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ventilate. In a close stove, no air passes through the room to
the flue of the chimney, except that which passes through the
fuel, and that is necessarily limited in quantity by the rate of
combustion maintained in the stove. In an open fire-place, on
the other hand, two independent currents of air pass into the
flue, one that which passes through the fuel and maintains the
combustion, and the other, which is far more considerable in
quantity, is that which passes through the opening of the fire-
place above the grate.

The temperature of the column in the flue is due entirely to
the former, and the activity of the combustion will be deter-
mined by the relative magnitudes of the grate and the space
above it ; these two magnitudes repi'esenting the proportion in
which the open stove serves the two purposes of warming and
ventilation, the grate representing the function of warming,
and the space above it the function of ventilating. Even when
there is no fire lighted in the grate, the column of air in the
chimney is in general at a higher temperature than the external
air, and a current will therefore in such case be established up
the chimney, so that the fire-place will still serve, even in the
absence of fire, the purposes of ventilation. In very warm
weather, however, when the external air is at a higher tem-
perature than the air within the building, the effects are
reversed ; and the air in the chimney being cooled, and there-
fore heavier than the external air, a downward current is
established, which produces in the room the odour of soot. To
prevent this, a trap or valve is usually provided in it, which
can be closed at pleasure, so as to intercept the current. It
should be observed, however, that this trap should only be
closed when a downward current is established ; since, at
other times, even in the absence of fire, the ventilation of the
apartment is maintained.

1387. Methods of warming apartments. In all apparatus
adapted to warm buildings, the fact that warm air is more
expanded, and therefore lighter bulk for bulk, than cool air.



40 HEAT.

requires to be attended to. It is usual to admit the warm air
through apertures placed in the lower parts of a room, because
it will ascend by its buoyancy and mix with the colder air,
whereas if it were admitted by apertures near the ceiling it
would form strata in the upper part of the room, and would
escape at any apertures which might be found there. But if
there be means of escape only in the lower part of the room,
then the strata of warm air let in above will gradually press
down upon the cool air below and force it out through the
chimney, doors, windows, or other apertures.

In general, the air contained in an apartment collects in
strata arranged according to its temperature, the hotter air
collecting near the ceiling, and the strata decreasing in tem-
perature downwards. Thermometers placed at different heights
between the floor and the ceiling would accordingly show
different temperatures. The difference of these temperatures is
sometimes so considerable that flies will continue to live in one
stratum which would perish in another.

If the door of an apartment be open it will be found that two
currents are established through it, the lower current flowing
inwards and the upper outwards. If a candle be held in the
doorway near the floor, it will be found that the flame will be
blown inwards ; but if it be raised nearly to the top of the
doorway, the flame will be blown outwards. The warm air in
this case flows out at the top, while the cold air flows in at the
bottom.

1388. Principle of an Argand lamp. The combustion
which produces the flame of an Argand lamp is maintained
upon the same principle as that by which the combustion is
maintained in a common fire-place. The wick, which is cylin-
drical, surrounds a brass tube which communicates at its lower
end with the external air. A glass chimney surrounds the
wick and the flame. The air ascending through the glass tube
passes the flame and is heated by it, and then ascends in the
glass chimney within which it is confined. This glass chimney
is therefore filled with a column of heated air which has a
buoyancy proportional to its expansion, and ascends with a
proportionate force, fresh air being supplied to the wick con-
tinually through the brass tube already mentioned. But as the
column of air ascending through this brass tube would only
touch the flame on its external surface, the internal parts of the



DILATATION OF GASES. 41

column would not be so strongly heated. To increase the heat
imparted to the air, therefore, a metal wire is placed in the
centre of the brass tube, which supports a button a little less in
diameter than the wick at the level of the flame. When the
column of air which ascends in the tube encounters this button,
the central parts of the column are intercepted, and can only
ascend by passing round the edge of the button, and therefore
in contact with the flame. By this expedient all the air which
ascends through the brass tube is made to pass in close contact
with the flame before it can enter the glass chimney above the
flame, and thus the intensity of the force of the draft is increased
and the combustion is augmented.

It will be explained hereafter that flame is gas heated to such
an intense degree as to become luminous. It is in consequence
of its levity that it always ascends in the atmosphere.

1389. Cause of atmospheric currents. The expansion and
contraction of different parts of the atmosphere consequent upon
the vicissitudes of temperature, produce the phenomena of the
winds. When any portion of the atmosphere becomes heated
it expands, and being lighter than the surrounding parts of the
air it rises ; immediately the adjacent air rushes in to fill its
place and produces a wind. The sun acting with greater effect
on the portions of the atmosphere around the equator than on
those near the poles, these portions become heated and lighter
than the former. They therefore ascend as air does in a chimney,
and the colder portions of the atmosphere around the poles rush
in to fill their place. There are, therefore, permanent atmo-
spheric currents established from the poles towards the equator.
These, combined with the effects of the rotation of the earth
upon its axis, produce the phenomena called the trade-winds,
which blow with such regularity and permanency, in the
northern hemisphere from the north-east, and in the southern
hemisphere from the south-east.

It must be observed, however, that the sun is not the only
cause which affects the temperature of the air. The different
degrees of heat reflected or radiated from the surface of the land
compared with the surface of the water, form another important
cause of the variation of the temperature of the air, and
therefore of the atmospheric currents.

1390. Experiments illustrating the expansion and contraction
of air. The expansion of air by heat and its contraction by



42 HEAT.

cold may be made manifest by a variety of simple and easily
executed experiments. If a common drinking glass be inverted
and held over the flame of a lamp or candle for some time, it
will be filled with air heated by the flame ; if it be then sud-
denly plunged with its mouth downwards in water, the water
will be found to rise in the glass to a height above the level of
the water outside the glass. The cause of this is that the air
which fills the glass, having been previously rarefied by heat
and afterwards cooled, when removed from the lamp is con-
tracted so as to fill a less space than the capacity of the glass
which it filled when heated previous to immersion.

This experiment may be rendered still more striking by using
a glass bulb blown at the end of a tube, like a thermometric
tube, instead of a glass. Let such a bulb be held for some
minutes over the flame of a spirit-lamp. The air which fills it
will become highly expanded and rarefied by the heat. Let the
open end of the tube be then plunged in water, the bulb
being presented upwards. After some time, when the tube has
cooled and the air within it contracted, the water will rise in
the tube and will nearly fill the bulb, the portion of the bulb
not filled being the space within which the air previously heated
had been contracted by cooling.



CHAP. V.

DILATATION OF LIQUIDS.

1391. Liquid a state of transition. The liquid state is one of
transition between the solid and the vaporous states. Solids by
heat are converted into liquids, and liquids into vapours.

The liquid state, therefore, is maintained between two limits
of temperature a lower limit, at which the liquid would solidify;
and a higher limit, at which it would vaporize. In different
liquids these limits are separated by a greater or less range of
temperature. In some, alcohol for example, the point of solidi-
fication stands at a low temperature on the scale ; while in others,
as in some of the oils, the point of vaporization is placed at a
very high limit. In others, as in mercury, these points are



DILATATION OF GASES. 43

widely separated, the vaporizing point being at a very high,
and the freezing point at a very low temperature.

1392. Rate of dilatation of liquids in general not uniform.
It is found in general that the rate of dilatation of liquids is not
uniform, like that of solids and gases, and that it not only in-
creases as the temperature is elevated, but is subject to certain
irregularities as it approaches the points at which the liquid
would pass, on the one hand, into the solid, and, on the other,
into the vaporous state.

1393. Specific gravity of a liquid varies with its temperature.
Since by dilatation and contraction the proportion of the
volume of the liquid to its weight is varied, all the methods
which have been explained in (763) et seq. for ascertaining
the specific gravity of liquids will be equally applicable to de-
termine their dilatation and contraction. If, for example, a
given volume of liquid at a certain temperature weigh 1000
grains, and the same volume at another temperature weigh only
950 grains, the proportion of the volumes which have equal
weights will be the inverse of those numbers, that is, of 950 to
1000.

1394. Rates of dilatation of liquids. The only body in the
liquid state whose variations of volume through a considerable
range of the thermometric scale are found to be exactly propor-
tional to its change of temperature, is mercury.

It has been ascertained that, from 13 below the freezing
point to 212, the increments of volume in this liquid for equal
increments of temperature are equal.

The principal liquids whose rates of dilatation have been sub-
mitted to exact experimental investigation, are, water, mercury,
and alcohol. The increment of volume which each of these
liquids receives from 32 to 212 is ^rd of the volume at 32
for water, ^th for mercury, and ith for alcohol.

1395. Exceptional phenomena manifested by water approach-
ing its freezing point, Water, as it falls in temperature towards
the freezing point, exhibits phenomena which form a striking ex-
ception to the general laws of dilatation and contraction by tem-
perature. As its temperatvire is lowered, the rate at which it
contracts is found to diminish until it arrives at the temperature
of 38-8 Fah. when all contraction ceases, and, if the tempera-
ture be further lowered, the volume is observed to remain sta-
tionary for some time ; but, on lowering it still more, instead of



44 HEAT.

contraction, a dilatation is produced, and this dilatation continues
at an increasing rate until the water is congealed. It appears,
therefore, that at the temperature of 38'8 the density of water
is a maximum. It is found that for a few degrees above and
below such temperature of greatest density the dilatation is the
same ; thus, at 1 above and 1 below 38-8, and at 2 above
and 2 below that point, the specific gravities are exactly equal.

1396. Temperature of greatest density. The experiments
of Blagdon and Gilpin fixed the temperature of greatest con-
densation at 39 ; those of Lefevre, Gineau, Halstrom, Hope,
and Rumford fixed it a little above 40. More recent experi-
ments, however, conducted under conditions of greater accuracy
by Miinke and Stampfer, have determined it at 38'8.

1397. Taken as the basis of the French metrical system.
Water, at its greatest density, is taken as the base of the uni-
form system of measures adopted in France, the unit of weight
being the weight of a cube of distilled water taken at its
greatest density, the side of the cube being the length of a
centimetre, or the one hundredth part of a metre, which is the
linear unit. The length of the metre is 39'37 English inches.

1398. Effect of the relative densities of different strata of the
same liquid. It has been already proved that if liquids having
different specific gravities be placed in the same vessel without
mixing with each other, they will arrange themselves in strata
according to their specific gravities, the heavier being below
the lighter. This principle will seem to explain several facts.
If cold water be poured into a vessel, a thermometer being
immersed in it, and hot water be carefully poured over it, so as
to prevent the liquids being mixed, the hot water will float on
the cold. The thermometer immersed in the cold water will
not rise, nor will a thermometer immersed in the hot water fall.
But if the water be agitated so as to mix the two strata, then
their temperatures will be equalized, and the lower thermometer
will rise and the upper fall. If, however, hot water be first
poured into the vessel, a thermometer being immersed in it,
and cold water be then carefully poured over it, so as to pre-
vent such agitation as would cause the fluids to mix, and a
thermometer be also immersed in it, it will be found that the
lower thermometer will rapidly fall and the higher one will
rise ; in fact, in this case the cold water descends through the
hot water by its superior gravity, and the two fluids of dif-



DILATATION OF GASES.



45



ferent temperatures, in passing through one another, become
mixed, and the whole mass takes an intermediate temperature.

1399. Process of heating a liquid. The process by which
water is boiled by heat applied to the bottom of a vessel, is ex-
plained on this principle. The water in contact with the bot-
tom of the vessel being heated, is expanded, and becomes
lighter bulk for bulk than the strata over it. It therefore rises,
and the water above it falls, and, in its turn being expanded by
heat, is made to rise. There is thus a continual current of the
water heated by the fire upwards, and a counter current of the
colder water forming the superior strata downwards ; and this
goes on until all the water in the vessel has been raised to the
boiling point.

1400. Heat does not descend in a liquid. It is easy to show
that any source of heat, however intense, applied to the upper
surface of water, would be incapable of raising the temperature
of the mass. Thus, if we suppose oil at the temperature of
300 poured upon the surface of water in a vessel at 50, the oil
will float upon the water, and a thin stratum of the water in
contact with ft will have its temperature raised, and will there-
fore be expanded; but, being lighter bulk for bulk than the
colder water under it, it will still float on the top. No inter-
change of currents will take place, by which the heated water
forming the upper stratum can be mixed with the water form-
ing the lower stratum ; and, as water is a non-conductor of
heat, as will hereafter be shown, the heat of the oil, and of the

stratum of water in immediate contact with it,
will not be propagated downwards. It would
be possible for a cake of ice to remain in the
bottom of such a vessel without being melted,
notwithstanding the stratum of oil at 300
floating upon its surface.

1401. Experiment showing the propagation
of heat through a liquid by currents. The
system of upward and downward currents pro-
duced by heat applied to the bottom of a vessel
containing a liquid, may be rendered manifest
by the following experiment. Let a tall jar,
fig. 43H., be filled with cold water, and let some
amber powder be thrown into it. The particles
Fig. 438. of this powder being equal in weight to water



i!




46 HEAT.

bulk for bulk, or nearly so, will remain suspended, and may
be seen through the sides of the vessel. Let this jar be im-
mersed to some depth in a vessel of hot water, so that the lowest
strata of the water in it may become gradually heated. The
water in the bottom of the jar will now be observed continually
to ascend, carrying the amber particles with it, while the colder
water in the upper part will descend. The contrary currents
will be rendered manifest to the eye by the particles of amber
which they carry with them.

If heat be applied to the sides of the cylindrical jar, but not
to the bottom, the water immediately in contact with the sides,
becoming heated, will ascend. The water in the centre of the
jar, on the other hand, being removed from the source of heat,
will retain its temperature, and will of course sink as the water
next the side rises. In this case, two distinct currents will be
seen, one immediately next the surface of the jar continually
ascending, and the other in the centre of the jar continually
descending.

This may be shown by placing the cylindrical glass jar within
another somewhat greater in diameter, and pourirfg a hot liquid
in the space between them.

1402. Method of warming buildings by hot water. On the
same principle is explained the method of warming buildings by
pipes filled with hot water.

A boiler is constructed in the lowest part of the building
completely closed at the top, but terminating in a tube or pipe,
which is conducted upwards, and carried through the different
apartments which it is intended to warm. This pipe terminates
in a funnel at the top of the building, the boiler and pipe being
filled with water up to the funnel. When fire is applied under
the boiler, the water, becoming heated, ascends, and the colder
water descends ; and these contrary currents continue until every
particle of water contained in the pipes carried through the
building is raised to whatever temperature, under 212, may be
desired.



CALORIMETRY. 47

CHAP. VI.

CALORIMETRY.

1403. Quantitative analysis of heat. The department of the
physics of heat devoted to the quantitative analysis of that agent
is called calorimetry, and the instruments by which its quantity
is measured are called calorimeters.

1404. Calorimetry and thermometry. If the same quantity
of heat always produced the same or equal thermometric changes,
every thermometer would be a calorimeter, and calorimetry
would not form a part of this subject distinct from ther-
mometry.

But not only do equal quantities of heat produce unequal
thermometric changes on different bodies, but even on the same
body at different points of the scale, and in some cases no
thermometric change whatever.

1405. Thermal unit. To reduce heat to arithmetical ex-
pression, it is necessary that some suitable thermal measure be
adopted, and a thermal unit selected.

It may be assumed as self-evident, that to produce the same
thermal effect on the same quantity of the same body under like
circumstances will ^always require the same quantity of heat.
Thus it is apparent, that to raise a pound of pure water from
32 to 33, or to liquefy a pound of ice, or to convert a pound
of water into vapour under a given pressure, will always require
the same quantity of heat, from whatever source such heat may
proceed.

Water has been selected as the standard of thermal measure,
for reasons nearly the same as those which have determined its
selection as the standard of specific gravity, (763, et seq.)

We shall therefore take as the thermal unit the quantity of
heat which is necessary to raise a pound of pure water from
32 to 33.

1406. Specific heat. The quantity of heat which is neces-
sary to raise a pound of any other body from 32 to 33, being
in general different from that which would produce the same
effect on water, and in general being different for different
species of bodies, is called their specific heat, for the same



48



HEAT.



reason that the weight they include under the same volume is
called their specific gravity.

1407. Uniform and variable. The specific heat of a body
is said to be uniform throughout any extent of the thermometric
scale when it requires the same quantity of heat to raise the
temperature one degree through such extent of the scale.

If H express the quantity of heat necessary to raise wlbs. of
a body from the temperature expressed by T' to the temperature
expressed by x, the specific heat being expressed by s and
being uniform, we shall therefore have

H=S X(T T') xw;

that is to say, the quantity of heat is found by multiplying
together the numbers expressing the specific heat, the elevation
of temperature, and the weight in Ibs.

When the quantity of heat necessary to raise a body one
degree is different in different parts of the scale, the specific heat
is said to be variable; and when it does so vary, it is in general
found to increase with the temperature.

1408. Method of solving calorimetric problems. Three
methods have been practised for the solution of calorimetric
problems: 1st, by measuring the heat by the quantity of ice
it liquefies ; 2ndly, by calculating it by means of mixing or
bringing into close juxtaposition bodies at different temperatures
so that their temperatures shall be equajjzed ; and 3dly, by

observing the rate at which
heated bodies cool.

1409. Calorimeter of La-
voisier and Laplace. The
calorimeter of Lavoisier and
Laplace is based upon the first
of these principles.

This apparatus is repre-
sented in fig. 439. Two simi-
lar metallic vessels, v and v',
are constructed, one a little
smaller than the other, so that,
when applied one within the
other, a small space A may be
left between them. From the
bottom of the external vessel




Fig 439.



CALORIMETRY. 49

v a discharge-pipe/ with a stop-cock K, proceeds. From the
bottom of the inner vessel v' a similar pipe proceeds, which
passes water-tight through the bottom of the vessel v, and is
also furnished with a stop-cock K'.

This pipe K' is inserted into a close vessel R. The external
vessel v has a close cover, by which all communication with
the external air is cut of and the inner vessel v' is likewise
furnished with a small cover, by which all communication
with the space A is intercepted. The space A between the
two vessels is filled with pounded ice; and if the apparatus be
placed in an atmosphere above 32, this ice will be gradually
liquefied, and the water produced by it will flow off through the
cock K, when the stop-cock is open, and will be received in
the vessel B. The space A being kept continually supplied
with ice, it is evident that the interior vessel v' will be main-
tained constantly at the temperature of 32, and the air included
in it, and any objects placed in it will be necessarily reduced to
that temperature.

A third vessel v" is now placed within the second v', and the
space B between the second and third is filled with pounded ice,
in the same manner as the vessel A. But it is evident that this
ice cannot be affected by the temperature of the external air,
since it is surrounded with the melting ice included in the space
A, which is continually at 32.

If any object at a temperature above 32 be placed at c,
within the vessel v", this object will gradually fall in its tem-
perature by imparting its heat to the ice in the space B ; and it
will continue to impart heat, and its temperature will continue
to fall, until it arrives at the temperature of 32, when it will
cease to liquefy the ice round it. The water proceeding from
the liquefaction of the ice in the space B, is discharged through
the pipe K', the stop-cock being opened, and is received in the
vessel R. The quantity of water thus received in R will there-
fore be proportional to the heat imparted by the body contained
in the vessel v" to the ice in the space B.

If this apparatus be applied to solid bodies, it will be suffi-
cient to introduce the body under experiment directly into the
interior of the vessel v" ; but if it be applied to liquids, it will be
necessary that the liquid under experiment should be contained
in a vessel, which vessel is introduced into v". In this case,
the vessel containing the liquid should be reduced to the tern-



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 7 of 45)