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and the increased volume which the liquid undergoes in evapo-
ration. Thus, if a cubic inch of a liquid swells by evaporation
into 2000 cubic inches of vapour having a pressure of 10 Ibs.
per square inch, it is easy to show that a mechanical force is
developed in such evaporation which is equivalent to 20,000 Ibs.
raised through one inch. For, if we imagine a cubic inch of
the liquid confined in a tube, the bore of which measures a
square inch, it will, when evaporated, fill 2000 inches of such
tube, and in swelling into that volume will exert a pressure of
10 Ibs., so that it would in fact raise a weight of 10 Ibs. through
that height. Now 10 Ibs. raised through the height of 2000
.inches, is equivalent to 20,000 Ibs. raised through the height of
one inch.



VAPORIZATION AND CONDENSATION. 93

Since, however, it is customary to express the mechanical
effect by the number of pounds raised through one foot, the
mechanical effect produced in the evaporation of each cubic
inch of a liquid will be found by multiplying the number
which expresses the volume of vapour produced by the unit of
volume of the liquid by the number expressing the pressure of
the vapour in pounds per square inch, and dividing the product
by 12.

In the following tables, the relation between the temperature,
pressure, density, volume, and mechanical effect of the vapour
of water are given as determined by observation so far as the
pressure of twenty-four atmospheres, and by analogy from that
to the pressure of fifty atmospheres.

TABLE I.

Showing the Pressure, Volume, and Density of the Vapour of
JVater produced at the Temperatures expressed in the first
Column, as well as the mechanical Effect developed in the
Process of Evaporation.



Temperature,
FarVnheit.


Pressure.


Volume of

v TnTuro a r

V6lume of


Densitv of Vapour
(Density of
Water =1).


Mechanical
Effect in Lbs.
raised 1 Foot.


Inches of
Mercury.


Lbs. per

Square Inch.


4


0-0^2


003


650588


0-00000154


1395


5


0-074


0-04


470898


212


1423


14


0-104


0-05


342984


292


1451


23


0-144


0-07


251358


398


1480


32


0-1 69


o-io


182323


540


1483


33-8


0-212


010


174495


673


1514


35-6


0-226


o-n


164332


609


1519


37-4


0-241


12


154*42


646


1525


392


0-257


0-13


145886


6S6


1531


41


0-274


0-13


137488


727


153G


42-8


0-291


0-14


129587


772


1542


44-6


0-310


0-15


122241


818


1549


46-4


0-330


0-16


115305


867


1555


48-2


0-351


0-17


108790


919


1559


50


O'o73


0-18


102670


974


1565


51-8


0-397


0-19


99202


0-00001032


1607


53-6


0-422


0-21


91564


1097


1577


55-4


0-448


0-22


86426


1157


1582


57-2


0-476


0-23


81686


1224


1588


59


0-505


025


77(08


1299


1590


60-8


0537


0-26


72913


1372


1598


62-6


0-570


0-28




1451


1604


64-4


0'6 n 4


0-30


65201


1534


1610


6C-2


0-641


0-31


61654


1622


1615


68


0-682


033


58224


1718


16-21


69-8


0-721


0-35


55206


1811


1626


71-6


0-764


0-37


52260


1914


1632


73-4


0-810


0-40


49487


2021


1638


75-2


0-858


0-42


46^77


2133


1(144


77


0-909


0-45


44411


2252


1649


78-8


0-963


0-47


42084


2376


1655


80-6


1-019


0-50


39895


2507


1661


82-4


1-078


0-53


37*38


S643


1667


84-2


1-143


0-56


35796


2794


1672



94



HEAT.



sssssr-


Pressure.


Volume of
Vapour contain-
ing Unit of
Volume of
Water.


TSST


Effect in'lJbs.
raised 1 Foot.


Inches of
Mercury.


Square Kch.


86


1-206


0-59


34041


0-00t029:t8


1678


87-8


1 276


063


32-291


3097


1634


89-6


1'349


0-G6


30650


3-J63


1689


91-4


1-425


0-70


29112


343>


1694


932


1-506


0-74


27636


3619


1700


95


1-591


0-74


26253


3809


1706


96-8


1-683


0-82


24*97


4017


1712


98-6


1-773


0-87


23704


4219


17i7


100-4


1-873


092


22513


4442


1722


102-2


1-974


0-97


21429


4666


1728


104


2-087


1-02


20343


4916


1734


10V8


2-196


1-08


19396


5156


1740


107-6


2-315


1 13


18469


5418


1746


109-4


2-439


1-20


17572


5691


1751


111-2


2-584


1-27


16805


6023


1774


113


2-707


1-33


15938


6274


17ti2


1148


2-850


1-40


15185


6585


1768


1166


3-OOfl


147


14472


6910


1774


118-4


3-158


1-55


13809


7242


1781


1202


3-322


1-63


13154


7602


1785


122


3-494


1-71


12546


7970


1791


123-8


3-673


1-80


11971


8354


1796


125-6


3861


1-89


11424


8753


1802


127-4


4-058


1-99


10901


9174


1807


129-2


4-263


2-09


10410


9606


1813


131


4-477


2-19


9946


0-0001 0054


1819


132-8


4-700


2-30


9501


10525


1824


131-6


4-934


2-42


9082


11011


1830


136-4


5-177


2-54


8680


11523


1836


138-2


5-431


2-66


8303


12044


1842


140


5-695


2-79


7937


12599


1847


141-8


5-973


2-93


7594


13179


1853


143-6


6-258


3-07


7-267


13760


IBM


145-4


6-558


3-21


b957


14374


1864


147-2


6-869


3-37


6662


15010


1869


149


7-193


3-53


<i382


15668


1875


150-8


7-530


369


6114


16356


1881


152-6


7-881


3-86


5860


17060


1*87


1544


8-246


4-04


5619


17797


1893


156-2


8624


4-23


5386


18566


1898


158


9-019


4-42


5167


19355


19(14


1598


9-4i7


4-62


4957


20174


1909


161 6


9-852


4-83


4759


21013


1915


163-4


10-293


5-05


4569


21889


I!i21


165-2


10-749


5-27


4387


2-2794


1926


167


11-223


5-50


4204


23789


1928


168-8


11-715


5-74


4043


24702


1937


170-6


12-224


5-99


3891


25699


1943


1724


12-752


6-25


3741


26739


1949


174-2


13-298


6-52


3599


27789


1955


176


13-862


6-80


3462


28889


1963


1778


14-449


7-08


3331


30025


1966


179-6


15-055


7-38


3206


31195


1972


1H1-4


15-680


769


3087


32399


1977


183-2


16-328


8-00


2973


33637


1983


185


16-996


833


2864


34916


1989


186-8


17-688


8-67


2760


36237


1994


188-6


18-401


902


MM


37590


2000


190-4


19-138


9-38


2.->65


3-984


2(105


192-2


19-897


W75


2474


40417


2011


194


20-680


10-14


2387


41891


2017


195-8


21-488


10-53


2304


43405


2023


197-G


22-321


10-94


2224


4 1956


2028


l!i9-4


23-179


1136


2148


46556


2034


201-2


24-062


11-80


2075


48201


2040


203


24-971


12-24


2015


49886


2045


204-8


25-!)08


12-70


1938


51613


2051


206-6


26874


13-17


1873


53388


5056


208-4


27-860


13-66


1812


55191


2062


210-2


28-877


14 16


1751


57(155


20C6


212


29-921


14-67


1696


58955


2073



VAPORIZATION AND CONDENSATION.



95



TABLE II.

Showing the Temperature, Volume, and Density of Vapour
of Water, corresponding to Pressures of from 1 to 50
Atmospheres.

From 1 to 24 Atmospheres obtained by Observation.
24 to 50 Analogy.





Pressure,
sphra.


Tempe-
raiure,
Fahrenheit.


Volume of

H&

V K-


W

tfKft


Pressure,
sphSe7.


Tempe-
Fahrenh'eit


Volume of
Vapour
produced by
Unit of
Volume of
Water.


Density of
Vapour
(Density of
Water =1).


!


212


1696


0-0005895


13


380-66


16374


0-006107


l|


233-96


1167-8


8563


14


386-96


153-10


6527


2


250-52


897-09


0-0011147


15


392-90


144-00


6944


N


263-84


731 -39


13673


16


398-48


135-90


7359


3*


27-V18


619 19


16150


17


403-88


12871


7769


3J


2*5-08


537-96


18589


18


408-92


12228


8178


4


293-72


476-26


20997


19


41378


116-51


8583


a


300-38


427-18


23410


20


418-46


111-28


8986


5


3(17 58


388-16


25763


21


422-96


106-53


9387


N


314-24


35.V99


209I


22


427-28


102 19


9785


6


3v('-:i6


328-93


30402


23


431-42


98-21


0-010182


65


326-30


305-98


3i683


24


435-56


9456


10575


r


331 70


286-12


3491 1


25


439-34


91-17


10!) 68


"i


336-92


26882


37217


30


457-16


77-50


12903


8


341-78


253-59


39434


35


472-64


68-20


14(63


9


3SO-78


227-98


43865


40


486-50


60f8


16644


10


35S-88


207-36


48220


45


499-10


5406


18497


11


366-80


190-27


52557


50


51062


49-31


20306


12


371-00


175-96


56834











1495. Vapour separated from a liquid may be dilated by heat
like any gaseous body. In these tables the vapour is con-
sidered as being in the state of the greatest density which is
compatible with its temperature. It must be remembered that
vapour separated from the liquid may, by receiving heat from
any external source, be raised like so much air, or other gaseous
fluid, to any temperature whatever, and that the elevation of
its temperature under such circumstances is attended with the
same effects as atmospheric air. If it be so confined as to be
incapable of expansion, its pressure will be augmented a ^^th
part by each degree of temperature it receives ; and if it be
capable of expanding under an uniform pressure, then its volume
will be augmented in the same ratio.

1496. Peculiar properties of superheated vapour. Vapour
which receives a supply of heat after it has been separated
from the liquid, and which may therefore be denominated
superheated vapour, has some important properties which dis-
tinguish it from the vapour which proceeds directly from the
liquid.

The vapour which proceeds directly from a liquid by the



96 HEAT.

process of evaporation, contains no more heat than is essential
to its maintenance in the vaporous form. If it lose any portion
of this heat, a part of it will become liquid ; and the more it
loses, the more will return to the liquid state, until, being de-
prived of all the heat which it had received in the process of
evaporation, the whole of the vapour will become liquid.

But, in the case of superheated vapour, the effects are
different. Such vapour may lose a part of its heat and still
continue to be vapour. In fact, no part of it can be reduced to
the liquid state until it lose all the heat which had been im-
parted to it after evaporation.

1497. Vapour cannot be reduced to the liquid state by mere
compression. It is sometimes affirmed that vapour may, by
mere mechanical compression, be reduced to the liquid state.
This is an error. It is true neither in relation to vapour raised
directly from liquids, nor of superheated vapour.

1498. Vapour which has the greatest density due to its tem-
perature under any given pressure, will have the greatest
density at all other pressures, provided it do not gain or lose
heat while the pressure is changed. If vapour raised directly
from a liquid, at any proposed pressure, be, after separation
from the liquid, either compressed into a diminished volume
or allowed to expand into an increased volume, its temperature
will be raised in the one case and lowered in the other ;
and, at the same time, its pressure will be augmented by the
diminution and diminished by the augmentation of volume. It
will be found, however, that the temperature, pressure, and
volume will in every case be exactly those which the vapour
would have had if it had been directly raised from the liquid
at that temperature and pressure.

Thus, the vapour raised from water at the temperature of 68
has a volume 58224 times greater than the water that produced
it (see Table I. p. 93.). Now let this vapour, being separated
from the water, be compressed until it be reduced to a volume
which is only 1696 times that of the water which produced it,
and its temperature will rise to 212, exactly that which it
would have had if it had been directly raised from the water
under the increased pressure to which it has been subjected.

In the same manner, whatever other pressure the vapour
may be submitted to, it will still, after compression, continue to
be vapour, and will be identical in temperature and volume



VAPORIZATION AND CONDENSATION. 97

with the vapour which would be raised from the same liquid
directly if evaporated under the increased pressure.

1499. Compression facilitates the abstraction of heat by
raising the temperature, and thus facilitates condensation. ,
Although mere compression cannot reduce any part of a volume
of vapour to the liquid state, it will facilitate such a change by
raising the temperature of the vapour without augmenting the
quantity of heat it contains, and thereby rendering it possible
to abstract heat from it. Thus, for example, if a volume of vapour
at the temperature of 32 be given, it may be difficult to convert
any portion of it into a liquid, because heat cannot be easily
abstracted from that which has already a temperature so low.
But if this vapour, by compression, and without receiving any
accession of heat, be raised to the temperature of 212, it can
easily be deprived of a part of its heat by placing it in contact
with any conducting body at a lower temperature; and the
moment it loses any part of its heat, however small, a portion
of it will be reduced to the liquid state.

1 500. Permanent gases are superheated vapours. It may be
considered as certain, that all that class of bodies which are
denominated permanent gases are the superheated vapours of
bodies which, under other thermal conditions, would be found
in the liquid or solid state. It is easy to conceive a thermal
condition of the globe, which would render it impossible that
water should exist save in the state of vapour. This would be
the case, for example, if the temperature of the atmosphere were
212 with its present pressure. A lower temperature, with the
same pressure, would convert alcohol and ether into permanent
gases.

1501. Processes by which gases have been liquefied and
solidified. The numerous experiments by which many of the
gases hitherto regarded as permanent have been condensed and
reduced to the liquid, and, in some cases, to the solid state,
have further confirmed the inferences based on these physical
analogies. The principle on which such experiments have in
general been founded is, that if, by any means, the heat which a
superheated vapour has received after having assumed the form
of vapour can be taken from it, the condensation of a part of
it must necessarily attend any further loss of heat, since, by
what has been explained, it will be apparent that no heat will

II. F



98



HEAT.



remain in it except what is essential to its maintenance in the
vaporous state.

The gas which it is desired to condense is first submitted to
severe compression, by which its temperature is raised either
by diminishing its specific heat or by developing heat that was
previously latent in it. The compressed gas is at the same
time surrounded by some medium of the most extreme cold ; so
that, as fast as heat is developed by compression, it is absorbed
by the surrounding medium.

When, by such means, all the heat by which the gas has been
surcharged has been abstracted, and when no heat remains save
what is essential to the maintenance of the elastic state, the gas
is in a thermal condition analogous to that of vapour which has
been directly raised by heat from a liquid, and which has not
received any further supply of heat from any other source. It
follows, therefore, that any further abstraction of heat must
cause the condensation of a corresponding portion of the gas.

1502. Gases which have been liquefied. The following
gases, being kept at the constant temperature of 32 by de-
priving them of heat as fast as their temperature was raised
by compression, have been reduced to the liquid state. The
pressures necessary to accomplish this are here indicated :



Names


of Oases


rondens


d.






Pressure under which
Condensation took place.


Sulphurous acid -












Adhere,


Cyanogen gas
Hydriodic acid -
Amraoniacal gas -
Hydrochloric add
Protoxide' of azote












2-3
4-0
4-4
8-0
37-0


Carbonic aci.i












39-0



If these substances be regarded as liquids, the above pressures
would be those under which they would vaporize at 32. If
they be regarded as vapours, they are the pressures under
which they would be condensed at 32.

M. Pouillet succeeded in condensing some of these gases
at the following higher temperatures and greater pressures :





Temperature,


Pressure,




Fahrenheit.


Atmospheres.


Sulphurous acid - .-..
Ammoniacal gas - - -
Protoxide of azote - -


4G'4
50
51-8


25
5
43


Carbonic acid - - -. -


50


45



VAPORIZATION AND CONDENSATION. 99

Hydrochloric acid has been reduced to a liquid at 50 under
a pressure of 40 atmospheres.

1 503. Under extreme pressures, gases depart from the common
Jaw of the density being proportional to the pressure. In these
experimental researches, it has been found that when the gases
are submitted to extreme compression, and deprived of a large
portion of the surcharged heat, they begin to depart from the
general law in virtue of which the density of gaseous bodies at
the same temperature is proportional to the compressing force,
and they are found to acquire a density greater than that
which they would have under this general law. This would
appear, therefore, as a departure from the law, preliminary to
the final change from the gaseous to the liquid state ; and in
this point of view, analogies have been observed which render
it probable that the point of condensation of several of the
gases not yet liquefied has been very nearly approached.

Thus, it has been found that the density of several of them,
among which may be mentioned light carburetted hydrogen
and defiant gas (heavy carburetted hydrogen), has been sen-
sibly greater than that due to the compressing force under
extreme degrees of compression.

1504. State of ebullition, boiling point. If heat be continu-
ally imparted to a liquid, its temperature will be augmented,
but will only rise to a certain point on the thermometric scale.
At that point it will remain stationary, until the whole of the
liquid shall be converted into vapour. During this process,
vapour will be formed in greater or less quantity throughout
the entire volume of the liquid, but more abundantly at those
parts to which the heat is applied. Thus if, as usually happens,
the heat be applied at the bottom of the vessel containing the
liquid, the vapour will be formed there in large bubbles, and
will rise to the surface, producing that agitation of the liquid
which has been called boiling or ebullition.

This limiting temperature is called the boiling point of the
liquid.

Different liquids boil at different temperatures. The boiling
point of a liquid is therefore one of its specific characters.

1 505. Boiling point varies with the pressure. Liquids in ge-
neral being boiled in open vessels, are subject to the pressure
of the atmosphere. If this pressure vary, as it does at dif-



100 HEAT.

ferent times and places, or if it be increased or diminished by
artificial means, the boiling point will undergo a corresponding
change. It will rise on the thermonietric scale as the pressure
to which the liquid is subject is increased, and will fall as that
pressure is diminished.

The boiling point of water is 212, when subject to a pres-
sure expressed by a column of 30 inches of mercury. It is
185, when subject to a pressure expressed by 17 inches of
mercury.

In general, the temperatures at which water would boil
under the pressures expressed in the second column of the table
(1494) are expressed in the first column.

1506. Experimental verification of this principle. Let
water at the temperature of 200, for example, be placed in a
glass vessel, under the receiver of an air-pump, and let the air
be gradually withdrawn. After a few strokes of the pump the
water will boil; and if the gauge of the pump be observed, it
will be found that its altitude will be about 23^ inches. Thus,
the pressure to which the water is submitted has been reduced
from the ordinary pressure of the atmosphere to a diminished
pressure expressed by 23^ inches, and we find that the tempe*
rature at which the water boils has been lowered from 212 to
200. Let the same experiment be repeated with water at the
temperature of 180, and it will be found that a further rare-
faction of the air is necessary, but the water will at length boil.
If the gauge of the pump be now observed, it will be found to
stand at 15 inches, showing that at the temperature of 180
water will boil under half the ordinary pressure of the atmo-
sphere. This experiment may be varied and repeated, and it
will always be found that water will boil at that temperature
which corresponds to the pressure given in the table.

1507. At elevated stations water boils at low temperatures.
It is well known, that as we ascend in the atmosphere, the pres-
sure is diminished in consequence of the quantity of air we leave
below us, and that, consequently, the barometer falls. It follows,
therefore, that at stations at different heights in the atmosphere,
water will boil at different temperatures ; and that the boiling
point at any given place must therefore depend on the eleva-
tion of that place above the surface of the sea. Hence the
boiling point of water becomes an indication of the height of
the station, or, in other words, an indication of the atmospheric



VAPORIZATION AND CONDENSATION.



101



pressure, and thus the thermometer serves in some degree the
purposes of a barometer.

1508. Table of the boiling points of water at various places.
In the following table the various temperatures are shown at
which water boils in the different places therein indicated.

Table of the boiling Points of Water at different Elevations
above the Level of the Sea.



Names of Places.


Above Level
of Sea.


Mean Height
of Barometer.


Thermometer.




Feet.


Incha.


D


Farm of Antisana -


13455


17-87


1874'


Town of Micuipampa (Peru)


11870
9541


19-02
2075


190-2
194-2


Town of Caxamarca (Peru)


9384


20-91


191-5


Santa Fe de Bogota


8731


21-42


195-6


Cuenca (Quito) -




21-50


195-8


Mexico ...


7471


22-52


198-1


Hospice of St. Gothard -


6808


23-07


199-2


St. Veron (Maritime Alps)


6693


23-15


199-4


Breuil ( Valley of Mont Cervin)
Maurin (Lower Alps)
St.Remi ... -


6585
6240
5265


2327
23-58
24-45


199-6
200-3
202-1


Heas (Pyrenees) -


48(7


24-88


202-8


Gavanne (Pyrenees)


4738


2-T96


203-0


Briancon -


4285


2539


203-9


Barege ( Pyrenees)
Palace of San Ildefonso (Spain)


4164
3790


25-51
25-87


204-1

204-8


Baths of Mont d'Or ( Auvergne)


3412


26-26


205-7


Pontarlier -


2717


26-97


206-8


Madrid


1995


2772


208'0


Innspruck -


1857


27-87


208-4


Munich -


1765


27-95


208-6


Lausanne -


1663


28-08


208-9


Augsburg - -


1558


28 19


209 1


Salzburg - -


1483


28-27


209-1


Neufchatel -


1437


2831


2093


Plombieres


1381


28-39


209-3


Clermont-Ferrand (Prefecture)
Geneva and Fribu g -


1348
1221


28-54


209-3

209-5


Ulm


1211


28-58


209-7


Ratisbon -


1188


28-58


209-7


Moscow -


984




210-2


Gotha -


935


2886


210-2


Turin - -


755


2906


210-4


Dijon


712


29-!4


210-6


Prague -


587


29-25


210-7


Macon (S:ione)


551


29-29


210-9


Lyons (Rhone)


532


29-33


210-9


Cassel


518


29-33


210-9


Gottingen - -


440


29-41


211-1


Vienna (Danube)


436


29-41


211-1


Milan (Botanic Garden) -


420


29-45


211-1


Bologna - -


397


29-49


211-1


Parma -


305


29-57


211-3


Dresden - -


295


29-61


211-3


Paris (Royal Obse vatory, first floor)
Rome (Capitol)



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 12 of 45)