coolness; and we know the vaporization of ether will freeze into
insensibility. When a fluid evaporates we can tell that the heat passes
away at the same time, for we cool water in porous jars, which permit
some of it to pass off in vapour, the remainder being cooled.
Sir John Leslie invented a method of freezing water by rapid
evaporation on sulphuric acid under the receiver of an air-pump, and
water has been frozen even on a _hot plate_ by these means. By pouring
sulphurous acid and water on this plate, the acid evaporates so quickly
that it produces sufficient cold to freeze the water it quitted into
solid ice.
We leave the phenomena of clouds and watery vapour in the atmosphere
for consideration on another opportunity, under the head of
_Meteorology_, _Rain_, etc.
[Illustration: Fig. 80.—Apparatus for freezing carafes of water.]
An experiment is often performed by which water is frozen in a vacuum.
By putting a saucer full of water under the receiver of an air-pump
it will first boil, and then become a solid mass of ice. It is not
difficult to understand the cause of this. The water boils as soon as
the air is removed; but in order to pass from the liquid to the gaseous
state without the assistance of exterior heat, it gives out heat to the
surroundings, and in so doing becomes ice itself. This fact Mr. Carré
has made use of in the apparatus shown above (fig. 80). A small pump
creates a vacuum in the water bottles, and ice is formed in them.
This apparatus might easily be adopted in country houses, and in places
where ice is difficult to procure in summer. The only inconvenience
attending it is the employment of sulphuric acid, of which a
considerable quantity is used to absorb the vapour from the water, as
already referred to. If proper precautions are taken, however, there
will be no danger in using the apparatus.
The mode of proceeding is as follows:—The bottle full of water is
joined to the air-pump by a tube, and after a few strokes the water
is seen in ebullition. The vapour thus disengaged traverses an
intermediate reservoir filled with sulphuric acid, which absorbs it,
and immediately condenses it, producing intense cold. In the centre of
the liquid remaining in the carafe some needles of ice will be seen,
which grow rapidly, and after a few more strokes of the pump the water
will be found transformed into a mass of ice. This is very easy of
accomplishment, and in less than a minute the carafe full of water will
be found frozen.
The problem for the truly economical formation of ice by artificial
means is one of those which have occupied chemists for a long time, but
hitherto, notwithstanding all their efforts, no satisfactory conclusion
has been arrived at. Nearly every arrangement possesses some drawback
to its complete success, which greatly increases the cost of the ice,
and causes inconvenience in its production. The usual mode in large
towns is to collect the ice, in houses constructed for the purpose,
during the winter, and this simple method is also the best, so far as
at present has been ascertained.
[Illustration: Fig. 81.—Retort and Receiver.]
In connection with vaporization we may now mention two processes
referred to just now (page 83); viz., sublimation and distillation.
The former is the means whereby we change solid bodies into vapour
and condense the vapour into proper vessels. The condensed substances
when deposited are called _sublimates_, and when we go into Chemistry
we shall hear more of them. The mode of proceeding is to place the
substance in a glass tube, and apply heat to it. Vapour will be formed,
and will condense at the cool end of the tube. The sublimate of sulphur
is called “Flowers of Sulphur,” and that of perchloride of mercury
“Corrosive Sublimate.”
Distillation is a more useful process, or, at any rate, one more
frequently employed, and is used to separate a volatile body from
substances not volatile. A distilling apparatus (_distillo_, to drop)
converts a liquid to vapour by means of heat, and then condenses it by
cold in a separate vessel.
The distilling apparatus consists of three parts,—the vessel in which
the liquid is heated (the still, or retort), the condenser, and the
receiver. The simple retort and receiver are shown in fig. 81. But when
very volatile vapours are dealt with, the arrangement shown on next
page is used (fig. 82). Then the vapour passes into the tube encased
in a larger one, the intervening space being filled with cold water
from the tap above (_c_), the warm water dropping from _g_. The vapours
are thus condensed, and drop into the bottle (or receiver) B.
[Illustration: Fig. 82.—Distilling apparatus.]
The apparatus for distilling spirits is shown below. The “still”
A is fitted into a furnace, and communicates with a worm
O in a metal cylinder filled with water, kept constantly
renewed through the tube TT′. This spirit passes through the
spiral, and being condensed, goes out into the receiver C.
[Illustration: Fig. 83.—Spirit still.]
There are even more simple apparatus for spirit distilling, but the
diagram above will show the principle of all “stills.” In former days,
in Ireland, whiskey was generally procured illegally by these means.