THE STEAM-ENGINE.
[Illustration: Fig. 381. Hancock's steam omnibus, which ran on the
common roads.]
It must be apparent to those who read popular works on science, that
they possess, at all events, one point of utility--viz., that they are
_indicative_ of the various subjects that may be selected in science for
special, searching and exhaustive study. The subject of steam and the
steam engine is not one that could be thoroughly treated of in the
narrow space allowed in this volume, but enough may be said to give some
instruction and to impart common principles, whilst the minute details
are better examined and learnt in the works of Bourne, Rankine, and
other authors who devote themselves specially to the important
commercial question of steam.
The first truth to be comprehended is, that all matter contains within
its substance the power of creating heat--or as it may be expressed more
plainly, solids, fluids, and gases contain what is termed _latent_ or
insensible heat, in contradistinction to the heat which is apparent when
we touch a vessel containing warm water or approach a cheerful fire;
this latter is termed _sensible_ heat, and has formed the subject of the
preceding chapters.
If a cold horse-shoe nail is applied to a thin dry slice of phosphorus
laid on a sheet of paper, no combustion of the phosphorus ensues,
because the temperature of the iron is not sufficiently high to affect
that combustible substance; but if the horse-shoe nail is vigorously
hammered on an anvil, the particles of the metal are brought closer
together, and if it is applied to the phosphorus, so much heat has been
generated, thrust or squeezed out by the hammering or _condensation_ of
the iron, that it is now sufficiently warm to set fire to it.
[Page 407]
The reverse or antithesis to this experiment--viz., the production of
cold--would be shown if it were possible to expand a mass of metal
suddenly, and this can be effected by first melting together
207 parts by weight of lead.
118 " " tin.
284 " " bismuth.
When these metals are in the liquid state and perfectly mixed, they are
poured from a sufficient height into a pail of cold water, for the
purpose of _granulating_ or dividing them into small fragments.
If the granulated compound metal is now mixed with 1617 parts by weight
of quicksilver, it becomes suddenly liquefied and expanded: liquefaction
is the reverse of solidification, and hence cold is produced from the
natural heat of the compound metals being rendered latent by the change
from the solid to the liquid state; so that a small quantity of water
placed in a glass tube, and surrounded with the metals whilst liquefying
in the mercury, becomes rapidly converted into ice, the fall of the
temperature, as shown by a thermometer, being from 60° Fahr. to 14°,
which is 18° degrees below the freezing point of water. In the former
case, by hammering the iron the _latent heat_ is made _sensible_; whilst
in the latter case, by the liquefaction of the compound metal in
mercury, the _sensible_ heat is rendered _latent_. The heat rendered
latent by melting different substances is not a constant quantity, but
varies with every special body employed, and the Drs. Irvine have proved
this fact by the following experiments:--
Ditto, reduced
Heat to the
of specific heat
fluidity. of water.
Sulphur 143.68° Fahr. 27.14
Spermaceti 145 " --
Lead 163 " 5.6.
Bees'-wax 175 " --
Zinc 493 " 48.3.
Tin 500 " 33.
Bismuth 550 " 23.25.
Every one of these substances requires more heat to bring them into the
liquid condition than ice, for which 140° of heat are sufficient, or are
rendered latent during its conversion into water.
In coining at the Mint, the cold blank pieces of gold, silver, or copper
become hot directly they have sustained the violent and sudden pressure
of the coining press, and they must be heated again, or annealed, to
restore the equilibrium of the heat disturbed by the violent blow, or
else they remain hard and unfit to sustain the finishing process of
milling.
The condensation of water when it assumes a smaller bulk by union with
sulphuric acid, is easily proved by measuring a pint of water and a pint
of acid, and mixing them together, when a very great increase of
temperature may be perceived; and by placing into the mixture a cold
copper wire that previously could not ignite phosphorus, it becomes
[Page 408] very hot, and when removed and wiped it will cause phosphorus
to fire directly it touches that substance. When the mixture of
sulphuric acid and water is measured after it has cooled, it has no
longer a bulk of two pints, but is found to have lost bulk equal to one
or more ounces by measure. The heat evolved by a mixture of four parts
of strong sulphuric acid and one part water is shown by the thermometer
to be 300° Fahr., and this mode of obtaining heat has been used by
aeronauts for the purpose of obtaining artificial warmth without the
danger of setting fire to the gas in the balloon.
[Illustration: Fig. 382. Aeronauts in the car warming their hands by a
bottle containing sulphuric acid and water.]
When alcohol and water are mixed a change of density occurs, and heat is
produced; and if equal measures of alcohol of a specific gravity of
.825, and water, each at 50° Fahr., are mixed, a temperature of 70°
Fahr. is obtained; if the mixture is made in a glass vessel, as shown in
the annexed cut, the combination is very apparent. To perform the
experiment properly, water is poured into the lower tube and bulb, and
alcohol into the top one; when this is done, the stopper is inserted,
and the whole thoroughly shaken and mixed together; the warmth which is
[Page 409] thus obtained is apparent to the hand, whilst the contraction
is shown after the mixture is cold, as it no longer fills the two bulbs
of the instrument. (Fig. 383.)
[Illustration: Fig. 383. Glass bulbs and tube to show the contraction in
bulk of a mixture of alcohol and water.]
The latent heat of gases is easily shown by suddenly condensing air in a
small syringe or pump, of which the piston contains a minute fragment of
amadou (a species of fungus, _Polyporus igniarius_; this, according to
Simmonds, after having been beaten with a mallet, and dipped in a
solution of saltpetre, forms the spunk or German tinder of commerce; it
is also used as a styptic, and made into razor strops), which takes
fire, and before the invention of vesta and other matches,
tobacco-smokers were in the habit of obtaining a light for their pipes
and cigars in this manner--viz., by the latent heat obtained from the
contraction or compression of air. Then, again, an instructive though
opposite parallel is afforded by suddenly expanding or rarefying air in
a glass receiver provided with a delicate thermometer. By pumping out
some of the air, a considerable diminution of the temperature occurs,
and equal to several degrees of the thermometer. Every child knows that
steam direct from the kettle will scald, but if it issues from a
high-pressure boiler, say at fifteen pounds on the square inch, the hand
may be held with impunity in the escaping steam, as it merely feels
gently warm, and not scalding. This is due partly to the loss of heat
rendered latent by the expansion of the high-pressure steam directly it
passes into the air, and partly to the currents of air that are dragged
into an escaping jet of steam. This tendency of the air to rush into a
jet of steam was discovered by Faraday, and explains those curious
experiments with a jet of steam by which balls, empty flasks, and
globular vessels are sustained and supported either perpendicularly or
horizontally.
[Illustration: Fig. 384. A. Jet discharging high-pressure steam B B.
Lighted torch held round the escaping steam the flames from the former
all rush into the latter.]
If steam at a pressure of about sixty pounds per inch is allowed to
escape from a proper jet, and a large lighted circular torch composed of
tow dipped in turpentine held over it, the course of the external air is
shown, by the direction of [Page 410] the flames, which are forcibly
pulled and blown into the jet of steam with a roaring noise, indicating
the rapidity of the blast of air moving to the steam jet. (Fig. 384.)
Egg-shells, empty flasks, india-rubber or light copper and brass balls,
are suspended in the most singular manner inside an escaping jet of
high-pressure steam; and before the explanation of Faraday, reams of
paper were used in the discussion of the possible theory to account for
this effect; and what made the explanation still more difficult, was the
fact that the jet of steam might be inclined at any angle between the
horizontal and perpendicular, and still held the ball, egg-shell, or
other spherical figure firmly in its vapory grasp. (Fig. 385.)
[Illustration: Fig. 385. A. Ball and socket jet at an angle, and
discharging steam. The egg-shells are supported by the enormous current
of air moving into the jet in the direction of the arrows.]
In consequence of the great rush of air towards a jet of escaping
high-pressure steam, Mr. Goldsmith Gurney has patented the application
of this principle in his ventilating steam jet, which he has already
successfully applied; in one case especially, where a coal-mine had been
on fire for several years, and the whole working of the coal-measures in
the pit was jeopardized by the spreading of the combustion to new
workings; the fire was first extinguished by carbonic acid gas, pulled,
as it were, into the coal-mine by a jet of steam blowing into the
_downcast_, but placed in connexion with a furnace of burning coke; and
the circulation of the carbonic acid, called _choke-damp_, through the
pit workings was further assisted by a jet of high-pressure steam
blowing upwards, and placed over the mouth of the _upcast_ shaft.
The experiment succeeded perfectly at the South Sauchie Colliery, near
Alloa, about seven miles from Stirling, where a fire had raged for about
thirty years over an area of twenty-six acres in the waste seam of coal
nine feet thick. (Fig. 386.)
For the general purpose of ventilating the coalmine, Mr. Gurney's plan
was tried at the Ebbw Vale Colliery, and very economically, the waste
steam alone being used. Experiments have also been satisfactorily made
with it for blowing a cupola for smelting iron, and with dry
steam--_i.e._, steam of a very high pressure--escaping through a warm
tube, the results were perfectly successful.
[Page 411]
[Illustration: Fig. 386. Gurney's steam jet. A. Furnace. B. Water tank.
C. Downcast stopping. D. Upcast stopping. E E E. Steam jets. F F.
Galleries from shaft to shaft.]
[Page 412]
With this digression from the subject of latent heat derived from the
compression of air, we return again to the subject with another case in
point, furnished by the Fountain of Hiero, as it is called, at
Schemnitz, in Hungary, described by Professor Brande; and it may be
observed that all the phenomena related would apply to the great
pressure of the water from the water-towers at the Crystal Palace, if
fitted with a similar air-vessel.
"A part of the machinery for working these mines is a perpendicular
column of water 260 feet high (the Crystal Palace water-towers are each
284 feet high), which presses upon a quantity of air enclosed in a tight
reservoir; the air is consequently condensed to an enormous degree by
this height of water, which is equal to between eight and nine
atmospheres; and when a pipe communicating with this reservoir of
condensed air is suddenly opened, it rushes out with extreme velocity,
instantly expands, and in so doing it absorbs so much heat as to
precipitate the moisture it contains in a shower of snow, which may
readily be gathered on a hat held in the blast. The force of this is so
great, that the workman who holds the hat is obliged to lean his back
against the wall to retain it in its position."
The best examples of latent heat are furnished by ice, water, and steam,
and we are indebted chiefly to Dr. Black for the elegant and conclusive
experiments demonstrating the important truths connected with the latent
heat of these three conditions of matter. When various solids are
heated, they frequently pass through certain intermediate conditions of
softness, terminating in perfect liquidity; but ice and many other
bodies change at once to the liquid state on the application of a
sufficient quantity of heat. The process of melting ice is very slow,
because every portion must absorb or render latent a certain quantity of
heat before it can take the liquid state--hence the difficulty of
melting blocks of ice when they are surrounded with non-conducting
materials; and this fact the author has proposed to take advantage of in
keeping water cool which is to be supplied to the ova of salmon whilst
taking them to stock the rivers of Australia.
In order to prove that heat is rendered latent by the liquefaction of
ice, it is only necessary to weigh a pound of finely-powdered ice and a
pound of water at 212° Fahr. (_boiling water_), and mix them together;
when the ice is all melted, the resulting temperature is only 52°,
therefore the boiling water has lost 160° of temperature, of which 20°
can be accounted for, because the resulting temperature of the melted
ice is 52°; but in the liquefaction of the pound of ice, 140° have
disappeared or become latent, or, as Dr. Black termed it, have become
_combined_.
1 lb. of ice at 32° + 20° = 52°, the resulting temperature.
1 lb. of water at 212° - 52° = 160° - 20° = 140°, rendered latent.
140° represents the result obtained from innumerable experiments made by
mixing equal parts of ice and boiling water, and it is this large
quantity of latent heat required by ice and snow that prevents their
sudden liquefaction, and the disastrous circumstances that would arise
from the floods that must otherwise always be produced.
[Page 413]
To put the fact beyond all doubt, it is advisable to mix together equal
weights of water at 32° and boiling water at 212°, and the result is
found by the thermometer to be the mean between the two, because half
the extremes are always equal to the mean; and if the two temperatures
are added together and divided by two, the result is a temperature of
122°, as shown below:--
1 lb. of ice water at 32° + 1 lb. of water at 212° = 244° ÷ 2 = 122°.
From similar experiments Dr. Black deduced the important truth, "that in
all cases of liquefaction a quantity of heat _not indicated by, or
sensible to_, the thermometer, is _absorbed_ or disappears, and that
this heat is _withdrawn_ from the _surrounding bodies_, leaving them
_comparatively cold_." At p. 79 it is shown how the sudden solution or
liquefaction of certain salts produces cold, and hence numerous freezing
mixtures have been devised. In olden times, when officials in authority
did what they pleased, without being troubled with disagreeable returns,
and colonels clothed their men, and were merchant tailors on the grand
scale, gun cartridges were not confined to practice on the enemy, but
they did duty frequently in the absence of ice as refrigerators of the
officers' wine, in consequence of the gunpowder containing nitre or
saltpetre; as a mere solution of this salt finely powdered will lower
the temperature of water from 50° Fah. to 35°; whilst a mixture of four
ounces of carbonate of soda and four ounces of nitrate of ammonia
dissolved in four ounces of water at 60°, will in three hours freeze ten
ounces of water in a metallic vessel immersed in the mixture during the
liquefaction or solution of the salts.
Fahrenheit imagined he had attained the lowest possible temperature by
mixing ice and salt together, and it is by this means that confectioners
usually freeze their ices, or ice puddings; the materials are first
incorporated, and being placed in metallic vessels or moulds, and
surrounded with ice and salt placed in alternate layers, and then well
stirred with a stick, they soon solidify into the forms which are so
agreeable, and so frequently presented at the tables of the opulent. The
temperature obtained is Fahrenheit's _zero_--viz., thirty-two degrees
_below_ the freezing point of water. According to the very wise police
regulation observed in London, all householders are required to sweep or
remove the snow from the pavement in front of their houses, and this is
frequently done with salt; should an unfortunate shoeless beggar, tramp
past whilst the sudden liquefaction is in progress, the effect on the
soles of his feet is evidently very disagreeable, and the rapidity with
which he retires from the _zero_ affords a thermometric illustration of
the most lively description.
_Heat the Cause of Vapour._
Every liquid, when of the same degree of chemical purity, and under
equal circumstances of atmospheric pressure, has one peculiar point of
temperature at which it invariably boils. Thus, ether boils at 96°
Fahr., and if some of this highly inflammable liquid is placed carefully
in a [Page 414] flask, by pouring it in with a funnel, and flame
applied within one inch of the orifice, no vapour escapes that will take
fire; but if the flame of a spirit lamp is applied, the ether soon
boils, and if the lighted taper is again brought near the mouth of the
flask, the vapour takes fire, and produces a flame of about two feet in
length. This fire only continues as long as the flame of the spirit-lamp
is retained at the bottom of the flask, and on removing it the vessel
rapidly cools. The length of the flame is reduced, and is gradually
extinguished for the want of that essence of its vitality, as it
were--viz., heat. (Fig. 387.) If a thermometer is introduced into the
flask, however rapid may be the ebullition or boiling of the ether, it
is found to be invariably at 96°. The heat carried off by evaporation is
most elegantly displayed by placing a little water in a watch glass, and
surrounded by charcoal saturated with sulphuric acid, in the vacuum of
an air-pump. The rapid evaporation and condensation of the water by its
affinity for the sulphuric quickly produces ice; and the pumps and other
apparatus of Knight and Co., Foster-lane, City, are greatly to be
recommended for this and other illustrations.
[Illustration: Fig. 387. Heat the cause of vapour.]
The illustration of the determination of the fixed and invariable
boiling point belonging to every liquid is further carried out by
introducing some water into a second flask standing above a lighted
spirit-lamp, with a small thermometer, graduated, of course, properly to
degrees above the boiling point of water; when the water boils, it will
be found to remain steadily at a temperature of 212°. And however
rapidly the water may be boiled, provided there is ample room for the
steam to escape, the heat indicated by the thermometer is like the law
of the Medes and Persians, which altereth not, and it remains standing
at the number 212°. The only exception (if it may be so termed) to this
law is brought about by the shape and nature of the containing vessel;
under a mean pressure the boiling point of water in a metallic vessel is
generally 212°; in a glass vessel it may rise as high as 214° or 216°,
but if some metallic filings are dropped in, the escape of steam is
increased, and the temperature may then drop immediately to 212°.
When a thermometer is inserted in a flask containing water in a state
[Page 415] of ebullition or boiling, so that the bulb does not touch the
fluid, but is wholly surrounded with steam, it will be found that the
temperature of the latter is exactly the same as that of the former; and
if the liquid boils at 96°, the vapour will be 96°, if at 212°, the
steam is 212°. Steam has therefore exactly the same temperature as the
boiling water that produces it. (Fig. 388.)
[Illustration: Fig. 388. Thermometer in the steam escaping from boiling
water.]
Whilst performing the last experiment, it may be noticed that the steam
inside the neck of the flask is invisible, and that it only becomes
apparent in that kind of intermediate condition between the vaporous and
liquid state called _vesicular vapour_--a state corresponding with the
"earth fog," and called by Howard the _stratus_. When a flask containing
boiling water is placed under the receiver of an air pump (as soon after
the ebullition has ceased as may be possible), and the air pumped out,
it will be noticed that the water again begins boiling as the vacuum is
obtained, showing that the boiling point of the same fluid varies under
different degrees of atmospheric pressure, and according to the height
of the barometer.
Height of Boiling point
barometer. of water.
26 204.91°
26.5 205.79
27 206.67
27.5 207.55
28 208.43
28.5 209.31
29 210.19
29.5 211.07
30 212
30.5 212.88
31 213.76
Alcohol and ether confined under an exhausted receiver boil violently at
the ordinary temperature of the atmosphere, and in general liquids boil
with 124° less of heat than are required under a mean pressure of the
air; water, therefore, in a vacuum must boil at 88° and alcohol at 49°.
On ascending considerable heights, as to the tops of mountains, the
boiling point of water gradually falls in the scale of the thermometer.
Thus, on the summit of Mont Blanc water was found by Saussure to boil at
187° Fahr. In Mr. Albert Smith's delightful narrative of his ascent of
Mont Blanc, he mentions the violent commotion and escape of the whole of
the champagne in froth directly the bottle was opened at the summit of
this king of mountains.
Dr. Wollaston's instrument for measuring the heights of mountains by
[Page 416] the variations of the boiling point of water has long been
known and used for this purpose.
If a Florence flask is first fitted with a nice soft cork, and this
latter removed, and the former half filled with water, which is then
boiled over a gas or spirit flame, the same fact already mentioned and
illustrated in the preceding table may be rendered apparent when the
flask is corked and removed from the heat. If it is now inverted, and
cold water poured over it, an ebullition immediately commences, because
the cold water condenses the steam in the space above the hot water in
the flask, and producing a vacuum, the water boils as readily as it
would do under an exhausted receiver on an air-pump plate. (Fig. 389.)
[Illustration: Fig. 389. The paradoxical experiment of water boiling by
the application of _cold_ water.]
Water may be heated considerably higher than 212°, if it is enclosed in
a strong boiler, and shut off from communication with the air; by this
means steam of great pressure is obtained.
Dr. Marcet has invented a very instructive form of a miniature boiler,
supplied with a thermometer and barometric pressure gauge, which can be
purchased at any of the instrument makers, and is figured and described
in nearly every work on chemistry.
The reason water boiled in an open vessel does not rise to a higher
temperature than 212° is because all the excess of heat is carried off
by the steam, and is said to be rendered latent in the vapour. The
fixation of caloric in water by its conversion into steam may be shown
by the following experiment. Let a pound of water at 212° and eight
pounds of iron filings at 300° be suddenly mixed together. A large
quantity of steam is instantly generated, but the temperature of the
water and escaping steam are still only 212°; hence the steam must
therefore contain all the degrees of heat between 212° and 300°, or
eight times 88. When the water is heated in the hydro-electric machine
or other boiler, to 322.7°, it very quickly drops to 212° when the steam
is allowed to blow off; yet if the latter is collected, it represents
but a very small quantity of water which constituted the steam, and it
has carried off and rendered latent the excess of heat in the
boiler--viz., the difference between 212° and 322.7°, or 110.7°
If steam can carry off heat, of course it may be compelled, as it were,
[Page 417] to surrender it again; and this important elementary truth is
shown by adapting a tube, bent at right angles, and a cork, to a flask
containing a few ounces of water, and when it boils, the steam issuing
from the end of the pipe may now be directed into and below the surface
of some water contained in a beaker glass; in a very short time the
water in the latter will be raised to the boiling point by the
condensation of the steam and the latent heat arising from it. (Fig.
390.) The amount of latent heat is enormous, when it is remembered that
water by conversion into steam has its bulk prodigiously enlarged--viz.,
1698 times, so that _a cubic inch_ of water converted into steam of a
temperature of 212°, with the barometer at thirty inches, occupies a
space of _one cubic foot_, and its latent heat amounts, according to
Hall, to 950°; Southeron, 945°; Dr. Ure, 967°. When we come to the
consideration of the steam-engine, it will be noticed that the question
of the latent heat of steam is one of the greatest importance.
[Illustration: Fig. 390. A. Flask for generating steam. B. Glass pipe
bent at right angles to convey the steam into the fluid containing some
cold water.]
Temperature of Elasticity in inches Latent Heat.
Steam. of Mercury.
229° 40° 942°
270 80 942
295 120 950
The same weight of steam contains, whatever may be its density, the same
quantity of caloric, its latent heat being increased in proportion as
its sensible heat is diminished; and the reverse. In consequence of the
enormous amount of latent heat contained in steam, it is advantageously
employed for the purpose of imparting warmth either for heating rooms or
drying goods in certain manufacturing processes. The wet rag-pulp
pressed and shaken into form on a wire-gauze frame or _deckle_, passes
gradually to cylinders containing steam, and is thoroughly dried before
the guillotine knife descends at the end of the paper machine, and cuts
it into lengths. In calico stiffening and glazing, also in calico
printing, steam-heated cylinders are of great value, because they impart
heat without the chance of setting the goods on fire. The elementary
principles already described with reference to heat, will prepare the
youthful reader for the application of the expansion of water into
steam, as the most valuable _motive power_ ever employed to assist the
labour of man.
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