Henry Watts.

A Dictionary of chemistry and the allied branches of other sciences, Volume 3 online

. (page 15 of 86)
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chloride of carbon), that substances which are capable of existing at the same tem-
perature either as solids or liquids, produce, in both states of aggregation, vapours of
equal tension (see next paragraph).

Fig. 640.



Fiff. 539.




Tension of Vapours. — The quantity of any given liquid which can evaporate in
an enclosed space, either previously vacuous or already containing a gas or vapour, is
limited, and depends on the nature of the liquid, on the temperature, and on the
extent of the enclosed space. When the liquid is in excess, that is, when some of it
remains after the formation of the largest quantity of vapour that is possible under



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the cmmmstanoes, eyaporation ceasee when the vapour exerts a certain preosniv npoa
the sides of the containixig vesseL The laws of evaporation are, therefore^ most eatHj
arrived at by studying, in the first instance, the phenomena of the formation of rapour
in a vacuum, in which case the pressure exerted by the vapour itself is the only one to
which the sides of the vessel are exposed.

If a glass tube, closed at one end. about a yard long and three-quarters of an inch
wide, is filled with mercuiy and inverted in a basin of that liquid, as shown in J!^. 639,
a vacuous space, about six inches in length, is formed at the top of the tube. On
now passing up a drop or two of ether or alcohol into this space, the liquid will imme-
diately evaporate, so that the surface of the mercury and ox the tube will remain dry,
but the mercury column will be depressed by the elastic force of the vapour which has
been formed. The amount of this elastic force, or of the pressure which the Ti^KXir
exerts upon the sides of the tube and upon the top of the column of mercury, is evi-
dently measured by the difference between the height of this column and that of the
barometer at the time of the experiment On transferring the tube in this condition
to a mercury-trough provided with a deep cylindrical well (Jig. 540), and depressing
it slightly, the space occupied by the vapour above the mercury will be diminished ;
the tension of the vapour will therefore increase, and the mercuiy column AC will be
still further depresseo. But on continuing to diminish in this way the volume of the
vapour, a point will soon be reached at which part of it returns to the liquid state and
condenses as a dew upon the inside of the tube and on the mercury ; and if the height
of the column A C be measured, as soon as the first trace of condensed liquid becomes
visible, it will be found to remain the same until the tube has been depressed so far
that the whole of the vapour has been converted into liquid.
Fig. 641. and the tube becomes mled with mercury, surmounted by a

drop of liquid ether or alcohol. The progress of the experi-
ment in this stage will be easily unaerstood from Jig. 641,
where C D represents the tube when the liquid first begins
to appear in the space above the mercury, and A B the level
of the mercury in the tube above that in the trough. Aa
the tube is depressed successively to the positions Cx X>i, and
Ca Ds, more and more of the vapour is hquefied, bat the re-
' maining portion exerts neither more nor less pressure than
the quantity which existed in the tube at the instant when
liquefaction first began to take place; and hence the .«ar-
fiice of the mejcury remains immovably at the level AR
Again, if the tube is raised up out of the mercuiy, the top oi
the mercury column will stifl remain at the level A B, unti
^ the whole of the liquid has a^ain evaporated into the space
^ above it, but from this point it will begin to rise higher and
higher as the tube is still further raised ; if, however, when
the tube, has attained its highest position (so that the open
end is only just covered by the mercury in the trough), an sdditional quantity of ethei
or alcohol is passed up into it, the column of mercury will sink ; and if the quantity oi
liquid passed up is more than can evaporate in the portion of the tube unoccupied bj
the mercury, the surface of the latter will sink again to the level A B (Jig. 641).

These experiments prove that when a vapour is compressed, its temperature remain-
ing always the same, its elastic force increases up to a certain limit which it canno<
exceed, the effect of any further compression being to change it to a liquid ; and that
when a vapour is in contact with any portion of the corresponding liquid, in a space
otherwise vacuous, its elastic force always attains this limit, whatever may be the
relative volumes of vapour and liquid. In this condition, a vapour is said to be satu-
rated, or to exert its maximum tension.

Maximum Tension. — The maximum tension which the vapour of any liquid can exert
depends upon the nature of the liquid, and upon its temperature.

If we pass up into the first of three barometric tubes, like that represented in
fig. 639, a small quantity of ether, into the second some alcohol, and into the third
some water, taking a larger quantity of each liquid than can evaporate complet^l^
within the tube, the mercury colunm will be depressed to a different level in each,
supposing all three to have the same temperature. If, for instance, the temperature ii
10** C, the mercury will be depressed about 11 J inches in the first tube, nearly 1 inch
in the second tube, and a little more than i inch in the third.

If the three tubes are now gradually heated, by surrounding them with warm water oi
oLberwise, more vapour will be formed from the liquid contained in each of them; the elastic
force or tension of each vapour will acconliugly increase, and the mercury will descend
lower and lower in the tubes as the temperature rises. At about 36°, the tension oi
the ether-vapour will have become equal to that of the atmosphere, and hence at this



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TENSION OF VAPOURS — EBULLITION. 83

tcoipermtiire, the mercniy in the tube containing it will have been driven down to as to
be at the same level as that in the reeenroir ; at aboat 78'a^ the aame will be the eaee
with the mercnzy in the aloohol-tobe ; and at about 100^ With that contained in the
wmtep-tube.

When a mtnrated vaponr is cooled, the pressnre npon it remaining unaltered, com-
plete liquefaction takes place, just as it does when the pressure is increased and the
temperature remains the same. The condition of saturation, or maximum tension, is
therefore a limiting condition, beyond which a vapour can neither be compressed nor
eooled without returning to the state of liquid.

On the other hand, when a saturated vapour not in contact with an excess of
liquid is heated, it ceases to be saturated (unless the pressure upon it is increased at
the same time to a corresponding extent). Hence, when a ftxrther quantity of liquid is
brought in contact with a vapour which has been thus heated, more v^wur is formed in
the same space, until saturation is again produced, and the tension increases at the same
time till it reaches the maximum tension corresponding to the higher temperature.

Distinetion between Vapours and Gaaes. — ^It has heea already stated, that when the
space into which a liquid can evaporate is increased until the whole quantity of liquid
has become vapour, the tension of the vapour diminishes with any further increase
of volume ; and that when a vapour, not in contact with the liquid from which it is
formed, is compressed, its tension augments until liquefetction begins. In proportion
as a vapour under these conditions is expanded, its tension becomes more and more
nearly inversely proportional to the vdume which it occi^es ; that is, its properties
approiaeh more and more nearly to those of a perfect gas. Similarly, when a saturated
Tapour, not in contact with an excess of liquid, is he^ed, its elastic force increases,
ana it expands if the pr e ssur e upon it is not |»oportionably augmented; and as the
temperature rises, the relation between the tension or elastic force, p^ of the vapour,
its volume, % and its temperature, ^ comes to be more and more nearly expressed by
the equation

pv^J(a +<);

which, as we have already seen (p. 45), expresses the relation between these three
quantities in the case of a perfect gas (see aiiK> p. 60).

Hence the physical properties of vapours, when sufficiently expanded, and at suffi-
ciently high temperatures, are identical with those of the permanent gases.

From this it is natural to conclude, conversely, that the so-called permanent gases
themselves are onlv vapours which, at ordinary pressures and temperatures, are very
far removed from toeir points of saturation, and that by exposing them to lower tem-
peratures and increasing the pressure, a point might be reached for each of them, at which
the pressure would be equal to the maximum tension which it was capable of exerting at
the temperature of the experiment, and therefore, that any fbrther oiminution of tem-
perature or increase of pressure would cause it to become liquid. This conclusion has
been actually rerified in the case of manv gases formeriy regarded as permanent, the
only gases which have hitherto resisted all attempts to Uqu^ them being hydrogen,
oxjTgen, nitrogen, nitric oxide, carbonic oxide and marsh-gas. (The methods em-
ployed for the liouefoction of gases will be i^irther considered in connection with the
processes adoptea for obtaining great reductions of temperature.)

Tenman of Vaponrt in presence of Permanent Gases. — The fiuniliar fact of the
evaporation of water in the open air ttflfbrds sufficient proof that the presence of a per-
manent gas in any given space does not prevent volatile liquids giving off vapour into
the same space. By measuring the volume and elastic force of a given quantity of dry
air, or other ^as, then introducing a little more of any liquid than can completely
evaporate in it, and when equilibrium h&s been re-established, a^ain measuring the
volume and elai^c force of the mixture of gas and vapour, the tension of the latter can
be asoertained. In this way it has been found that, when the liquid exerts no solvent
or chemical action upon the gas, the combined tension of gas and vapour is nearly equal to
the separate tension of the gas, increased by the maximum tension which the v^Kmr
is capable of exerting in an otherwise vacuous space at the temperature of the experi-
ment In other words, the vapour given off by a liquid at any temperature has neariy
the same maximum tension, whether it is formed in a space previously vacuous or filled
with a permanent gas. The only essential difference betw6en the evaporation of a
liquid in a vacuum, and its evaporation in a gas, is that, in the former case, the vapour
attains the condition of saturation in an inappreciably short time, while in the latter,
this condition is arrived at more slowly. Hegnault*s experiments (M^m. Acad.
Sdenoes, xxvi.) prove, however, that b'quids do not give off vapour of quite so j?reat a
tension in a space occupied by a permanent gas, as they do in a vacuum, and that the
difference increases as the temperature rises.

Boiling Points,^ EMlitum.—Ytom the facts stated in the last paragraph, it foUowi

o 2



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that the temperature at which the rapoor of a liquid introduced into the baromHritf
vacuum would exert sufficient elastic force to drive the mercury down to the bottom of
the tube, so afl to make it stand at the same level inside and outside, is also the t<^m-
perature at which the vapour formed on heating the liquid in the air would exert aa
elastic force equal to the atmospheric pressure. As already stated, the first of tha^
effects is produced by ether-vapour at 35**, by alcohol at 785^, and by water at 100=* :
these temperatures, however, are those at which the liquids respectively boil when heat«d
in the open air, the barometer being at its average height The boiling point of t
liquid is, therefore, the temperature at which the tension of its vapour becomes equal to
the atmospheric pressure. This temperature is evidently not absolutely constant far eadb
liquid, but varies more or less with alterations of the pressure of the atmosphere. A
liquid at its boiling point is in a limiting condition, comparable to that of a saturated
vapour; any diminution of pressure or increase of t^^mperature equally causes it to
pass from the liquid to the vaporous state. The two conditions are in fitet conter-
minous ; and the temperature at which a liquid produces vapour of any given maxi-
mum tension, is also the temperature at which the liquid would boil under an atmo-
spheric pressure e^ual to that tension. Hence the observation of the pressures under
which a liquid boils at various temperatures constitutes a method of aetermining the
maximum tension of its vapour at those temperatures.

The phenomenon of ebullition, which presents itself when heat is applied to the
lower part of a mass of liquid, already at such a temperature that the tension of its
vapour is equal to the pressure of the atmosphere, results from the transfomnatioD of
the liquid into vapour at the points where the heat is applied, and the escape of thi^
vapour in the form of bubbles through the superincumbent liquid.

The temperatures at which different liquids boil, under the ordinary atmo-
spheric pressure, vary very greatly. They will be found given for each liquid in the
article of this dictionary wherein it is specially described ; in the fqllowing table a few
boiling points are given in order to illustrate the range of temperature through which
they occur.

Table of Boiling Points.



Liquid.


BoiUng Point.


PreMure in miillmeUes
of mercury.


Authority.


Nitrous oxide


-87-90


769


Regnault


Carbonic anhydi


ide . -78-2


767-3


n


Ammonia .


-33-7


749-3


Bunsen


Sulphurous anhy


dride -10-6


744


n


Chloride of ethyl


+ 110


768


Pierre


Oxide of ethylen


e . 13-5


746-6


WurtB


Aldehyde .


19-8


734


Kopp


Ether


34-2


742


tt


Sulphide of carb


on . 47-9


766-8


Pierre


Methylic alcohol


610


764


Delfb


Bromine .


630


760


Pierre


Alcohol .


. . 78-4


760


6ay-Lussae


Benzene .


80-4


762


Kopp


Water


1000


760


II


Acetic add


116-9


760


n


(>mene .
Naphthalene


177-6


744


II


216-8


747-6




Phosphorus


290


• •


Pelleticr


Oil of vitriol .


326


, ,




Mercury .


360


, ,


Regnault


Sulphur .


440


. .


< Dumas; Deville
andTroost


Cadmium .


860


, ,


Deville and Troost


Zinc .


1040


• •


If



Dfierminaiion of Boiling Points. — The boiling points of different liquids being anoong
their most characteristic properties, the determination of them becomes a very &eqTien1
and important operation in chemical research. The method recommended by Kopp, ii
order to ensure as much accuracy as possible in these observations, is as follows : —

The liquid to be examined is placed in a cylindrical glass vessel, containing a few
scraps of freshly ignited platinum foil, the diameter of which, when the quantity of liquid
is small, need not much exceed that of the bulb of the thermometer. This vessel ii
closed by a cork, through the centre of which the thermometer is inserted, in such s
way that it can ht raised or lowered, so that the bulb may dip either into the liquid oi



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EBULLITION. 85

noTply into the vapour. It is generally advisable to give the tbermometejr the latt«r
position, since, us wfll be seen by what follows, the temperature of the liquid may,
nnder dnnunstances which not unfrequently occur, rise somewhat above the> true
boiling point ; but even when this is the case, a thermometer in the vapour will show
tlie real boiling point of the liquid : under nearly all circumstances, the thermometer will
stand lower in the vapour than it does in the liquid, if this is a mixture of two or more
liquids of different boiling points instead of a pure, homo-
geneous substance. Through a second hole in the cork is Fig, 642.
inserted a glass tube, open at both ends, and bent at a riglit
angle, as shown in fig, 542 ; b^ connecting this tube with a
condenser, the loss of the liquid used for the experiment can
be prevented. The liquid is heated either by applying a
small flame to the outside of the vessel, or by means of a
water-bath or sand-bath, care being taken that the sides of the
vessel above the liquid do not get over-heated. The indications
of the thermometer are observed during the whole time that
the liquid is beii^ slowly boiled away, until only a small Quan-
tity remains. "Die temperature thus observed is not, how-
ever, in most cases, the true boiling point of the liquid : usually, ^ /-s^
part of the mercury column in me thermometer lises above j j^ ''vi^i-^
the cork, and is therefore exposed to a lower temperature than *■ ^^^^
that of the boiling liquid ; consequently, the upper extremity
of the column stands at a lower point tnan it would do if the
thermometer were completely immersed in the liquid. In
order to find the correction which it thus becomes necessary
to apply, a second thermometer is placed so that its bulb is in
contact with the stem of the thermometer inserted into the
cork of the boiling vessel, and is half way between the top of
the mercury column of the latter thermometer and the middle
of the cork. The temperature indicated by this second thermometer may be taken
as the mean temperature of that portion of the mercury column of the principal ther-
mometer which is not heated by the vapour of the boiline liquid. Let this temperature
be ^ ; let the uncorrected boiling point, directly indicated by the principal thermometer,
be 7^ ; let -AT be the difference between T and the point of the scale situated at the
middle of the cork, that is to say, the length, expressed in degrees of the scale, of that
portion of the mercury column of the prmcipal thermometer of which the mean tem-
|)eratnre is f^ ; lastly, let Z be the coefficient of i^parent expansion of mercury in the
glass of which the thermometer is constructed. The correction to be applied to the
directly observed temperature T^ is then

As already stated (p. 57), 9 may always be taken, in calculating the value of this
expression, as » 0'0001645.

The table which follows on p. 86 gives the amounts of the correction in question for
various values of A and of T— t. The amounts corresponding to other values of these
factors can be easily deduced by interpolation from the numbers g^iven in the table.

This table sufficiently shows that the correction in question can never be neglected in
accurate experiments, and that in the case of liquids of high boiling points, its value
may become very considerable.*

Since the boiling point of a liquid depends on the pressure to which the liqmid is
subjected, another correction becomes necessary in order to reduce determinations
made under the varying pressure of the atmosphere, to the values which would be
found if the atmosphere exerted always its normal pressure, equal to that of 760
millimetres of mercury at 0^. Strictly speaking, the correction to be applied to the
boiling point of a liquid observed under an atmospheric pressure differing by a given
amount from the above standard pressure, varies with the nature of the liquid ; since
equal alterations of pressure do not cause precisely equal changes in the boiling points
of different liquids. Nevertheless, the greatest variations which ever occur in the
pressure of the atmosphere are relatively so small, that they may, without any appre-
ciable error, be regarded as affecting the boiling points of all liquids equally : to the
extent, namely, of 0*1° for a variation of pressure of 2*7 millimetres of menmiy, this
number being deduced from direct determmations of the boiling point of water under
different pressures.

In what follows, whenever the boiling point of a liquid is spoken of withont further
explanation, it is to be understood to mean the boiling point under a pressure equal to
tliat of 760 millimetres of mercury at 0^.

• It Is o viiMis thftt R precisely simUar corr»Ttion oiiRht to be applied to all thermometric observatinns
in which any fiorttuc of the mercury in the stem of the thermometer b at a different temperature from
tliHt lu the bulu.



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EBULLITION. 87

Ciftumstanees wkick modify the Boiling Point, — Although, when a b'qtiid is he«ted
in rach a manner that Ta^xmr can escape freely from seme part of its surfiace, the
Taponr fo formed has a tension equal to the pressure upon the free sur&ce of the liquid
as soon aa the temperature of the latter reaches the boiling point, this temperature
may nerertheless he attained, and even considerably exceeded, without the formation
of a trace of vapour, if no portion of the surface of the liquid is freely exposed. These
conditions can be reah'sed by suspending the liquid to be examined in a second liquid
of equal specific grayi^, but higher boiling point

The phenomena which take place under these circumstances have been particularly
studied by Dufour (Ann. Ch. Phys. [3] Ixriii S78). In order to examine them in
the case of water, he employed a mixtu]i« in the requisite proportions of oil of dores
(preriously heated alone to about 200^) and linseed oil. The water, already heated
to 80® or 90°, was dropped gently into the mixture of oils, so as not to disturb the
film which coated the bottom of the vessel, and the temperature of the bath was
gradually raised. Under these circumstances the ordinaiy boiling point of water, 100**,
was passed without the occurrence of any perceptible change, and traces of ebullitaon
scarcely began to show themselves below 110^ or 116°. Even at these temperatures,
ebullition seldom began except when the globules of water came in contact with the
sides of the vessel or with the thermometer. A burst of vapour then occurred, and the
globule, more or less diminished in size, was driven rapidly away, like a pith ball after
touching an electrified conductor. These contacts were of course more dimcult to avoid
in the case of large than of small globules ; hence the latter remained liquid, as a rule,
to higher temperatures than the former.

In these experiments, it was a rare exception when ebullition occurred between 100**
and 110°; veiy commonly globules of 10 mm. in diameter reached 120° or 130**, and
in one experiment the last temperature was attained by a globule of 18 mm. dia-
meter, and therefore containing more than 3 c c of water. Spheres of 10 or 12
mm. diameter often reached 140**; those of 6 or 6 mm. reached 166®; and others
of from 1 to 3 mm. attained 176** or even 178®, temperatures at which the elastic force
of the vapour which forms at the freely exposed surface of water is between 8 and 9
atmospheres.

At these high temperatures, the contact of a solid body very generally occasioned
the sudden partial or complete vaporisation of the globules, accompanied by a hissing
sound like that produced on immersing red-hot iron in water. This invariably
occurred when the globules were touched with pieces of wood or chalk, shreds of cotton,
paper, &c., but not always on contact with a ^ass rod or metallic wire, the difierence
appearing to depend on the porous structure of the former substances. A platinum



Online LibraryHenry WattsA Dictionary of chemistry and the allied branches of other sciences, Volume 3 → online text (page 15 of 86)