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1509. Latent heat of vapour. When a liquid is converted
into vapour, a certain quantity of heat is absorbed and rendered
latent in the vapour.



The vapour which proceeds from the
liquid has the same temperature as
the liquid. It can be shown, how-
ever, experimentally, that, weight fur
weight, it contains much more heat.
To render this manifest, let B, Jig.
446., be a vessel containing water,
which is kept in the state of ebulli-
tion and at the temperature of 212
by means of a lamp, or any other source of heat. Let the steam
be conducted by a pipe c to a vessel A, which contains a quantity
of water at the temperature of 32. The steam issuing from the
pipe is condensed by the cold water, and mixing with it, gra-
dually raises its temperature until it attains the temperature of
212, after which the steam ceases to be condensed, and escapes
in bubbles at the surface, as common air would if driven into
the water from the pipe.

If the quantity of water in A be weighed before and after this
process, its weight will be found to be increased in the ratio of
11 to 13. Thus 11 Ibs. of water at 32, mixed with 2 Ibs. of
water in the form of steam at 212, have produced 13 Ibs. of
water at 212, so that the 2 Ibs. of water which were introduced
in the form of steam at 21 2 have been changed from the va-
porous to the liquid state, retaining however their temperature
of 212, and have given to 11 Ibs. of water which were pre-
viously in A at 32 as much heat as has been sufficient to raise
that quantity to 212.

It follows, therefore, that any given weight of water in the
form of steam at 212 contains as much heat latent in it as is
sufficient to raise 5^ times its own weight of water from 32 to
212, that is, through 180 of the thermometric scale.

If it be assumed that to raise a pound of water through 180
requires 180 times as much heat as to raise it one degree, it
will follow that the quantity of latent heat contained in a pound
of Avater in the form of steam at 212 is 5 x 180990 times as
much as would raise a pound of water through one degree.

This fact is usually expressed by stating that steam at 212
contains 990 degrees of latent heat.

The same important fact can also be made manifest in the
following manner. Let a lamp, or any source of heat which acts


in a regular and uniform manner, be applied to a vessel con-
taining any given quantity of water which is at 32 when the
process commences, and let the time be observed which the
lamp takes to raise the water to 212. Let the lamp continue
to act in the same uniform manner until all the water has been
converted into steam, and it will be found that the time neces-
sary for such complete evaporation will be exactly 5^- times
that which was necessary to raise the water from the freezing
to the boiling point. In a word, it will require 5^ times as
long an interval to convert any given quantity of water into
steam as it will take to raise the same quantity of water, by the
same source of heat, from the freezing to the boiling point ; and
consequently it follows, that 5 times as much heat is absorbed
in the evaporation of water, as is necessary to raise it without
evaporation through 180 of temperature.

1510. Different estimates of the latent heat of the vapour of
water. Different experimental inquirers have estimated the
heat rendered latent by Avater in the process of evaporation at
2 12 as follows:






- 950

- 945

- 1000

- 1004'8

- 955'8

Kegnault - - - 967 -5

Fabre and Silbermann - - - 964-8

In round numbers, it may therefore be stated that as much
heat is absorbed in converting a given quantity of water at
212 into steam as would be sufficient to raise the same quantity
of water to the temperature of 1200 when not vaporized.

1511. Heat absorbed in evaporation at different temperatures.
It was observed at an early epoch in the progress of dis-
covery, that the heat absorbed in vaporization was less as the
temperature of the vaporizing liquid was higher. Thus a given
weight of water vaporized at 212, absorbs less heat than would
the same quantity vaporized at 180. It was generally assumed
that the increase of latent heat, for lower as compared with
higher temperatures, was equal to the difference of the sensible
heats, and consequently, that the latent heat added to the sensible



heat, for the same liquid, must always produce the same sum.
Thus, if water at 212 absorb in vaporization 950 of heat,
water at 262 would only absorb 900, and water at 162 would
absorb 1000.

The simplicity of this result rendered it attractive, and, as
the general result of experiments appeared to be in accordance
with it, it was generally adopted. M. Regnault has, however,
lately submitted the question, not only of the latent heat of
steam, but also its pressure, temperature, and density, to a
rigorous experimental investigation, and has obtained results
entitled to more confidence, and which show that the sum of
the latent and sensible heats is not rigorously constant.

1512. Latent heat of vapour of water ascertained by Regnault.
The pressures and densities obtained by M. Regnault are
in accordance with those given in (1494). The latent heats
are given in the following table, where I have given their
sums, and shown what does not seem to have been hitherto
noticed, that they increase by a constant difference.

Sum of

Sum of



Latent Heat


ifeat 1 *

Latent Heat

Sensible Heat.

Sensible Heat.


















1 198-4























































It appears, therefore, that the sum of the latent and sensible
heats is not constant, but increases by a constant difference, a
difference however which, compared with the sum itself, is very
small, and for limited ranges of the thermometric scale, when
extreme accuracy is not required, may be disregarded.

1513. Latent heat of other vapours ascertained by Fabre
and Silbermann. The latent heat of the vapours of other
liquids have been ascertained by MM. Fabre and Silbermann,
and are given, as well as the specific heats, in the following
table :



Names of Substances.








Carburetted hydrogen




Pyroligneous acid
Alcohol, absolute












Ether, sulphuric




Acid, formic
















Ether, acetic




Butyrate of Metylfi





Essence of turpeni





Terebene -
Oil of lemons




1514. Condensation of vapour. Since by continually impart-
ing heat to any body in the liquid state it at length passes into
the form of vapour, analogy suggests that by continually with-
drawing heat from a body in the vaporous state, it must ne-
cessarily return to the liquid state ; and this is accordingly
generally true. The vapour being exposed to cold is deprived
of a part of that heat which is necessary to sustain it in the
aeriform state, and a part of it is accordingly restored to the
liquid form, and this continues until by the continual abstrac-
tion of heat the whole of the vapour becomes liquid; and as a
liquid, in passing to the vaporous form, undergoes an immense
expansion or increase of bulk, so a vapour in returning to the
liquid form undergoes a corresponding and equal diminution of
bulk. A cubic inch of water, transformed into steam at 212,
enlarges in magnitude to nearly 1700 cubic inches. The same
steam being reconverted into water, by abstracting from it the
heat communicated in its vaporization, will be restored to its
former bulk, and Avill form one cubic inch of water at 212.
Vapours arising from other liquids will undergo a like change,
differing only in the degree of diminution of volume which they
suffer respectively. The diminished space into which vapour
is contracted when it passes into the liquid form, has caused this
process to be called condensation.

1515. Why vessels in which liquids are boiled are not de-
stroyed by extreme heat. The absorption of heat in the process
by which liquids are converted into vapour will explain why a
vessel containing a liquid that is constantly exposed to the action

106 HEAT.

of fire can never receive such a degree of heat as would destroy
it. A tin kettle containing water may be exposed to the action of
the most fierce furnace, and I'emain uninjured ; but if it be ex-
posed without containing water to the most moderate fire, it
will soon be destroyed. The heat which the fire imparts to the
kettle containing water, is immediately absorbed by the steam
into which the water is converted. So long as water is contained
in the vessel, this absorption of heat will continue ; but if any
part of the vessel not containing water be exposed to the fire,
the metal will be fused, and the vessel destroyed.

1516. Uses of latent heat of steam in domestic economy.
The latent heat of steam may be used with convenience for
many domestic purposes. In cookery, if the steam raised from
boiling water be allowed to pass through meat or vegetables,
it will be condensed upon their surface, imparting to them the
latent heat which it contained before its condensation, and thus
they will be as effectually boiled as if they were immersed in
boiling water.

1517. Method of warming dwelling -houses. In dwelling-
houses where pipes convey cold water to different parts of
the building, steam pipes carried through the building will
enable hot water to be procured in every part of it with speed
and facility. The cock of the steam pipe being immersed in a
vessel containing cold water, the steam which escapes from it
will be condensed by the water, which receiving the latent heat
will soon be raised to any required temperature below the
boiling point. "Warm baths may thus be prepared in a few
minutes, the water of which would require a long period to

1518. Effects of the temperatures of different climates on
certain liquids. The variations of temperature incident to any
part of the globe are included within narrow limits, and these
limits determine the bodies which are found to exist there most
commonly in the solid, liquid, or gaseous state.

A body whose boiling point is below the lowest temperature
of the climate must always exist in the state of vapour or gas ;
and one whose point of fusion is above the highest tem-
perature must always be solid. Bodies whose point of fusion is
below the lowest temperature, while their boiling point is above
the highest temperature, will be permanent liquids. A body
whose point of fusion is a little above the lowest limit of the


temperature, will exist generally as a liquid, but occasionally as
a solid. Water in these climates is an example of this. A
liquid, on the other hand, whose boiling point is a little below
the highest limit of temperature, will generally exist in the
liquid, but occasionally in the gaseous form. Ether in hot
climates is an example of this, its boiling point being 98.

Some bodies are only permanently retained in the liquid
state by the atmospheric pressure. Ether and alcohol are
examples of these. If these liquids be placed under the re-
ceiver of an air-pump, and the pressure of the air be par-
tially removed, they will boil at the common temperature of
the air.



1519. Good and bad conductors. When heat is imparted to
one part of any mass of matter, the temperature of that part
is raised above that of the other parts. This inequality,
however, is only temporary. The heat gradually diffuses
itself from particle to particle throughout the volume of the
body, until a perfect equilibrium of temperature has been
established. Different bodies exhibit a different facility in
this gradual transmission of heat. In some it passes more
rapidly from the hotter to the colder parts than in others.
Those bodies in which it passes easily and rapidly, are good
conductors. Those in which the temperature is equalized
slowly, are bad conductors.

1520. Experimental illustration of conduction. Let AB,

Fig. 447.

Jig. 447., be a bar of metal having a large cavity formed at its
extremity A, and having a series of small cavities formed at

108 HEAT.

equal distances throughout its length at TJ, T 2 , T 3 , &c. Let
the bulbs of a series of thermometers be immersed in mercury
in these cavities severally. These thermometers will all
indicate the same temperature, being that of the bar AB.

Let the large cavity A, at the end of the bar, be filled with
mercury at a high temperature, 400 for example.

After the lapse of some minutes the thermometer T will
begin to rise ; after another interval the thermometer T 2 will
begin to be affected ; and the others, T 3 , T 4 , &c., will be suc-
cessively affected in the same way ; but the thermometer T I} by
continuing to rise, will indicate a higher temperature than T 2 ,
and T 2 a higher temperature than T 3 , and so on. After the
lapse of a considerable time, however, the thermometer TJ will
become stationary. Soon afterwards T 2 , having risen to the
same point, will also become stationary; and, in the same
manner, all the others having successively risen to the same
point, will become stationary.

If several bars of different substances of equal dimensions be
submitted to the same process,
the thermometers will be more or
less rapidly affected according as
they are good or bad conductors.
An apparatus by which this is ex-
Fig. 448. hibited in a striking manner is
represented in Jig. 448. A series

of rods of equal length and thickness are inserted at the same
depth in the side of a rectangular vessel, passing across the inte-
rior of the vessel to the opposite side. The rods, which are silver,
copper, iron, glass, porcelain, wood, &c., are previously covered
with a thin coating of wax, or any other substance which will
melt at a low temperature. Boiling water or heated mercury
is poured into the vessel, and imparts heat to those parts of the
rods which extend across it. It is found that the heat as it
passes by conduction along the rods, melts the wax from their
surface. Those which are composed of the best conductors
silver, for example will melt off the wax most rapidly; the less
perfect conductors less rapidly ; and on the rods composed of the
most imperfectly conducting materials, such as glass or porce-
lain, the wax will not be melted beyond a very small distance
from the point where the rod enters the vessel.

1521. Table of conducting powers. By experiments con-


ducted on this principle, it has been found that the conducting
powers of the subjoined substances are in the ratio here ex-
pressed, that of gold being 100.

Gold - - - 100-00
Platinum - - 98'10

Silver - - - 97-30
Copper ... 89-82
Iron ... 37-41
Zinc ... 36-37

Tin ... 30-38
Lead - - - 17"96
Marble - - - 2-34
Porcelain - - 1'22
Brick earth - - M3

It is evident, therefore, that metals are the best conductors of
heat, and in general the metals which have the greatest specific
gravity are the best conductors, as will appear by comparing
the preceding numbers with those given in the table of specific
gravity (782). It is also found that among woods, with some
exceptions, the conducting power increases with the density.
The conducting power of nut wood, however, is greater than
that of oak.

Bodies of a porous, soft, or spongy texture, and more espe-
cially those of a fibrous nature, such as wool, feathers, fur, hair,
&c., are the worst conductors of heat.

1522. Liquids and gases are non-conductors. Liquids are
almost absolute non-conductors. Let a tall narrow glass vessel
having a cake of ice at the bottom be filled with strong alcohol
at 32. Let two thermometers be immersed in it, one near the
surface, and the other at half the depth. If the alcohol be in-
flamed at the surface, the thermometer near the surface will
rise, but that which is at the middle of the depth will be
scarcely aiFected, and the ice at the bottom will not be dis-

Bodies in the gaseous state are probably still more imperfect
conductors than liquids.

1523. Temperature, equalized in these by circulation The

equilibrium of temperature is, however, maintained in liquid and
gaseous bodies by other principles, which are more prompt in
their action than the conductibility even of the solids which
possess that quality in the highest degree. When the strata of
fluids, whether liquid or gaseous, are heated, they become by
expansion relatively lighter than those around them. If they
have any strata above them, which generally happens, they
rise by their buoyancy, and the superior strata descend. There
are thus two systems of currents established, one ascending and

110 HEAT.

the other descending, by which the heat imparted to the fluid is
transfused through the mass, and the temperature is equalized.

1524. Conducting power diminished by subdivision and pul-
verization. The conducting power of all bodies is diminished
by pulverizing them, or dividing them into fine filaments.
Thus sawdust, when not too much compressed, is one of the
most perfect non-conductors of heat. A casing of sawdust is
found to be the most effectual method of preventing the escape
of heat from the surface of steam boilers and steam pipes.

If, however, the sawdust be either much compressed on the
one hand, or too loosely applied on the other, it is not so perfect
a non-conductor. In the one case, the particles being brought
into closer contact, transmit heat from one to another ; and in
the other case, the air circulating too freely among them, the
currents are established by which the heat is transfused through
the mass.

To produce, therefore, the most perfect non-conductor, the
particles of the body must have naturally little conductibility,
and they must be sufficiently compressed to prevent the circu-
lation of currents of air among them, and not sufficiently com-
pressed to give them a facility of transmitting heat from par-
ticle to particle by contact.

1525. Beautiful examples of this principle in the animal
economy. The animal economy presents numerous and beau-
tiful examples of the fulfilment of these conditions. It is
generally necessary to the well-being of the animal to have a
temperature higher than that of the medium which it in-
habits. In the animal organization, there is a principle by
which heat is generated. This heat has a tendency to escape
and to be dissipated at the surface of the body, and the rate
at which it is dissipated depends on the difference between the
temperature of the surface of the body and the temperature of
the surrounding medium. If this difference were too great, the
heat would be dissipated faster than it is generated, and a loss
of heat would take place, which, being continued to a certain
extreme, would destroy the animal.

Nature has provided an expedient to prevent this, which
varies in its efficiency according to the circumstances of the
climate and the habits of the animal.

1526. Uses of the plumage of birds. The plumage of birds
is composed of materials which are bad conductors of heat, and


are so disposed as to contain in their interstices a great quantity
of air without leaving it space to circulate. For those species
which inhabit the colder climates a still more effectual provision
is made, for, under the ordinary plumage, which is adapted to
resist the wind and rain, a still more fine and delicate down is
found, which intercepts the heat which would otherwise escape
through the coarser plumage. Perhaps the most perfect insu-
lator of heat is swansdown.

1527. The wool and fur of animals The wool and fur of

animals are provisions obviously adapted to the same uses.
They vary not only with the climate which the species inhabits,
but in the same individual they change with the season. In
warm climates the furs are in general coarse and sparse, while
in cold countries they are fine, close, light, and of uniform tex-
ture, so as to be almost impermeable to heat.

1528. The bark of vegetables. The vegetable, not less than
the animal kingdom, supplies striking illustrations of this prin-
ciple. The bark, instead of being hard and compact, like the
wood which it clothes, is porous, and in general formed of dis-
continuous laminas and fibres, and for the reasons already ex-
plained is a bad conductor of heat, and thus prevents such a
loss of heat from the surface of the wood under it as would be
injurious to the tree.

A tree stripped of its bark perishes as an animal would if
stripped of its fleece, or a bird of its plumage.

1529. Properties of the artificial clothing of man. Man is
endowed with faculties which enable him to fabricate for him-
self covering similar to that which nature has provided for
other animals; and "where his social condition is not sufficiently
advanced for the accomplishment of this, his object is attained
by the conquest of inferior animals whose clothing he appro-

Clothes are generally composed of some light non-conducting
substances which protect the body from the inclement heat or
cold of the external air. In summer clothing keeps the body
cool ; in winter, warm. Woollen substances are worse con-
ductors of heat than cotton, cotton than silk, and silk than
linen. A flannel shirt more effectually intercepts heat than
cotton, and a cotton than a linen one.

1530. Effects of snow on the soil in winter. What the
plumage does for the bird, wool for the animal, and clothing for

112 HEAT.

the man, snow does in winter for the soil. The farmer and the
gardener look with dismay at a hard and continued frost which
is not preceded by a fall of snow. The snow is nearly a non-
conductor, and, when sufficiently deep, may be considered as
absolutely so. The surface may therefore fall to a temperature
greatly below 32, but the bottom in contact with the vegetation
of the soil does not share in this fall of temperature, remaining
at 32, a temperature at that season not incompatible with the
vegetable organization. Thus the roots and young shoots are
protected from a destructive cold.

1531. Matting upon exotics. The gardener who rears exotic
vegetables and fruit-trees, protects them from the extreme cold
of winter by coating them with straw, matting, moss, and other
fibrous materials which are non-conductors.

1532. Method of preserving ice in hot climates. If we would
preserve ice from dissolving, the most effectual means would be
to wrap it in blankets. Ice-houses may be advantageously sur-
rounded with sawdust, which keeps them cold by excluding
the heat, by the same property in virtue of which it keeps steam
boilers warm by including the heat.

Air being a bad conductor of heat, ice-houses are sometimes
constructed with double walls, having a space between them.
This expedient is still more effectual, if the space be filled with
loose sawdust.

1 533. Glass and porcelain vessels, why broken by hot water.
Glass and porcelain are slow conductors of heat, which explains
the fact that vessels of this material are so often broken by sud-
denly pouring hot water into them. If it be poured into a glass
tumbler, the bottom, with which the water first comes in contact,
expands, but the heat not passing freely to the upper part, this
expansion is limited to the bottom, which is thus forced from the

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 13 of 45)