Dionysius Lardner.

Hand-book of natural philosophy and astronomy (Volume 2) online

. (page 11 of 45)
Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 11 of 45)
Font size
QR-code for this ebook

blast furnaces. In a certain sense, salt may be said to be a flux
for ice ; but this term flux is usually limited in its application
to bodies which are only fused at very elevated temperatures :
for example, in enamelling, and in the manufacture of glass and
of the paste by which precious stones are imitated, siliceous
sand is employed in greater or less proportion, about one-third
for enamel, and nearly three-fourths for plate glass. Now
silica is not fused at any heat attainable by commen furnaces.
M. Gaudin lately succeeded in its fusion, by means of the oxy-
hydrogen blow-pipe, and drew it into threads as fine as the
filaments of silk. When combined, however, with proper
fluxes, it fuses readily in the furnace. The fluxes used vary
according to the purposes for which the silica is applied, but
they consist generally of soda, potash, and lime, with the addi-
tion of lead for flint glass, and stannic acid for enamel. The
compound which results from the mixture of these ingredients,
by their exposure to intense heat, is reduced to a sort of pasty
fusion, but can never be said to undergo positive liquefaction.
Nevertheless, the beautiful transparency of Bohemian glass,
plate glass, flint glass, and the factitious diamonds, show that
the constituents must be combined in a very intimate manner.

Fine earthenware and porcelain are also fabricated by means
of fluxes ; for although fusion is not actually produced, nor is
there the same intimate combination of the constituents as takes
place in vitrefaction, still there is a partial combination, and an
incipient fusion. The fluxes in this case consist also of soda,
potash, lime, and sometimes magnesia, the soda and potash
however being used in their combined form of feldspar.

1475. Infusible bodies. Infusible bodies may be resolved
into two classes, those which are refractory, and which alone
can be properly said to be fusible, and those whose fusion is pre-
vented by their previous chemical decomposition or composition.
Before the invention of the oxy-hydrogen blowpipe, and other
scientific expedients for the production of intense heat, the
number of refractory substances was much more considerable
than it is at present. Scarcely any body can be said to be ab-
solutely infusible except charcoal, which under all its forms of
pure carbon, anthracite, graphite, and diamond, has resisted
fusion at the highest temperature which has yet been produced.


The term refractory, however, is still applied to those classes
of substances which resist fusion by ordinary furnaces.

When certain compound bodies are exposed to an intense
heat, they are resolved into their constituents before they attain
the point of fusion ; and in other cases simple bodies enter into
chemical combination with others which surround them, or are
in contact with them before the fusion takes place.

The fusion, however, may in some cases of both of these classes
of bodies be effected by confining them in some envelope which
will resist the separation of their constituents if they be com-
pound, or exclude them from the contact with bodies with which
they might combine if they be simple.

1476. Marble may be fused, If marble be exposed under
ordinary circumstances to an intense heat, it will be resolved
into its constituents, lime and carbonic acid ; but if it be con-
fined in a strong gun-barrel, for example, it may be fused.

1477. Organic bodies are decomposed before fusion. Almost
all organic solids, except the resins and the fats, are infusible
before they are decomposed ; we cannot melt a piece of wood,
a leaf, a flower, or a fruit ; but after having evaporated their
liquid constituents, and dried them, the influence of heat causes
their constituents to enter into combination, and produces new
substances, which are generally volatile, and which have nothing
in common with the original substances.

1478. Water separated from matter held in solution by con-
gelation. When water holding any body in solution has its
temperature sufficiently lowered, its congelation takes place in
one or other of three ways : first, the water may congeal inde-
pendently of the body which it holds in solution ; secondly, the
body which it holds in solution may congeal, leaving the water
still liquid ; thirdly, the water and the body it holds in solution
may congeal together.

The congelation of the water independent of the substance
it holds in solution is presented in the case of the very weak
solutions. In this case, the point of congelation is always below
the freezing point. Thus, if water hold in solution a small
quantity of alcohol, acid, alkali, or salt, it will be necessary to
reduce the whole to the freezing point to produce its congelation ;
but when ice has been formed upon it, this ice will consist of
pure water, without the mixture of any proportion of the sub-
stances which the water held in solution. Thus, sea-water

84 HEAT.

freezes at 27-^, being 44 below the freezing point of pure
water; and if the ice produced upon it be withdrawn and melted,
it will produce pure water. In the same manner, if weak wine
be frozen, the ice formed upon it will be the ice of pure water,
and the wine which still remains liquid will be proportionally
stronger. This method is sometimes practised to give increased
strength to wine.

1479. Saturated solutions partially decomposed bij cooling.
Water is generally capable of holding in solution only a certain
quantity of any solid substance, and when all the substance has
been dissolved in it which it is capable of taking, the solution
is called a saturated solution. Now, it is found that the quan-
tity of solid matter of any kind which water is capable of hold-
ing in solution, increases with the temperature. Thus, water
at 212 will hold more of any given salt in solution, than would
water at 50. Let us suppose, then, that a saturated solution of
any salt is made at 200. If this solution be allowed to cool, a
part of the salt which it contains must return to the solid state,
since at lower temperatures it cannot hold in solution the
same quantity ;. and in proportion as the temperature of the so-
lution falls, the quantity of solid matter which will be formed in
it will increase. In this case, the cooling accomplishes a partial
decomposition of the solution. If the cooling be accomplished
suddenly, the salt is precipitated tumultuously and in a confused
mass, without form or cohesion ; but if the solution is allowed to
cool slowly and without agitation, the molecules of the salt col-
lect into regular crystals.

Even after the temperature of the solution has ceased to fall,
the decomposition and crystallization will continue, if the vessel
containing the solution be in a position favourable to superficial
evaporation. The water which evaporates from tiie surface
taking with it none of the salt, all that portion of salt with
which it was combined will receive the solid form, and will
collect into crystals ; and this process may be continued until, by
superficial evaporation, all the water shall have disappeared, and
nothing be left in the vessel except a collection of crystals of
the salt.

1480. Anomalous case of anhydrous sulphate of soda. The
solution of anhydrous sulphate of soda presents some remark-
able exceptional phenomena. At the temperature of 91'4 it
has a maximum of saturation, that is to say, above this point the


proportion of salt which it contains diminishes instead of in-
creasing as the temperature is raised. However, at the boiling
point, it contains much more salt than at the common tempe-
rature. If the solution be boiled in a large tube, and when it is
well purged of air the tube be closed at the top, so as to ex-
clude the atmosphere, the cooling will take place without any
solidification ; but when the top of the tube is broken so as to
admit the air, the salt is suddenly congealed in a mass, with so
great a disengagement of heat, that the tube becomes warm to
the touch.

1481. Case in which the matter held in solution congeals with
the water. In some cases the water and the salt which it holds
in solution are solidified together. This happens when the
salts contain their water of crystallization. The phenomena are
produced in the same manner as in the case just described,
with this difference, that the molecules of salt in collecting
carry with them the molecules of the water of crystallization,
which pass also to the solid state, taking the place which be-
longs to that in the crystals. Nevertheless, the solidification of
the water disengaging in general much more latent heat than
the solidification of the salt, the crystals undergo a less rapid
increase, whether formed by mere cooling, or by evaporation
of a part of the dissolving mass.

1482. Dutch tears. When bodies liquefied by
heat are suddenly cooled, some remarkable and ex-
ceptional phenomena are often produced. Thus,
if large drops of glass in a state of fusion be let fall
into a vessel of cold water, the solidification of their
superficial parts is immediate ; that of their interior
is much more slow. There results from this a sort
of forced and unnatural arrangement of the mole-
cules of the drop, which explain the singular pheno-
menon produced by Dutch Tears, so called from
the form they assume, as represented in Jig. 442.
' If the extremity of the tail of one of these be broken,
in an instant the entire mass cracks, and is reduced to powder.
This arises from the fact that, the glass not being cooled slowly
and gradually, the molecules in solidifying have not had time
to assume their natural position, and, being in a forced position,
on the least disturbance separate.

1483. Use of annealing in glass manufacture and pottery.

86 HEAT.

To prevent this, articles manufactured of glass are submitted to
the process called annealing after their fabrication, a process
in which, being again raised to a certain temperature, they are
allowed to cool very'slowly. Pottery iu general is submitted
to the same process.

1484. Tempering steel. The temper of steel is a quality
analogous to this. Being heated almost to the point of fusion,
and being plunged in water, it becomes as brittle as glass. In
this state, it is said to have the highest temper. If it is tem-
pered only at a cherry red, it is less hard and less brittle. This
is what is called the ordinary temper. In short, it may be
annealed in an infinite variety of degrees over a fire of small
charcoal, according to the temper which it is desired to impart
to it. The oxidation which it suffers at the surface indicates
by the colour which it gives to it the degree of annealing which
it has received : thus it sometimes acquires a blue colour and
sometimes a straw colour, the latter colour indicating a harder
and less elastic quality.



1485. Evaporation of liquids in free air. If a liquid be
exposed in an open vessel, it will be gradually converted into
vapour, which mixing with the atmosphere will be dissipated,
and after a certain time the liquid will disappear. This pheno-
menon, called evaporation, was formerly explained by the sup-
position that the air had a certain affinity for the liquid in
virtue of which the air dissolved it, just as water dissolves
sugar or salt.

A conclusive proof of the falsehood of this hypothesis was
presented by the fact, that the vaporization of the liquid takes
place in a vacuum, and that the presence of air not only does
not cause more of the liquid to be evaporated than would have
been evaporated in its absence, but actually retards and obstructs
the evaporation.

1486. Apparatus for observing the properties of vapour.





Fig. 443.

To be enabled to examine and observe with clear-
ness and precision the mechanical properties of the
vapour of any liquid, it is necessary to provide
means by which such vapour can be separated from
air and all other gases and vapours, since, being
mixed with these, its properties would be modified,
so that it would be difficult to determine what ef-
fects are due to the vapour, and what to the gases
with which it is combined.

This object has been attained by apparatus, the
principle of which we shall now explain.

Let A B, fig. 443., be a glass bulb and tube, the
bore of the tube being very small compared with
the capacity of the bulb. Let the tube be widened
into a sort of bell-shaped mouth at the end B, and
let a graduated scale be engraved upon it, the zero
being near the bulb.

Let the tube, held with the open end B upwards,
be filled with pure mercury well
purged of air, as described in (714)
et seq. Placing the finger on B
to prevent the escape of the mer-
cury or the entrance of air, let the
tube be inverted, and the end B
immersed in a trough of mercury,
as represented in fig. 444. If it be
immersed to such a depth that the
height of the top of the bulb A
above the level L L of the mercury
in the trough is less than the height
of the barometric column, the mer-
cury will not fall from the bulb,
being sustained there by the atmo-
spheric pressure.

But if the bulb be raised to a
greater height A above L i/, the
column of mercury will not rise with
it, but will stand at the height of
the barometric column.

Let the bulb be raised to such a
Fig. 444. height A, that the zero of the scale

88 HEAT.

engraved on the tube shall be at a height above L i/ equal to
the barometric column. In that case the level of the column
of mercury in the tube will coincide with the zero of the scale,
and the space in the bulb and tube above this level will be a
vacuum. Let this space be s A', and let s M represent the
column of mercury which corresponds in height with the baro-

Let C v,Jig- 445., be a small iron cylinder containing mercury,
above which is a piston by which it can be pressed downwards.
This piston is urged by a screw, so as to
be capable of being moved with accu-
racy through any proposed space, however
small. Attached to the bottom of the
cylinder c D is a very fine tube D P, bent
into a rectangular form so as to present its
mouth upwards. This capillary tube is
filled with the liquid, the vapour of which
' Fio . 445 it is desired to submit to observation. By

means of the screw acting on the piston,
any proposed quantity of this liquid can be expelled from the
mouth P of the tube.

This instrument being immersed in the trough L \!,fig. 444.,
and the mouth of the tube P being directed into the bell-shaped
end of the tube B, a certain small quantity of the liquid is
expelled by means of the screw, and issues from p. It rises by
its relative levity through the mercury, and arrives at the top s
of the column. There it instantly disappears, and at the same
time the mercury falls to a lower level.

1487. Vapour of a liquid an elastic, transparent, and invisible
fluid like air. The cause of this will be easily understood.
The minute drop of liquid which rises to the surface is con-
verted into vapour on arriving there, and is diffused in that
state throughout the entire capacity of the tube and bulb. It
is transparent and invisible like air ; and therefore, notwith-
standing its pressure, the bulb and tube appear to be empty,
as they would if they were filled with air.

1488. How its pressure is indicated and measured. But
this vapour being, like air, an elastic fluid, exercises a certain
pressure upon the mercurial column s M, which pressure is
manifested and measured by the fall of that column. The
summit, which before stood at the zero of the scale, now stands


at a lower point, and the number of the scale indicating it3
position, expresses the pressure of the vapour in inches of
mercury. Thus, if the summit s of the column stand at half
an inch below zero, the pressure of the vapour in the bulb is
such as would support a column of mercury half an inch in

Now let us suppose another small drop of the liquid to be in-
jected by the apparatus^. 444. Like effects will ensue, and
the summit s of the column will fall still lower, showing that
the pressure of the vapour is augmented.

1489. When a space is saturated with vapour. By repeat-
ing this process, it will be found, that when a certain quantity
of the liquid has been injected, no more vapour will be produced,
and the liquid will float on the summit s of the mercurial
column without being vaporized. The summit of the column will
not be further depressed.

It appears, therefore, that the space in the bulb and tube is
then saturated with vapour. It has received all that it is capable
of containing. That this is the case will be rendered manifest
by elevating the tube. The summit s of the column still main-
taining its height above L i/, a greater space will be obtained
above s, and it will be accordingly found, that a portion of the
liquid which previously floated on s will be vaporized, and if
the tube be still more elevated, the whole will disappear.

Since during this process the height s M of the mercurial
column in the tube remains unaltered, it follows that the pressure
of the vapour remains the same.

By comparing the volume of the liquid ejected from ?,fig. 444.,
with the volume of the tube and bulb tilled by the vapour into
which it is converted, the density of the vapour, or, what is the
same, the column of vapour into which one unit of volume of the
liquid is converted, may be ascertained.

There are, however, other circumstances connected with this
process, which are not rendered apparent, and which it is im-
portant to observe and comprehend.

When the liquid rises to the surface of the mercurial column
and expands into vapour, it absorbs a certain quantity of heat
which becomes latent in it. This heat must be supplied by the
tube, the bulb, and the mercury ; and as the temperature of
these does not permanently fall, this heat is replaced, and
their temperature restored by the surrounding air. The

90 HEAT.

quantity of heat absorbed in the evaporation of the liquid will
be presently shown. Meanwhile it must be observed, that the
supply of the latent heat is essential to the evaporation of the
liquid. If the mercury on which the liquid floats, and the glass
by which it is inclosed, were absolute non-conductors, and could
impart no heat whatever to the liquid, then the evaporation
could not take place.

It appears from what has been explained, that when the space
above the mercury has been charged with a certain quantity of
liquid in the state of vapour, or, what is the same, when the
vapour it contains has attained a certain density, all further
evaporation ceases ; and any liquid which may be injected will
remain in the liquid state, floating on the mercury. So long as
the temperature of the surrounding medium, and consequently
that of the bulb and its contents, remain unaltered, and so long
as any liquid remains floating on the mercury, the pressure and
the density of the vapour in the bulb will be unaltered. If the
bulb be raised, so as to give more space for the vapour, a pro-
portionally increased quantity of the liquid will be vaporized ;
and if by depressing the tube the volume of the vapour be
diminished, a corresponding part of it will return to the liquid
state. In the one case, heat will be absorbed by the liquid
evaporated ; and in the other, heat will be developed by the
vapour condensed. This heat is borrowed from the surrounding
atmosphere in the one case, and imparted to it in the other ;
since, otherwise, the bulb and its contents must undergo a change
of temperature, contrary to what was supposed.

1490. Quantity of vapour in saturated space depends on tem-
perature But let us now consider what will be the effect of

raising or lowering the temperature of the bulb and its contents.
The bulb being charged with vapour, and a stratum of uneva-
porated liquid floating on the mercury, let the temperature of the
medium surrounding the bulb be raised through any proposed
number of degrees of the thermometric scale. This will be im-
mediately followed by the evaporation of a part of the liquid
floating on the mercury, and a depression of the column. An
increased volume of vapour is therefore now contained in thebulb
and tube ; but if this increase of volume be compared with the
increased quantity of liquid evaporated, it will be found to be less
in proportion ; and it consequently follows, that the density of


the vapour is augmented ; and since the column of mercury has
been more depressed, and since this depression measures the
pressure of the vapour, it follows that this pressure has been
also augmented.

1491. Relation between pressure, temperature, and density.
Thus it appears that the pressure and density of the vapour
produced from the liquid floating on the mercury are augmented
as the temperature of the liquid is augmented, and consequently
diminished as that temperature is diminished.

In short, a certain relation subsists between the temperature,
pressure, and density, such that when any one of these are
known, the other two can always be found. If this general re-
lation were known, and could be expressed by an arithmetical
formula, the pressure and density of the vapour corresponding
to any proposed temperature, or the temperature corresponding
to any proposed density or pressure, could always be ascer-
tained by calculation. But the theory of heat has not supplied
the means of determining this relation by any general principles ;
and, consequently, the pressures and densities of the vapour of
liquids at various temperatures have been determined only by
experiment and observation.

1492. Pressure, temperature, and density of the vapour of
water. Different liquids at the same temperature produce
vapours having different densities and pressures. But of all
liquids, that of which the vaporization is of the greatest
physical importance, and consequently that which has been
the subject of the most extensive system of observations, is

If water be introduced above the mercurial column in the
apparatus above described, and be exposed successively to
various temperatures, the pressures and densities of the vapour
it produces can be observed and ascertained.

It is thus found that, in all cases, water passing into the
vaporous state undergoes an enormous enlargement of volume,
and that this enlargement increases as the temperature at which
the evaporation takes place is diminished. Thus, if the tem-
perature be 212, a cubic inch of water swells into 1696 cubic
inches ; and if the temperature be 77, it swells into 23090 cubic
inches of vapour.

1493. Vapour produced from water at all temperatures, how-
ever low. There is no temperature, however low, at which

92 HEAT.

water will not evaporate. If the bulb and tube be exposed to
the temperature of 32, the mercurial column in the tube will
be lower than the barometric column by two-tenths of an inch,
a small, but still observable quantity ; and even if the tem-
perature be reduced still lower, so that the liquid floating on
the mercury shall become solid ice, there will still be a vapour
in the bulb, of appreciable pressure and density. Thus, a piece
of ice at the temperature of 4, (that is, 36 below the freezing
point) produces a vapour whose pressure is represented by a
column of mercury of a twentieth of an inch.

The relation between the temperature, pressure, and density
of the vapour of water, from the lowest temperatures and pres-
sures to temperatures corresponding to a pressure twenty-four
times greater than that of the atmosphere, has been ascertained
by direct observation ; and by the comparison of these observ-
ations, an empirical formula has been found, which expresses
the general relation of the temperature and pressure with such
precision, within the range of the temperatures and pressures
observed, that it may be applied without risk of any important
error to the computation of the pressures, temperatures, and
densities, through a certain range of the scale, beyond the limits
to which observation and experiment have extended. In this
way, the temperatures and densities of the vapour, corresponding
to all pressures up to fifty times the pressure of the atmosphere,
have been computed and tabulated.

1494. Mechanical force developed in evaporation. When a
liquid expands into vapour, it exerts a certain mechanical force,
the amount of which depends on the pressure of the vapour,

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 11 of 45)