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Principles of physics, or, Natural philosophy: designed for the use of colleges and schools online

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because the atmospheric pressure is less, and conversely in descending
into mines, it rises. Accurate observations show, that a difference of
about 543 feet in elevation produces a variation of 1 F. in the boiling
point of water. The metastatic thermometer (579) is used in these
observations. Fahrenheit first proposed determining the heights of
mountains by the depressed temperature cf boiling water.



462



PHYSICS OF IMPONDERABLE AGENTS.



Regnault has designed an apparatus called a hypsometer, fig. 487, for
determining elevations by the boiling point of water. It consists of a copper
vessel, C, containing water. This is surmounted by a brass cylinder which
supports and encloses a thermometer. The upper part of this cylinder is formed
in pieces, t, which slide into each other like the tubes of a telescope, and serve
to confine the steam about the thermometer tube, as in fig. 443. Air is supplied
to the lamp, I, by the holes, o, o. The steam escapes by a lateral 487

orifice in the upper part of the instrument. 488

680. High pressure steam. The

boiling point rises as the pressure in-
creases. This fact is readily demon-
strated in a general way by Marcet's
apparatus, fig. 488.

A spherical boiler is supported over a lamp
upon a tripod of brass. A thermometer, t,
enters the upper hemisphere, and its bulb is
exposed directly to the steam. A stop-cock
and safety valve, V, opens a communication
to the outer air. A manometer tube, A, with
confined air (280) descends into some mercury
placed in the boiler (whose lower hemisphere
is for that reason made of iron). The boiler
is filled with water to the equator. When the
water boils and the air has been expelled, the
open stop-cock is closed and the steam com-
mences to accumulate. The thermometer,
which stood previously at 212, begins to riso
higher and higher as the column of mercury
rises in the gauge. When the mercury has
risen in the gauge a little less than half
the height of the tube, the thermometer will
indicate 249-5 F., when two-thirds of the way 273-3, and so on. Table XIX.
gives the boiling point of water at different atmospheric pressures as ascer-
tained by Regnault.

Advantage is taken of the temperature of high steam in the arts to extract
gelatine from bones, and to perform other difficult solutions and distillations
which, at 212, would be impossible. Papin, a French physicist, who died in
1710, first studied these effects of high steam with an apparatus known as
Papin's digester. It is only a boiler, of great strength, provided with a safety
valve (then first used).

681. Production of cold by evaporation. A liquid grows sen-
sibly colder, if while evaporating it does not receive as much heat as it
loses, and the more sensibly so, as the evaporation is move rapid.

Eau de cologne, bay-rum, or ether, evaporating from the surface of the skin,
produces very sensible coldness, due to the rapid absorption of the bodily heat
in the evaporation. Portions of body may be thus benumbed and rendered
insensible to pain during surgical operations.




HEAT.



463



489




A summer shower cools the air by absorbing heat from the earth and the air
during evaporation. Curtains wet with water, called tatties, much used in India j
leafy branches of trees, mossy banks, and fountains draped by climbing plants,
are cool for the same reason. Fanning the surface produces coolness both from
conduction and evaporation. Wet clothes are pernicious, chiefly from the rapid
loss they cause of animal heat during evaporation, thus impeding the circula-
tion. In hot climates, where ice is rare, water is cooled to an agreeable
temperature by the use of jars of porous earthenware placed in a draught of
air. The surface moisture is rapidly evaporated by the dry air, and the water
in the vessels falls 20 or 30 degrees below the exterior air, even at 80 or 90
degrees. Water is readily frozen in a thin narrow test-tube by the constant
evaporation of ether from a muslin cover drawn over the outside of the tube.
In the East Indies, water is frozen by its own evaporation, aided by radiation,
in cool serene nights, when the external air is not below 40. For this purpose
shallow earthen pans are used, placed in a slight pit or depression of the earth
upon straw to cut off terrestrial radiation.

Water is endowed with a remarkable emissive
power, and will, as shown by Melloni, lose 7 be-
low the atmosphere by simple radiation in serene
nights. Compared to this remarkable Indian
result, Leslie's experiment of freezing water in
the vacuum of an air-pump (over sulphuric acid
to absorb the vapor, fig. 489) seems simple; and easier still is the same effect
produced in the cryophorus (or frost-bearer) of Dr. Wollaston, fig. 490, where a
portion of water in one 490

bulb of a vacuous glass
tube is frozen by its own
rapid evaporation due to
cooling the empty bulb in
a freezing mixture.

Twining's ice ma-
chine. An apparatus has been successfully contrived by Prof. Alex. Twining
for producing ice upon a commercial scale in those hot climates where it cannot
be carried from colder countries, by the rapid evaporation of a portion of ether
confined in metallic chambers contiguous to the water vessels the process, by
aid of an air-pump and condenser, being continuous and without sensible loss
of ether. This plan is equally applicable to cooling the air of apartments, either
for the preservation of provisions or for the comfort of the occupants.

682. Latent heat of steam. A large amount of heat disappears
or is rendered latent during evaporation. According to Regnault, the
latent heat of steam is 967'5. Its determination is made in a number
of ways.

If a vessel containing water at the temperature of 32 is placed over a steady
source of heat, it receives equal additions of heat in equal times. Let the time
bo noted that is required to raise the temperature to 212. If now the heat is
continued until all the water is converted into steam, it will be found that the
time occupied in the evaporation was 5J times that required to heat the water
through the first 180, . e., from 32 to 212. Consequently 5J times as much
heat is absorbed during the evaporation of water as is required to bring it to
boiling point. The latent heat of steam is therefore about (180 X H) 990.




464 PHYSICS OF IMPONDERABLE AGENTS.

Again, the latent heat of steain is determined by distilling a certain amount
of water and condensing the steam in a large volume of the same liquid. If
the temperature be noted before and after the experiment, it will be found that
the heat from the steam formed from a pound of water, was sufficient to raise
the temperature of ten pounds of water 99. The latent heat of steam is there-
fore again found to be (99 X 10) 990.

Experiments conducted in the simple manner just mentioned cannot be
entirely accurate, owing to a certain loss of heat by vaporization, conduction,
and radiation. Numerous precautions are therefore to be adopted to insure the
accuracy science demands in such an investigation, the details of which are
inconsistent with our limited space.

The latent heat of steam obtained by different experimenters, varies
somewhat as follows : Watt, 950 ; Lavoisier, 1000 ; Despretz, 955'8 ;
Brix, 972 ; Regnault, 967'5 ; Fabre and Silbermann, 964'8.

683. Latent and sensible heat of steam at different tempera-
tures. The whole amount of heat in steam is the latent heat, plus the
sensible heat. Thus the heat of steam at the temperature of ebullition
is 967'5 + 212 = 1179-5. It has heretofore been generally stated,
that the heat absorbed in vaporization is less as the temperature of the
vaporizing liquids is higher. So that if the sensible heat of steam at
any temperature is subtracted from the constant 1179'5, the remainder
is the latent heat of steam at that temperature. For example : the latent
heat of steam at 279-5, is 900, at 100, 1079'5, &c. This statement
however is found to be somewhat inaccurate, although in practice it
may be assumed to be nearly correct.

From the experiments of Regnault, it appears that the sum of the
latent and sensible heat increases with the temperature by a constant
difference of 0'305 for each degree F., as is shown in Table XXII.

684. Mechanical force developed during evaporation. During
the conversion of a liquid into vapor, a certain mechanical force is exerted.
The amount of this force depends on the pressure of the vapor and the
increase in volume which the liquid undergoes.

Equal volumes of different liquids produce unequal amounts of vapor at their
respective boiling points.

1 cubic inch of water expands into 1696 cubic in. vapor at boiling point.
1 " " alcohol " " 528 " " " " "

1 " " ether " " 298 " " " " "

1 " " turpentine " 193 " " " "

Now although the latent heat of equal weights of other vapors is less than
that of steam, yet no advantage would arise in generating vapor from them in
place of water in the steam-engine. For equal volumes of alcoholic and aqueous
vapor contain nearly the same amount of latent heat at their respective boiling
points, and such is the case to a great extent with other liquids. The cost of
the fuel in generating vapor would be in proportion to the amount of latent
aeat in equal vdumes of the vapor.



HEAT 465

685. Liquefaction of vapors, or the conversion of vapors into liquids,
is accomplished ia three ways. 1st, by cooling; 2d, by compression;
and 3d, by chemical affinity. Only the first two of these methods will
be spoken of. When vapors or gases are condensed into liquids, the
same amount of heat is given out as sensible heat which was absorbed
and rendered latent when they assumed the aeriform condition.

686. Distillation is the successive evaporation and condensation of
liquids. The process depends on the rapid formation of vapor during
ebullition, and the condensation of the vapor by cooling.

Distillation is used, first, for the separation of fluids from solids, as the dis-
tillation of ordinary water, to separate the impurities contained in it; 2d, for
the separation of liquids unequally volatile, as in the distillation of fermented
liquors, to separate the volatile spirits from the watery matter.

687. Distilling apparatus of various kinds is employed according
to the special purpose to which it is applied. The most ancient is
the alembic ; its invention is attributed to the Arabs. It consists of a
boiler of copper or iron, furnished with a dome-shaped head ; to the
upper part of this is attached a metal tube which passes through a
vessel of cold water, whereby the vapor (as it passes over when heat
is applied to the boiler) is condensed, and flows into a proper receptacle.

Where small quantities 491

of liquid are to be distilled,
glass retorts, fig. 491, or
flasks are used. These are
heated by alcohol lamps, or
by small charcoal furnaces.
The receiver may consist
of a small flask connected
with the neck of the retort,
as represented by S. By
means of water flowing
continually on it from B, a
proper cooling is effected.

688. Physical identity of gases and vapors. The difference
between gases and vapors is merely one of degree, and their identity
in many physical properties has already been shown. Thus the ratio
of their expansion by heat is the same as that of the permanent gases.
A permanent gas may be considered as a super-heated vapor ; the vapor
of a liquid which volatilizes at very low temperatures.

Theory of the liquefaction and solidification of gases. By
the last section, if the excess of heat is removed from a gas, it is in the
same condition as an ordinary vapor, containing only sufficient heat to
42




466



PHYSICS OF IMPONDERABLE AGENTS.




maintain it in the aeriform condition. By the compression of a gas,
heat is evolved, by rendering sensible the heat before latent. If the
compressed gas is then surrounded by a freezing mixture, the further
abstraction of heat causes the condensation of a corresponding portion
of gas into a liquid. It is thus by condensing and cooling gases, that
their liquefaction and solidification have been effected.

689. Methods of reducing gases to liquids. In 1823, Faraday
liquefied chlorine, cyanogen, ammonia, carbonic acid, and some other
gases, by the following simple means.

The materials from which the gas was to be evolved, provided they were solids,
were placed in a strong glass tube, 492

bent at an obtuse angle near the
middle, fig. 492, and the open ends
hermetically sealed. Heat was then
applied to the end containing the
materials (e. g. cyanid of mercury),
while the empty end was cooled in a
freezing mixture. The pressure of the gas evolved in so small a space, united
with the cold, liquefied a portion of it. Otherwise, if fluids were to be employed,
the tube had the shape seen in fig. 493. The fluids were introduced by the small
funnel o n, into the curves c and b, and the ends, a, d, were then sealed by the
blow-pipe. By a simple turn of the tube, all the fluid
contents are transferred to the end, a, fig. 494, and the
empty end, d, is placed in a freezing mixture where the
liquid gas collects. Any fluid which distils over from
a, collects in the bottom of the middle curve. A minute
manometer was introduced by Faraday into these tubes,
in order to determine the pressure at which liquefaction
occurred. The manometer was a small glass tube sealed
at one end, and holding a drop of mercury; the mode
of reading the pressure has been before explained (280).

Later researches of Faraday. In 1845,
Faraday published the results of his experiments
on the liquefaction of gases by means of solid
carbonic acid. A mixture of this solid with ether,
in the vacuum of an air-pump, gave him a tempe-
rature as low as 166 F.

In such a bath, at the ordinary pressure of the
atmosphere, chlorine, oxyd of chlorine, cyanogen, am-
monia, sulphuretted hydrogen, arseniuretted hydrogen, hydriodic acid, hydro-
bromic acid and carbonic acid, were obtained in the liquid form under moderate
pressures. These liquids were colorless, with the exception of those from chlo-
rine and oxyd of chlorine, which are colored gases in the ordinary state. A
number of the liquefied gases were solidified. The results obtained by Faraday
on the liquefaction and solidification of gases may be found in Table XX.

690. Thilorier's and Bianchi's apparatus for condensation of




494






HEAT. 467

gases.- To avoid the danger of explosion in the use of glass tubes,
and at the same time to obtain large supplies of liquid gases in a
manageable form, a powerful apparatus of iron has been contrived by
Thilorier; and, more lately, another by Bianchi with mechanical com-
pression, for a description of which reference may be had to the Author's
Chemistry.

691. Properties of liquid and solid gases. Liquid carbonic acid
is colorless, like water, and has a density of 0'83. Its coefficient of
expansion is more than four times that of air. Twenty volumes of the
liquid at 32, becoming 29 volumes at 86.

The solidified acid obtained by the evaporation of a portion of the liquid,
appears in the form of snow; when congealed by intense cold alone, it is clear
and transparent like ice. It rnelts at a temperature of 70 F., and is heavier
than the liquid bathing it. The solid acid may be preserved for many hours if
it be surrounded with cotton or some other poor conductor of heat. It gradually
vaporizes without assuming the liquid form. The temperature of this solid, as
determined by Faraday's experiments, is about 106 below F. Although so
intensely cold, it may be handled with impunity, and when thrown into water,
the latter is not frozen. By moistening it with ether, to which it has a strong
adhesion, its low temperature is at once manifested. If mercury is placed in a
wooden basin and covered with ether, and then solid carbonic acid be added,
the mercury will soon be frozen. The temperature required to freeze the mer-
cury is about 40 F. This frozen mercury may be drawn into bars, or moulded
into bullets, or beaten into thin plates, if the operations be performed with wooden
instruments.

NATTERER, with a mixture of liquid protoxyd of nitrogen and bisul-
phid of carbon, records a temperature of 220 F. Even at this low
temperature, liquid chlorine and bisulphid of carbon preserve their
fluidity.

In protoxyd of nitrogen gas, combustibles burn with nearly as great intensity
as in pure oxygen ; combustion also takes place in liquid protoxyd of nitrogen,
notwithstanding the intense cold. A fragment of burning charcoal, thrown into
this liquid, burns with brilliant scintillations, and thus almost at the same point
there is a temperature of about 3600 above and 180 below Fahrenheit's zero.

692. Latour's law. From his experiments on the conversion of
liquids into vapors, Caignard de Latour announced the following law :

There is for every vapor izable liquid a certain temperature and pres-
sure at which it may be converted into the aeriform state, in the same
space occupied by the liquid.

In these experiments, strong glass tubes, furnished with interior manometer
gauges, were partially filled with water, alcohol, ether, and other liquids, and
hermetically sealed. The temperature of the tubes was then gradually raised.
Ether becomes a vapor at 328, in a space equal to double its original bulk,
exerting a pressure of 37'5 atmospheres; alcohol at a temperature of 404-5,
with a pressure of 119 atm<j spheres, and water disappeared in vapor, in a space



468 PrfYSICS OE IMPONDERABLE AGENTS.

four times its own bulk, at the temperature of about 773. If Mario tte's law
held good in these cases, the pressures exerted would have been very much
greater than were actually observed. Even before a liquid wholly disappears,
the elasticity of the vapor is found to increase in a proportion far greater than
is the case with air at equally elevated temperatures. It is not therefore sur-
prising that mere pressure fails to liquefy many bodies which exist ordinarily
as gases. Compare the statements respecting Mariotte's law in $ 274-277.

G93. Density of vapors. The accurate determination of the density
of vapors, is of much importance in Chemical Physics. It is accom-
plished by filling a globe, or other vessel of glass, with the vapor at a
given temperature, and weighing it ; this weight, divided by the weight
of an equal volume of air, under the same circumstances of tempera-
ture and pressure, gives the density of the vapor. The details of
the methods in use for this purpose, belong more appropriately to
chemistry.

$ 9. Spheroidal condition of Liquids.

694. Spheroidal state. Drops of water scattered on a polished
surface of heated metal do not immediately disappear, but assume the
form of flattened spheres, rolling quietly about, until they gradually
evaporate. . If the metal has not a certain temperature, it is wetted by
the water with a hissing sound. This observation was made in 1746,
and ten years after, Liedenfrost called particular attention to the phe-
nomenon. Dobereiner, Laurent and others, also experimented upon
this subject. They found that saline solutions, as well as simple
liquids, would act in the same manner as water. It is, however, to
Boutigny that we are particularly indebted for the investigation of the
phenomena of the spheroidal state of liquids.

Illustration of the spheroidal state. The above experiment may
be variously performed, according to the ingenuity of the experimei ter.

A small smooth brass or iron capsule is heated over a lamp, fig. 495,^and a few
drops of water allowed to fall upon it from a pipette ; the
drops do not wet the metallic surface, but roll about in
spheroidal globules, uniting together after a time into a
single mass, which, it will be seen, has the form of an
oblate spheroid, and evaporates but slowly. This is the
condition distinguished by Boutigny as the spheroidal
state. If the metal is allowed to cool gradually, when
the temperature falls to a certain point, the liquid will
burst into violent ebullition and quickly evaporate.

The spheroidal state may be produced in a vacuum as well as in the
air, upon the smooth surface of most solids, and also upon the surface
of liquids.

Noticeable phenomena connected with the spheroidal state.
There are several important points to be noticed as regards this
curious subject. The chief of these are, that,





HEAT. 469

1. The temperature of the plate must be greater than the boiling point of the
liquids, in order to produce the spheroidal state, and it varies with the boiling
point of the liquid employed.

Thus, with water, the spheroidal state is produced when the plate is at a tem-
perature of 340, and may attain it even at 288 ; with alcohol 4&g
and ether, the plate must have at least the temperature of 273
and 142 respectively.

2. The temperature of the xpheroid* is always lower than the
boiling points of the liquids. This was determined by Boutigny,
by immersing a delicate thermometer in the spheroid, as shown
in fig. 496.

Thus, 205-7 is the temperature of the spheroid of water; 168-5
that of alcohol; 93-6 that of ether; 13-1 that of sulphurous
acid.

The temperature of a spheroid is not quite as definite as the
temperature of ebullition of the liquid, but rises somewhat as the
plate upon which it rests is more intensely heated.

3. The temperature of the vapor from a spheroid is nearly the
same as that of the plate upon which it rests, which proves that
the vapor is not disengaged from the mass of the liquid.

4. The rapidity of evaporation from a spheroid, increases with
the temperature of the plate upon which it rests, as is proved by
the following experiments of Boutigny. The same quantity of
water (0-10 gramme, or 1-534 grs.) was evaporated in each case.

With the plate at the temperature of 392, the water evaporated in 207 seconds.
With the plate at the temperature of 752, the water evaporated in 91 seconds.
With the plate at dull red heat, the water evaporated in 73 seconds. With the
plate at bright red heat, the water evaporated in 50 seconds.

Water, in the spheroidal state, evaporates much more slowly than at the tem-
perature of ordinary ebullition. Thus, when the plate was at the temperature
of 212, 0-10 grins, of water evaporated in 4 seconds ; and when at the tempera-
ture of 392, in 207 seconds, or about one-fiftieth part as rapidly.

695. Spheroidal state produced upon the surface of liquids.

A highly heated liquid may cause the spheroidal state in another liquid of lower
boiling point than itself.

Thus, Pelouze found that water assumed the spheroidal state on very hot oil of
turpentine, although the water is the denser liquid. Boutigny has thus sustained
water, alcohol, and ether on sulphuric acid, nearly at its boiling point. With
sufficient precautions, a number of liquids may be thus piled one upon the other.

696. A. liquid in a spheroidal state is not in contact with the
heated surface beneath. This must appear, on reflection upon the
facts already stated, and may be demonstrated as follows :

A horizontal silver plate is surmounted by a tube of the same metal, fig. 497,
whose lower edges have two longitudinal slits opposite to each other. The plate
is placed upon the eolipile (704) containing alcohol, which is nicely adjusted to
a perfect level by the screws in the triangular base. Silver is employed to avoid
the formation of scales of oxyd of copper, which would interfere with the obser-
vation by interposing themselves to the light.

When the plate heated over the lamp reaches the proper temperature, a por-
tion of water is placed upor its centre, and immediately assumes the spheroidal
42*



470



PHYSICS OF IMPONDERABLE AGENTS.



condition. Placing the eye on a level with the surface of the plate, and looking
through the apertures in the sides of the tube, the flame of a candle opposite

497






may be distinctly seen. This could not happen if the liquid was in contact
with the plate. If a thick and heavy silver capsule is heated to full whiteness
over the eolipile, it may, by an adroit movement, be filled entirely with water,
and set upon a stand, some seconds before the heat declines to the point when
contact can occur between the liquid and the metal. When this happens, the
water, before quiet, bursts into steam, with almost explosive violence, and is
projected in all directions, as shown in fig. 498.

697. A repulsive action is exerted between the spheroid and
the heated surface. This proposition follows, indeed, as a conse-
quence of the last. It has already been demonstrated, that a liquid does



Online LibraryBenjamin SillimanPrinciples of physics, or, Natural philosophy: designed for the use of colleges and schools → online text (page 51 of 78)