large mass of sealing-wax were to be gradually melted by
agitation in a pan of liquid sealing-wax over the fire. As
the heat was conveyed to the lump of wax, envelope after
envelope would become liquid and drop off, mixing "with the
liquid mass until a very small solid nucleus was left : but as
long as there was left a solid nucleus, however small, we
should surely be entitled to assume that the temperature of the
centre of this nucleus was lower than that of the melted wax.
We imagine that there is no impossibility in conceiving
that something of the same kind takes place in ice. Heat
:
102 CHANGE OF STATE.
will no doubt be conveyed into the interior of a block of ice
that is left for a long time in water a little above o, but this
heat (as remarked by Principal Forbes) will exhibit its action
rather in diminishing the size of the block of ice than in
completely equalizing its temperature throughout.
105. Assuming this objection to be answered by these
remarks, there are three questions started by the hypothesis
which can only be decided by experiment.
1. Is the interior of a block of ice in fact colder than
the exterior?
2. Is the interior of such a block harder than the exterior ?
3. Does soft ice possess more latent heat than hard ice ?
With regard to the first of these points certain experi-
ments made by Principal Forbes would seem to indicate that
the interior of a block of ice is slightly colder than the ex-
terior. For, in the first place, he found that a thermometer
buried in the heart of a block of ice fell decidedly below
oC, and he also found that rapidly-pounded ice was colder
than melting ice. This last experiment has been tried by
the author of this work with the same result. With re-
gard to the second point, Principal Forbes has remarked
that the surface of a block of ice is much softer than hard
cold ice. With respect to the third point, Person's deduc-
tions from Regnault's experiments are in favour of the view
that soft ice possesses more latent heat than hard ice. On
the whole, we think the gradual liquefaction of ice is a view
which appears not only to be supported by analogy, but to
be the best explanation of observed facts : nevertheless it
would be desirable that this view should be confirmed by
further experiments.
106. Substances which change their composition
in passing from the liquid to the solid state. When
a solid is dissolved in a liquid until it refuses to dissolve
any further, we have what is termed a saturated solution.
LIQUEFACTION AND SOLIDIFICATION. 103
But what is a saturated solution at one temperature will not
be so at another. In general, a hot liquid dissolves more
than a cold liquid. The consequence is that, if the tem-
perature of a saturated solution be diminished, we have a
deposition of solid matter in the shape of crystals, and the
liquid which is left behind is saturated for the reduced tem-
perature. If the solution contain two salts of unequal solu-
bility, of different crystalline .forms, and having no chemical
action upon each other, a greater or less separation of these
two salts may be produced by crystallization; by this means
nitre is purified from common salt.
107. Solutions are subject to the same anomalies as water
and the like liquids. Thus if we have a solution of Glauber's
salt at a high temperature, and if it be allowed to cool
gradually and at rest without the admission of air, it will
retain the salt in solution, even though the temperature be
much reduced. But if it be agitated, or if air be admitted,
or, better still, if a crystal of Glauber's salt be dropped into
it, crystallization will immediately commence, attended, as
in the case of water, with a rise of temperature.
Mr. C. Tomlinson has lately made a series of very inter-
esting experiments on supersaturated saline solutions, and
has come to the conclusion that those substances which
induce crystallization do so in consequence of not being
chemically clean. The subject has likewise been inves-
tigated by Professor Liversidge and Mr. John M. Thomson.
This last experimenter believes, as the results of his expe-
riments, that truly isomorphous bodies that is substances
possessing the same chemical composition and crystalline
form are active in causing the sudden crystallization of
each other.
108. If instead of a saturated solution we have a weak
solution of certain salts, such as sea water, this, when lowered
in temperature, will change its state in a different way. At
304 CHANGE OF STATE.
a temperature which is always lower than the freezing point
of water such a solution will freeze, producing a nearly pure
ice and leaving the salt behind. We have already recorded
the experiments made by Professor Guthrie on this subject.
CHAPTER VII.
Change of State. Production of Vapour and its Condensation.
109, When sufficient heat is applied to a body it generally
assumes the gaseous state; unless it be of such a nature
that it will under ordinary circumstances be decomposed
before assuming this state. By means of a certain appli-
cation of electricity, it is probable that the most refractory
substances, such as carbon, can be made to appear as gases,
although only in very small quantity.
Generally when a solid passes into the gaseous state it
first assumes the intermediate state of a liquid, but some-
times its passage into the gaseous state is completed without
the intermediate form of liquidity being assumed. This is
called sublimation; while the passage of a liquid to the
gaseous state goes under the general name si vaporization.
In whatever way the gaseous condition is produced it always
requires a considerable amount of latent heat. Thus a pound
of water at iooC will absorb a great quantity of heat before
it is entirely converted into steam, although the steam does not
possess a higher temperature than 100. In the same manner
as before we may apply the following formula, and say
Steam at 100 = water at 1 00 + latent heat of steam.
The latent heat of gases is greater than that of liquids,
and we shall afterwards shew how it may be measured.
This latent heat has to be disposed of in some sensible
form, when the gas which possesses it is reconverted into
PRODUCTION AND CONDENSATION OF VAPOUR. 105
a liquid, and thus the latent heat of gases is of great
service in retarding the change from the liquid to the
gaseous or from the gaseous to the liquid state, which, but for
the great latent heat of gases, would be inconveniently sudden.
Elastic fluids have been divided into gases and vapours,
but the distinction between these is merely conventional.
A vapour denotes a substance in the gaseous form which
at ordinary temperatures appears as a liquid or solid, while
a gas denotes a substance which under ordinary circum-
stances appears in the gaseous form, and which can only be
reduced to the solid or liquid form by intense pressure or
intense cold. Our subject may be divided into the following
parts.
1. Vaporization, or the conversion of a liquid into a gas;
and sublimation, or the conversion of a solid into a gas.
2. Liquefaction and solidification of vapours and gases.
3. Pressure and density of vapours and gases, with a few
remarks upon hygrometry.
VAPORIZATION AND SUBLIMATION.
110. Vaporization is the general name for a process of
which there are three varieties, namely
1. Evaporation, where a liquid is converted into a gas
quietly, and without the formation of bubbles.
2. Ebullition, where bubbles of gas are formed in the
mass of the liquid itself.
3. Vaporization in the spheroidal condition, where a liquid
evaporates slowly, although in apparent contact with a very
hot substance.
111. Vapours are formed in vacuo more readily
than in air. The presence of air or of any foreign gas
retards the formation of vapours, but in vacuo a liquid is very
quickly converted into vapour. If a small quantity of water,
alcohol, or ether be introduced up through a barometer tube
106 CHANGE OF STATE.
into the Torricellian vacuum at the top, as soon as it reaches
this it is converted into vapour, which shews itself by
lowering the column of mercury by means of the pressure
which it exerts. This column, which originally denoted the
pressure of the atmosphere, now denotes the pressure of the
atmosphere minus the pressure of the vapour of the liquid.
112. Maximum of pressure in vacuo. If we continue
to introduce an additional quantity of the volatile fluid into
the Torricellian vacuum of a barometer, we shall at first
probably perceive an additional depression; but as we go
on we shall find that the depression does not increase
beyond a certain limit, or, in other words, the pressure of
the vapour we have introduced has reached a maximum,
and the introduction of more liquid will not increase the
density of the vapour. We shall further find that the
maximum of pressure is regulated by the temperature in
such a manner that the higher the temperature the higher
is the maximum pressure, so that we are enabled to deduce
the following law, first discovered by Dalton : In space
destitute of air the vaporization of a liquid goes on only until
the vapour has attained a determinate pressure dependent on
the temperature, so that in every space void of air which is
saturated with vapour determinate vapour pressure corresponds
to determinate temperature.
Mr. John Aitken has recently made some experiments
which tend to show that small particles have an action in
promoting the condensation of aqueous vapour somewhat
similar to that which they have in promoting crystallization
from a super-saturated solution. The dust of the air may
thus possibly be a factor of some importance in meteorology.
113. Mixtures of gas and vapour in a confined
space. The experiments of Dalton lead to the following
law : In a space filled with air the same amount of water
evaporates as in a space destitute of air ; and precisely the
PRODUCTION AND CONDENSATION OF VAPOUR. 107
same relation subsists between the temperature and the pres-
sure of the vapour ', whether the space contains air or not.
Thus if a closed space contain air of the pressure of
30 inches at a temperature for which the pressure of aqueous
vapour is 2 inches, and if a little water be introduced, the
pressure will rise to 32 inches; while if the same space be
void of air the pressure of the aqueous vapour will of course
be 2 inches. This law of Dalton has been verified by
Gay Lussac. More lately, Regnault has made experiments
on this subject, and has investigated the pressures of the
vapours of water, ether, bisulphide of carbon, and benzole,
both in vacuo and in air. He has found that the pressure
in air is always slightly less than it is in vacuo, the difference
being greater for volatile liquids ; but he is inclined to
believe that Dalton's law is true in principle, and that the
differences which he observed were caused by the hygro-
scopic properties of the sides of the chamber which contained
the vapours.
114. Mixed liquids in a confined space. Where a
mixture of liquids is allowed to evaporate in a closed space,
Gay Lussac inferred that the pressure of the mixed vapour
was equal to the sum of the pressures of the two vapours
taken separately.
Magnus and Regnault have found that this holds for a
mixture of bisulphide of carbon and water, or of benzole
and water, of which the components do not dissolve each
other ; but in other cases it does not hold.
Thus for a mixture of ether and water the pressure is
scarcely higher than for ether alone. If the liquids mix
readily together in all proportions, then the vapour pressure
is generally less than that of the one liquid and greater than
that of the other.
115. Effect of chemical affinity upon evaporation.
If water be put into a confined space along with some sub-
T08
CHANGE OF STATE.
stance which has a great attraction for it and does not
readily part with it, the vapour density may be much di-
minished. Thus if a small quantity of water be mixed with
a large quantity of sulphuric acid, the acid will retain the
water and will not suffer any of it to evaporate.
On the other hand, if we have a large quantity of water
and an exceedingly small quantity of acid, we shall have very
nearly the usual pressure of vapour.
Between these two extremes we may prepare solutions of
intermediate strength which will diminish to a greater or less
extent the pressure of aqueous vapour corresponding to the
temperature of observation.
A similar rule will hold for other solutions; and if the
substance mixed with the liquid whose pressure in a state
of gas is sought be a fixed and not a volatile body, its
tendency will generally be to prevent the liquid from eva-
porating, and thus to diminish the pressure due to vapour.
116. Pressure when two vessels at different
Fig. 24.
temperatures are in communication with each other.
In Figure 24, let us first suppose that the stop-cock at C
is shut, and that two similar vessels A and B are en-
tirely filled with water and vapour of water to the exclu-
sion of air or any other gas. Also let A be surrounded
PRODUCTION AND CONDENSATION OF VAPOUR. 109
with ice, and let heat be applied to B, so that we may sup-
pose A to be at the temperature of melting ice and B to be
at iooC. In this case the pressure of the vapour in A will
hardly be one-fifth of an inch, while in B it will be 30 inches.
Now on opening the stop-cock, there will of course be a rush
of vapour from B to A, and we may suppose that for a
moment the pressure of the vapour will be the mean between
the two original pressures, but the effect of the cold surface
of A will be to condense this vapour and to render it as
nearly as possible equal to the pressure at o, viz. one-fifth
of an inch. If there be water in B more vapour will rise
and pass to A, there to be condensed as before. In fact,
the apparatus will now act as a still, and the water of B will
be gradually transferred to A. The latent heat, set free by
the large quantity of vapour which is condensed at A, will
of course tend to raise the temperature of A ; but provided
this temperature be kept steadily at or near o by a suffi-
ciently powerful application of cold, the pressure in A will
by this arrangement be kept very low, while the pressure of
vapour in B will be somewhat higher than in A, and the
dynamical effect of this inequality of pressure in these two
vessels will be represented by the rush of vapour from B to
A . The intensity of this rush will depend on the intensity
of the source of heat : if the heat which enters B be suffi-
cient to produce a large quantity of vapour in a short time,
this vapour will rush very fast towards A, and a powerful
freezing mixture will have to be applied in order to keep
down the temperature of A, but if the source of heat be
feeble the rush will be feeble also ; in fact, by this arrange-
ment the vapour of water may be regarded as a vehicle for
transferring the heat from the source at B to be spent in
liquefying the ice or freezing mixture, or to be otherwise
disposed of at A.
If in the first part of this experiment, when the cock C is
110 CHANGE OF STATE.
shut, the vessel B contains no water in a liquid form, but is
entirely filled with the vapour of water at 100, then when
C is opened this vapour will be almost immediately con-
densed at A, and an approximate vacuum will be formed.
We shall afterwards see how this principle has been applied
by Watt in the steam-engine.
It will be evident that the perfection of vapour as a vehicle
for carrying heat, as described above, depends upon the
absence of air in the arrangement of the experiment.
For if A and B be filled with air, each particle of vapour
which is carried from B to A must pass through all this air,
and the transmission of vapour will in this case be very
difficult. Professor O. Reynolds (Pro. R. S., May, 1873)
shows that a small quantity of air in steam does very much
to retard its condensation upon a cold surface.
The following are various useful applications of the pro-
cess described above.
117. Distillation. The subjoined figure will represent
this process. The liquid to be distilled away is contained
in A. It generally exists combined either with some fixed
impurities or with some other liquid less volatile than itself,
and the object of distillation is to separate it from these.
This is done by applying heat to A and by attaching to it a
tube, as in Fig. 25, the other extremity of which passes in
coils through a vessel of cold water. The liquid is vaporized
by the applied heat, and is then driven through this tube, but
as it passes through the coils immersed in the cold water
(technically called the worm), a comparatively large surface
is exposed to the cooling agent, and the vapour is rapidly
condensed, passing in drops from B into a bottle prepared
to receive it. The vessel C through which the worm of the
still passes must be kept cool ; this is done by constantly
supplying it at a low level with cold water by means of a
tube at D, and by withdrawing the hotter and therefore
PR OD UCTION A ND CONDENSA TION OF VAPO UR. Ill
lighter layers of the water at C ; a constant current of cold
water is thus made to circulate through the vessel.
Fig. 25.
118. Cold due to evaporation. Freezing apparatus.
Whenever vapour is produced a quantity of heat is rendered
latent. This heat is necessary to the formation of vapour,
and must be supplied either from some foreign source, or,
if this be not available, from the very liquid which is being'
evaporated. In this last case the temperature of the liquid
falls in order to supply the heat necessary to the existence
of vapour.
Leslie was the first to freeze water by means of the drain
of heat caused by its own evaporation.
In his experiment a vessel, Fig. 26, containing strong
sulphuric acid is placed under the receiver
of an air-pump, and above it a thin metallic
vessel containing a little water. As the
receiver becomes exhausted the water eva-
porates more and more rapidly, and the
vapour, as fast as it is formed, is absorbed
by the sulphuric acid. The vapour thus
becomes a vehicle for carrying heat from
the metallic vessel, and the consequence Fig. 26
112, CHANGE OF STATE.
is a diminution in temperature until ice is formed. Air-
pumps are now constructed by means of which water may
be frozen almost in the very act of boiling. They contain
reservoirs of sulphuric acid into which the vapour drawn
from the water is forced at each stroke of the pump.
An instrument called the cryophorus, or frost carrier
(Kpvos frost, (f)op6s bearing), very similar to that of Fig. 24,
is sometimes used to show the freezing of water from its own
evaporation. Thus if we suppose all the water to be in B,
and only vapour of water without air in A, and if A is cooled
by a powerful freezing mixture, while B is not exposed to a
source of heat, then rapid evaporation of the water in B will
take place, and this vapour will go to A and be condensed
there as fast as it is formed.
Heat is thus carried, as before, from B to A, but as there
is now no source of heat at B the water there must part with
its own heat in order to furnish that which is necessary for
evaporation, in consequence of which it will be frozen. In
this experiment it is well to protect B from the influence of
currents of air.
When other liquids and mixtures more volatile than water
are used in this manner, a very intense cold may be pro-
duced. Thus by the evaporation of liquid sulphurous acid a
degree of cold is obtained sufficiently strong to freeze mercury.
By a mixture of solid carbonic acid and ether Faraday
obtained a degree of cold which he estimated at iioC;
and more recently, Natterer, by mixing liquid nitrous oxide
with bisulphide of carbon, and placing them both in vacuo,
has obtained I4O C C.
In hot climates porous vessels called alcarazas are used
for cooling water. The water reaches the outside through
the pores, and hence a continual evaporation is going on,
especially when the vessels are placed in a current of air.
MM. Carre* and Co. of Paris have invented a very in-
PRODUCTION AND CONDENSATION OF VAPOUR. 113
Fig. 27.
genious freezing machine, which was exhibited in London
at the International Exhibition of 1862.
This apparatus is represented in Figs. 27 and 28. A is a
strong vessel of wrought
iron three quarters filled
with a concentrated solu-
tion of ammonia. B is
a strong wrought iron
circular condenser hav-
ing a central space suffi-
ciently large to receive
the vessel D. The pipes
are so arranged as to
prevent the liquid from '
boiling over into the con-
denser. Before using the
instrument it is laid upon its side, boiler downwards, for
about 10 minutes, so as to allow any liquid that may be in
the condenser to drain back into the boiler, and this is
facilitated by heating
the condenser slightly
with a lamp. The
process consists of two
parts. In the first of
these, Fig. 28, the
boiler is heated very
gradually by a char-
coal chauffer, or other
source of heat, while
the condenser B is
kept in a vessel through
which a stream of
cold water is constantly flowing. As the result of this
process, the ammoniacal gas separates from the water and
i
114 CHANGE OF STATE.
is condensed by its own pressure in B, and the heating is
allowed to go on until a thermometer attached to the boiler
indicates about i32C, at which temperature it is presumed
that nearly all the ammoniacal gas is condensed in B, while
all the water remains behind in A : the second part of the
process next begins.
The apparatus is now withdrawn from the fire ; the water
is allowed to run out of the orifice B through a hole in the
bottom ; this orifice is then stopped with a cork, and the
cylinder D containing the liquid to be frozen is put into B, a
little alcohol having been previously introduced in order to
establish a liquid communication between the sides of B and
D; the vessel A (Fig. 27) is now plunged into water which
is kept cool, while the condenser is wrapped round with
flannel, which is well known to be a non-conductor. The
temperature of A now falls very rapidly, and as the water
in A reacquires its power of absorbing ammoniacal gas, this
gas rises very abundantly from B, and is condensed in A.
In consequence of this rapid evaporation B becomes
intensely cold, and if it contains water this will be frozen.
Mercury may also be frozen by this means.
As the success of this instrument depends upon its being
devoid of air, there is an arrangement of the following kind,
by which any air can be got rid of. G is a small cup which
is always kept full of water, and in it works a screw, so that
when relaxed it opens up an exceedingly small entrance into
the interior. When the temperature of the boiler has risen to
about 6oC the screw is slightly loosened, and the disen-
gaged ammoniacal gas is rapidly absorbed by the water in G.
If any air be present, this will be seen by its rising to the
surface, and the channel must be kept open so long as such
an appearance of air continues, but when the gas is wholly
dissolved by the water, the screw must be again tightened
and the operation of heating continued.
PR OD UCTION AND CONDENSA TION OF VAPO UR. 115
A portion of the boiler at D, Fig. 28, is made of metal
which will fuse below that temperature at which the pressure
of the steam would burst the boiler. This arrangement acts
therefore as a safety-valve.
Quite recently Mons. Cailletet of Paris and Mons. Raoul
Pictet of Geneva have been able to obtain in the solid or
liquid state the gases hitherto uncondensed.
Their plan was to subject the gas at once to great pressure
and great cold, and then by suddenly relieving the pressure
the expansion produced so diminished the temperature as to
condense the gas.
119. Pressure in communicating vessels filled with
air. Dalton, as we have already mentioned, was the first to
shew that in a space filled with air the same amount of
water evaporates as in a space destitute of air ; and that
precisely the same relation subsists between the temperature
and the maximum vapour pressure whether the space con-
tains air or not. Unfortunately there has been based upon
this experimental result a theory which is only a possible but
not a necessary result of these experiments. It has been sup-
posed that no mutual relation whatever exists between vapour
and air, and that they remain near each other without pro-
ducing the slightest mechanical effect upon one another.