HEAT.
[Illustration: Fig. 342. James Watt.]
Throughout the greater number of the preceding chapters it will be
evident that the active properties of matter may be summed up under one
general head, and may be considered as varieties of attraction--such as
the attraction of gravitation, cohesive attraction, adhesive attraction,
attraction of composition (or chemical attraction), electrical
attraction, magnetical attraction.
The absolute or autocratic system does not, however, prevail in the
works of nature; and she seems ever anxious, whilst imparting great and
peculiar powers to certain agents, to create other forces which may
control and balance them. Thus, for instance, the great force of
cohesive attraction is an ever-present power discernible, as has been
shown, in solids and liquids; but if this agent [Page 352] were allowed
to run riot in its full strength and intensity, it would tyrannically
hold in subjection all liquid matter, and every drop of water which is
at present kept in the liquid state, would succumb to its iron rule, and
retain the solid state of ice. Hence, therefore, the wise creation of an
antagonistic force--viz., heat; which is not provided in any niggardly
manner, but is liberally bestowed upon the globe from that
all-sufficient and enormous source, the sun. And it is by the softening
and liquifying influence of his rays that the greater proportion of the
water on the surface of the globe is maintained in the fluid condition,
and is enabled to resist the power of cohesion, that would otherwise
turn it all, as it were, to stone.
Cohesion, electricity, and magnetism fully embody the notion of powers
of attraction, or _a drawing together_; whilst heat stands almost alone
in nature as the type of repulsion, or _a driving back_.
Mechanically, repulsion is demonstrated by the rebound of a ball from
the ground; the parts which touch the earth are for the moment
compressed, and it is the subsequent repulsion between the particles in
those parts which causes them to expand again and throw off the ball.
The development of heat is produced from various causes, which may be
regarded as at least four in number. Thus, it was shown by Sir Humphrey
Davy, that even when two lumps of ice are rubbed together, sufficient
heat is obtained to melt the two surfaces which are in contact with each
other. Friction is therefore an important source of heat, and one of the
most interesting machines at the Paris Exposition consisted of an
apparatus by which many gallons of water were kept in the boiling state
by means of the heat obtained from the friction of two copper discs
against each other. The machine attracted a good deal of attention on
its own merits, and especially because it supplied boiling water for the
preparation of chocolate, which the public was duly informed was boiled
by the heat _rubbed out_ of the otherwise cold discs of copper. When
cannon made on the old system are bored with a drill, it is necessary
that the latter should be kept quite cool with a constant supply of
water, or else the hard steel might become red-hot, and would then lose
its _temper_, and be no longer capable of performing its duty.
Count Rumford endeavoured to ascertain how much heat was actually
generated by friction. When a blunt steel bore, three inches and a half
in diameter, was driven against the bottom of a brass cannon seven
inches and a half in diameter, with a pressure which was equal to the
weight of ten thousand pounds, and made to revolve thirty-two times in a
minute, in forty-one minutes 837 grains of dust were produced, and the
heat generated was sufficient to raise 113 pounds of the metal 70°
Fahrenheit--a quantity of heat which is capable of melting six pounds
and a half of ice, or of raising five pounds of water from the freezing
to the boiling point. When the experiment was repeated under water, two
gallons and a half of water, at 60° Fah., were made to boil in two hours
and a half.
Chemical affinity has been so often alluded to in these pages, that it
[Page 354] may be sufficient to mention only one good instance of its
almost magical power in evoking heat. When a bit of the metal sodium is
placed on the tip of a knife, and thrust into some warm quicksilver, or
if a pellet of sodium and a few globules of mercury are placed on a hot
plate just taken from the oven, and then gently squeezed together, a
vivid production of heat and light is apparent; and when the mixture of
the two metals is cold, it will be found that the quicksilver has lost
its fluidity, and a solid amalgam of sodium and mercury is obtained,
which gradually, by exposure to the air, returns to the liquid state,
the mercury being set free, whilst the sodium is oxidized, and forms
soda. Just as an ordinary alloy of copper and gold used by jewellers
would lose its colour and brilliancy by the oxidation of the copper; and
when the rusty, dirty film is removed by rubbing and polishing, the
surface is again brilliant, and remains so until another film of the
exposed copper is attacked: in like manner the sodium is attacked and
changed by the oxygen of the air, whilst the mercury being unaffected
retains its brilliancy, and at the same time regains its fluidity. The
evolution of heat in the above case indicates that a chemical union has
taken place between the two metals.
Examples of the production of heat by electricity and magnetism have
been abundantly shown in the chapters on these subjects; and one of the
best illustrations of this fact has been shown on the occasion of the
opening of the telegraphic communication between France and England by
means of the submarine cable, when cannon were fired alternately at both
ends of the conducting cable by means of electricity, and the event thus
inaugurated in both countries.
That heat is a product of living animal organization is shown, as it
were, visibly by the marvellous phenomena that proceed in our own
bodies. People do not very often trouble themselves to ask where the
heat comes from, or even to think that this invisible power must be
maintained in the body, and that slow combustion, or, as Liebig terms
it, _eremacausis_, must continually go on inside our frail mortal
tenements; and more than this, that we cannot afford to waste our heat.
If the body is deprived of heat faster than it can be generated, death
must inevitably occur; and a very melancholy instance of this remarkable
mode of death has lately occurred in Switzerland to a Russian gentleman.
Such another instance of a man being slowly frozen to death within sight
and sound of other beings, through whose veins the blood was flowing at
its accustomed temperature (about 90° Fahr.), it would be difficult to
find, and it stands forth, therefore, as a marked example and
illustration of the statement already made, that living animal organisms
are truly a source of heat, which is as essential to the well-being of
the body as meat, drink, and air.
Heat is of two kinds, and may be either apparent to our senses, and
therefore called _sensible_ heat; or it may be entirely concealed,
although present in solids, liquids, and gases, and is then termed
_insensible_ or _latent_ heat.
[Page 355]
_Sensible Heat._
The first effect of this force is a demonstration of its repulsive
agency, and the dilatation or expansion of the three forms of matter
whilst under the influence of heat, admits of very simple illustrations.
The expansion of a solid substance, as, for instance, a metal, on the
application of heat, is apparent by fitting a solid brass cylinder into
a proper metal gauge, which is accurately filed so as to admit the
former when perfectly cold. If the brass rod is then heated, either by
plunging it into boiling water or by the application of the flame of a
spirit lamp, its particles are separated from each other; they now
occupy a larger space, and expansion is the result, and this is clearly
proved by the application of the gauge, which is no longer capable of
receiving it. (Fig. 343.) When, however, the latter is cooled, the
opposite result occurs, the particles of brass return to their old
position, and _contraction_ takes place; hence it is stated that "Bodies
expand by heat and contract by cold;" and it is proper to state here
that the term "_cold_" is of a negative character, and simply means the
absence of heat.
[Illustration: Fig. 343. A B. Cylinder of brass. C D. Iron gauge,
admitting A B longitudinally, and also in the hole E when cold, but
excluding A B when the latter is heated and expanded.]
Solid bodies do not expand equally on the application of the same amount
of heat; thus, a bar of glass one inch square and one thousand inches
long would only expand one inch whilst heated from the freezing to the
boiling point of water. A bar of iron one inch square and eight hundred
inches long would expand one inch in length, through the same degrees of
heat; and a bar of lead one inch square and three hundred and fifty
inches long would also dilate one inch in length. Hence,
Lead expands in volume 1/350th.
Iron 1/800th.
Glass 1/1000th.
The unequal expansion of the metals is well illustrated by an experiment
devised by Dr. Tyndal, the respected Professor of Natural Philosophy in
the Royal Institution of Great Britain, and is arranged as follows:--A
long bar of brass and another of iron are supported on the [Page 356]
edges of two pieces of wood placed at an angle, and resting against the
sides of a mahogany framework. The metallic bars only touch one end of
the frame, and are in metallic communication with a piece of brass
inserted there, and forming part of a conducting chain connected with a
voltaic battery; when heat is applied to both bars they expand
unequally; the brass bar dilates first, and filling up the minute space
left between the two ends of the frame, touches another brass plate and
instantly completes the voltaic circuit, when a coil of platinum wire
becomes ignited, showing the fact of expansion; and secondly, the
difference in the power of dilatation possessed by each is clearly shown
by removing the two angular supports of wood, when the iron falls away,
whilst the brass remains and still completes the voltaic circuit. (Fig.
344.)
[Illustration: Fig. 344. A A. The brass bar which has expanded by the
heat from the gas jet B, and making the contact between the brass plates
in connexion with the binding screws C C, the voltaic circuit is
completed, and a coil of platinum wire in the glass tube D, is
immediately ignited. The iron bar at E E has not expanded sufficiently,
which is shown afterwards by removing the angular wooden supports K K,
when the iron falls off, and the brass remains on the two ledges of the
mahogany framework L L L.]
The force exerted by the expansion of solids is enormous, and reminds us
again of the amazing power of all the imponderable agents; and it is
truly wonderful to notice how the entry of a certain amount of heat into
and between the particles of metals, or other solids, endues them with a
mechanical force which is almost irresistible, and is capable of
working much harm. Kussné made an experiment with an iron sphere,
which he heated from a temperature of 32° Fahr. to 212° Fahr., and
he found that the expansion of the ball exerted a force equal to 4000
atmospheres--_i.e._ 4000 × 15--on every square inch of surface, or a
pressure equal to thirty millions of pounds; the entry of only 180° of
heat into the iron sphere produced this remarkable result, just as
Faraday has calculated that a single drop of water contains a sufficient
quantity of electricity to produce a result equal to the most powerful
flash of lightning, provided the electricity of quantity in the drop of
water is converted into electricity of high tension or intensity.
The practical applications of this well-known property of solids with
respect to heat are very numerous; thus, the iron bullet-moulds are
always made a little larger than the requisite size, in order to allow
for the expansion of the hot liquid lead, and the contraction of the
cold metal. The tires of wheels and the hoops of casks are usually
placed on whilst hot, in order that the subsequent contraction may bind
the spokes [Pg 357] and fellies, or the staves, closely together. If an
allowance was not made for the expansion and contraction of the iron
rails on the permanent ways of railroads, the regularity of the level
would be constantly destroyed, and the position of the rails, chairs,
and sleepers would be most seriously deranged; indeed it is calculated
that the railway bars between London and Manchester are five hundred
feet longer in the summer than in the winter.
The walls of the Cathedral of Armagh, as also those of the Conservatoire
des Art et Métiers, were brought back to a nearly perpendicular
position, by the insertion (through the opposite walls) of great bars of
iron, which being alternately heated, expanded, and screwed up tight,
then cooled and contracted, gradually corrected the bulging out of the
walls or main supports of these buildings. The principle of these famous
practical experiments is neatly illustrated by means of an iron
framework with a bar of iron placed through both its uprights, and
screwed tight when hot; on cooling, contraction occurs, which is shown
by a simple index. (Fig. 345.)
[Illustration: Fig. 345. The iron frame, with C C, wrought-iron bar
heated by putting on the semicircular piece of iron E E, which is first
made red-hot, and as the heat is communicated to the wrought iron rod C
C, it is screwed up tight by the nut K. G G. The index attached to the
iron frame screwed up when hot; the arms come together at P, and
separate further to H H as the contraction takes place by cooling the
bar C D.]
It has often been remarked that there is no rule without an exception,
and this applies in a particular instance to the law that "bodies expand
by heat and contract by cold"--viz., in the case of Rose's fusible
metal, which consists of
Two parts by weight of bismuth,
One part " lead,
One part " tin.
To make the alloy properly, the lead is first melted in an iron ladle,
and to this are added first the tin, and secondly the bismuth; the whole
is then well stirred with a wooden rod, and cast into the shape of a
bar.
[Page 358]
When placed in the pyrometer and heated, the bar expands progressively
till it reaches a temperature of 111° Fahr.; it then begins to
_contract_, and is rapidly _shortened_, until it arrives at 156° Fahr.,
when it attains a maximum density, and occupies no more space than it
would do at the freezing-point of water. The bar, after passing 156°,
again expands, and finally melts at about 201°, which is 11° below the
boiling-point of water. Fusible metal is sometimes made into teaspoons,
which soften and melt down when stirred in a cup of hot tea or basin of
soup, to the great surprise and bewilderment of the victim of the
practical joke.
Unequal expansion is familiarly demonstrated with a bit of toasted
bread, which curls up in consequence of the surface exposed to the fire
contracting more rapidly than the other; and the same fact is
illustrated with compound flat and thin bars of iron and brass, which
are fixed and rivetted together; when heated, the compound bar curves,
because the iron does not expand so rapidly as the brass, and of course
forms the interior of the curve, whilst the brass is on the exterior.
The experiment with the compound bar is made more conclusive and
interesting by arranging it with a voltaic battery and platinum lamp.
One of the wires from the battery is connected with the extremity of the
compound bar, and as long as it remains cold, no curve or arch is
produced, but when heat is applied, the bar curves upwards, and touching
the other wire of the battery, the circuit is completed, and the
platinum lamp is immediately ignited. (Fig. 346.)
[Illustration: Fig. 346. A B. Compound bar resting on two blocks of
wood. The end A is connected with one of the wires from the battery. The
circuit is completed and the platinum lamp D ignited directly the bar
curves _upwards_ by the heat of the spirit lamp, and touches the wire C
C connected with the opposite pole of the battery.]
The expansion and contraction of liquids by heat and cold is also
another elementary truth which admits of ample illustration, and indeed
introduces us to that most useful instrument called the thermometer.
If a flask is fitted with a cork through which a long glass tube, open
[Page 359] at both ends, is passed, and then carefully filled with water
coloured with a little solution of indigo, so that when the cork and
tube are placed in the neck, all the air is excluded, a rough
thermometer is thus constructed, which, if placed in boiling water,
quickly indicates the increased temperature by the rising or expansion
of the coloured water inside the flask. (Fig. 347.)
[Illustration: Fig. 347. Expansion of liquids shown at A by the coloured
water rising in the tube from the flask, which is quite full of liquid,
and heated by boiling water. B. The expansion of the water heated by the
spirit-lamp is shown by the rising of the piston and rod C C. D
represents a retort filled up like A to show the expansion of a liquid
by heat.]
The thermometer embraces precisely the same principle as that already
described in Fig. 347, with this difference only, that the tube is of a
much finer bore, and the liquid employed, whether alcohol or mercury, is
boiled and hermetically sealed in the tube, so that the air is entirely
excluded. To make a thermometer, a tube with a capillary bore is
selected of the proper length; it is then dipped into a glass containing
mercury, so that the tube is filled to the length of half an inch with
that metal. The half-inch is carefully measured on a scale, and the
place the mercury fills in the tube marked with a scratching diamond;
the mercury is then shaken half an inch higher, and again marked, and
this proceeding is continued until the whole tube is divided into half
inches. The object of doing this is to correct any inequalities [Page
360]in the diameter of the bore of the glass tube, because if wider at
one part than another, the spaces filled with the mercury are not equal;
as the bore is usually conical, the careful measurement of the tube with
the half inch of mercury in the first place gives the operator at once a
view of the interior of his tube, and enables him to graduate it
correctly afterwards. (Fig. 348.)
[Illustration: Fig. 348. A B. Magnified view of the bore of one of the
thermometer tubes which are made by rapidly drawing out a hollow mass of
hot glass whilst soft and ductile, consequently the bore must be
conical, and larger at one end than the other.]
The next step is to heat one extremity by the lamp and blowpipe, and
whilst hot, to blow out a ball upon it; if this operation were performed
with the mouth, moisture from the breath would deposit inside the fine
bore of the glass tube, and injure the perfection of the thermometer
afterwards. In order to prevent any deposit of water, the bulb is blown
out, whilst red-hot, with the air from a small caoutchouc bag fitted on
to the other extremity of the tube. The operator now marks off the
intended length of his thermometer, and above that point the tube is
again softened with the flame and blowpipe, and a second bulb blown out.
(Fig. 349 _a_.)
[Illustration: Fig. 349 _a_.--No. 1. First bulb. The intended length of
the thermometer is shown at the little cross.--No. 2 is the second bulb
placed above the cross.]
The open end of the tube is now placed under the surface of some pure,
clean, dry quicksilver, and heat being applied to the upper bulb, the
air expands and escapes through the mercury, and as the tube cools a
vacuum is produced, into which the mercury passes. By this simple
method, the mercury is easily forced into the tube, as otherwise it
would be impossible to _pour_ the quicksilver into the capillary bore of
the intended thermometer. (Fig. 349 _b_.)
[Illustration: Fig. 349 _b_. Heating and expanding the air in the top
bulb, so that when cool the mercury in the glass A, may rise into the
tube and fill the bulb B.]
The tube is now taken from the glass containing the mercury, and simply
inverted; but in consequence of the very narrow diameter of the bore the
air will not pass out of the first bulb until heat is applied, when the
air expands, and the [Page 361] mercury, first stationary in the second
bulb, will now displace the air, and fall into the first bulb when the
tube is again cool.
The ball, No. 1 (Fig. 349 _a_), is now full of mercury, and there is
also some left in No. 2; in the next place, the tube is supported by a
wire, and held over a charcoal fire, when it is heated throughout its
entire length, and the mercury being boiled expels the _whole of the
air_, so that there is nothing inside the bulbs and capillary bore but
mercury and its vapour. (No. 1, Fig. 350.) The open end of the intended
thermometer is now temporarily closed with sealing-wax, and the whole
allowed again to cool with the sealed end uppermost, so that the ball
No. 2, Fig. 350, and the tube above it, are quite filled with
quicksilver.
After cooling, the tube is placed at an angle with the sealed end
uppermost, and, guided by experience, the operator heats the lower bulb
so as to expand enough mercury into the upper one to leave space for the
future expansion and contraction of the mercury in the tube, which has
now to be hermetically sealed. This is done by dexterously heating the
tube at the cross whilst the mercury in the first bulb is still
expanded; and by drawing it out rapidly with the help of the heat
obtained from the lamp and blowpipe, the second bulb is separated from
the first at the little cross (B, No. 3, Fig. 350), and the thermometer
tube at last properly filled with quicksilver, and hermetically closed.
(No. 4, Fig. 350.)
[Illustration: Fig. 350.--No. 1. Boiling quicksilver in the tube with
two bulbs.--No. 2. Tube cooled, with the sealed end uppermost.--No. 3.
Mercury in first bulb expanded by lamp A, and at the proper moment
hermetically sealed by the flame urged by the blowpipe at B. The upper
bulb and tube to the cross being drawn away and separated.--No. 4.
Thermometer tube containing the requisite quantity of mercury,
hermetically sealed, and now ready for graduation.]
[Page 362]
In order to procure a fixed starting-point, the thermometer tube is
placed in ice, with a scale attached; the temperature of ice never
varies, it is always at 32 degrees. When, therefore, the mercury has
sunk to the lowest point it can do by exposure to this degree of cold,
the place is marked off in the scale, and represents that position in
the graduated scale where the freezing point of water is indicated.
The tube is placed in the next place in a vessel of boiling water, care
being taken that the whole tube is subject to the heat of the water and
the steam issuing from it, and when the mercury has risen to the highest
position attainable by the heat of boiling water, another graduation is
made which indicates 212 degrees--viz., the boiling point of water. This
graduation should be made when the barometer stands at 30 inches,
because the boiling point of water varies according to the weight of the
superincumbent air pressing upon it.
Between the graduation of the freezing and the boiling point of water
the space is divided into 180 parts, which added to 32 make up the
boiling point of water to 212 degrees, being the graduation of
Fahrenheit, who was an instrument-maker of Hamburg. Why he divided the
space between the freezing and boiling point of water nobody appears to
know, unless he took a half circle of 180 degrees as the best division
of space. If the thermometer contains air the mercury divides itself
frequently into two or three slender threads, each separated from the
other in the capillary bore, and thus the instrument is rendered useless
until the threads again coalesce. If the thermometer has been well made,
and is quite free from air, it may be tied to a string and swung
violently round, when the centrifugal force drives the slender threads
of mercury to their common source--viz., the bulb containing the
quicksilver, and the whole is again united. The string must be attached,
of course, to the top of the thermometer scale.
When travelling on the Continent it is sometimes desirable to be able to
read the thermometers which are graduated in a different manner to that
of Fahrenheit. In France the Centigrade scale is preferred, and in many
parts of Germany Reaumur's graduation. The difference of the graduation
is seen at a glance.
In the Centigrade the freezing point is 0, the boiling point 100°.
" Reaumur " 0, " 80°.
" Fahrenheit " 32°, " 212°.
The number of degrees, therefore, between boiling and freezing is 100 in
the Centigrade, 80 in Reaumur, and (212-32, that is) 180 in Fahrenheit.
If, then, the letters C, R, F, be taken to denote the _number_ of
degrees from the freezing point at which the mercury stands in the
Centigrade, Reaumur, and Fahrenheit thermometers, we have the following
proportions:--
(1.) 100: 80 :: C: R, whence C = 5/4 of R, or R = 4/5 of C.
(2.) 180:100 :: F: C, whence F = 9/5 of C, or C = 5/9 of F.
(3.) 180: 80 :: F: R, whence F = 9/4 of R, or R = 4/9 of F.
[Page 363]
The following examples will show how to apply these formulæ:--
(1).--Suppose the Reaumur stands at 28°, at what height does the
Centigrade stand? We have C = 5/4 of R (in this case), 5/4 of 28 = 35:
that is, the Centigrade stands at 35°.
(2).--Suppose Fahrenheit to stand at 41°, what will Reaumur stand at? R
= 4/9 of (41-32) (that is, the number above freezing in Fahr.) = 4/9 of
9 = 4. Reaumur stands at 4.
(3).--Suppose Fahrenheit stands at 23°, what will the Centigrade stand
at? C = 5/9 of F = 5/9 of (32-23) = 5/9 of 9 = 5 below freezing (or-5).
(4).--If Fahrenheit stands at 4 below 0, what will Reaumur indicate? R =
4/9 of F = 4/9 of (32 + 4) = 4/9 of 36 = 16 below 0 (or-16).
The only liquid which has the exceptional property of expanding by cold
is water, and it will be seen presently that this curious anomaly is of
the greatest importance in the economy of nature.
If a box containing a mixture of ice and salt is placed round the top of
a long cylindrical glass containing water at a temperature of 60° Fahr.,
the intense cold of the freezing mixture, which is zero--that is to say,
32° below the freezing point of water--very soon reduces the temperature
of the water contained in the glass, and as it becomes colder it
contracts, is rendered heavier, and sinks to the bottom of the vessel,
and its place is taken by other and warmer water. This circulation
commencing downwards, proceeds till the water has attained a temperature
of about 40° Fahr., when the maximum density is obtained and the
circulation stops, because after sinking below 40° the cold water
becomes lighter, and continues to be so until it freezes, and of course,
being of a less specific gravity than the warmer water, it floats (like
oil on water) upon its surface; so that a small thermometer placed at
the bottom of the jar indicates only 40° Fahr., whilst the solid ice
enveloping the other or second thermometer placed at the top may be as
low as 29°, or even lower, according to the quantity of ice and salt
used in the box surrounding the top of the glass. (Fig. 351.)
[Illustration: Fig. 351. A B. Long cylindrical glass containing water
and two thermometers; the one at the bottom shows a temperature of 40°;
the other at the top 32°, or even lower, C C C C. Section of box
containing the ice and salt, and standing on four legs, two of which are
shown at D D.]
[Page 364]
The importance of this curious anomaly cannot be overrated. If water did
not possess this rare property, all the seas, rivers, canals, lakes,
&c., would _gradually_ become impassable from the presence of enormous
blocks of ice formed during the winter. The whole bulk of water
contained in them would have to sink below 32° before it could solidify
provided water increased in density or continued to contract by cold.
Having once solidified, the warmth of the rays from a summer's sun would
certainly melt a great deal of the ice, but not the whole, and winter
would come again before the solid masses had disappeared. The ocean
could not be navigated in safety even near our own shores, in
consequence of the vast icebergs that would be formed, and float about
and jostle each other even in the British Channel.
The earth has been wonderfully prepared for God's highest work--Man, and
in nothing is this supreme wisdom more apparent than in the fact that
water offers the only known exception to the law "that bodies expand by
heat and contract by cold."
The expansion of gases by heat and contraction by cold take place in
obedience to a law to which there is no exception, except in degree. It
was discovered in 1801 by M. Gay Lussac, of Paris, and also about the
same period by the famous English philosopher who established the atomic
theory--viz., by Dr. Dalton. Since these experiments and calculations
Rudberg, Magnus, and Regnault have made other researches, and their
successive experiments give the following results:--
Vols. of air. Volumes.
Dalton, Gay Lussac 1000 heated from 32° to 212° became 1375
Rudberg 1000 " " " 1365
Magnus, Regnault 1000 " " " 1366.5
As a natural result, air at 32° Fahr, expands 1/491 part of its volume
for every degree of heat on the scale of Fahrenheit; and a volume of air
which measures 491 cubic inches at 32° will measure 492 at 33°, 493 at
34°, and so on. The exception is only in degree, and Magnus and Regnault
discovered by their searching experiments that the gases easily
liquified are more expansible by heat than air and those gases (such as
oxygen, hydrogen, and nitrogen) which have never been liquified.
The expansion of air is easily shown by placing the open end of a tube
with a large bulb blown at the other extremity, under the surface of a
little coloured water; on the application of heat the air expands and
escapes, and its place is taken, when cool, by the coloured liquid. Such
an arrangement represents the first thermometer constructed by Sanctorio
about A.D. 1600, which might certainly answer for rough purposes, but as
the ascent and descent of the fluid depend on the bulk of air contained
in the bulb, and as this is affected by every change of the height of
the barometer, no satisfactory indication of an increase or decrease of
temperature could be obtained with it, although the instrument itself is
interesting in an historical point of view, and in a [Page 365]
modified form as an air thermometer has been employed by Sir John
Leslie, under the name of the "Differential Thermometer," in his refined
and delicate experiments with heat.
[Illustration: Fig. 352 A. Sanctorio's original air thermometer; the
expansion and contraction of the air in the bulb indicate the rise or
fall of the temperature. The cork is merely a support, and is not fitted
into the bottle air-tight. B C. The differential thermometer. When both
bulbs are subjected to a uniform temperature, no movement of the fluid
shown at D occurs; but if the bulb B is put into any place warmer than
the position of the bulb C, then the air expands in B, and drives the
coloured liquid, which consists of carmine dissolved in oil of vitriol,
up the scale attached to the stem of the bulb C.]
Fire balloons are a good example of the expansion of gases, and the
levity of the air thus increases in bulk was taken advantage of by
Montgolfier in the construction of his famous balloon, which, with a
cage containing various animals, ascended, in the presence of the King
and royal family of France, at Versailles; and in spite of huge rents in
two places, it rose to a height of 1440 feet, and after remaining in the
air for eight minutes, fell to the ground at the distance of 10,200 feet
from the place whence it started, without injury to the animals. When it
is considered that a volume of air heated from 32° to 491° is doubled,
and tripled when heated to 982°, it will at once be understood how great
must be the ascending power of such balloons, provided the air within
them is kept sufficiently hot.
That gallant aëronaut, Pilate de Rozier, offered himself to be the first
aërial navigator; and having joined Montgolfier, they made three
successful ascents and descents with a large oval-shaped balloon,
forty-eight feet in diameter, and seventy-four feet high. On the fourth
occasion he ascended to a height of 262 feet, but in the descent a gust
of wind having blown the machine over some large trees of an adjoining
garden, the situation of the brave aëronaut was extremely dangerous, and
if he had not possessed the strongest presence of mind, and at once
[Page 366] given the balloon a greater ascending power, by rapidly
supplying his stove with some straw and chipped wood, he might on this
occasion have met with that untimely end which subsequently, in another
rash aëronautic adventure, befell this brave but foolhardy Frenchman.
On descending again, he once more, and without the slightest fear,
raised himself to a considerable height by feeding his fire with chopped
straw. Some time after he ascended, in company with M. Giroud de
Vilette, to the height of 330 feet, hovering over Paris at least nine
minutes, in sight of all the inhabitants, and the machine keeping all
the while perfectly steady.
The danger in using this method of inflating the balloon arises from the
possibility of generating gas, which escaping unburnt into the body of
the balloon, may accumulate and blow up, or burn afterwards.
[Illustration: Fig. 353. A B. Wessel's gas stove, with ring of gas jets
lighted inside; the air rushes in the direction of the arrows, C C, and
escaping at the top of the chimney, D D, soon fills the air or fire
balloon, which is usually made of paper.]
Fire balloons, as usually made, are very dangerous toys, and may
sometimes prove rather costly to the person who may send them off, in
consequence of their being blown by the wind on a hay or corn rick, or
other combustible substances. The safest mode of using fire balloons is
to fill them with hot air from a lighted gas stove (Wessel's, for
instance); the balloons may then be used in large rooms, or out in the
air, without fear of doing any harm to neighbouring property, as of
course the stove and the fire remain behind, and will fill any number of
air balloons. (Fig. 353.)
After all the fuss made about the novelty of the American hot-air
engine, it is somewhat amusing to look back to the records of civil
engineering, and in the "Transactions of the Institution of Civil
Engineers," to read Mr. James Stirling's account of his improved air
engine, in which the great expansion of air mentioned at p. 365 has been
successfully applied. The engine was constructed about the year [Page
367] 1843, and the principle, discovered thirty years before by Mr. R.
Stirling, will be comprehended by reference to the cut. (Fig. 354.)
[Illustration: Fig. 354. Stirling's air engine.]
Two strong air-tight vessels are connected with the opposite ends of a
cylinder, in which a piston works in the usual manner. About four-fifths
of the interior space in these vessels is occupied by two similar
air-tight vessels or plungers, which are suspended to the opposite
extremities of a beam, and capable of being alternately moved up and
down to the extent of the remaining fifth. By the motion of these
interior vessels, which are filled with non-conducting substances, the
air to be operated upon is moved from one end of the exterior vessel to
the other, and as one end is kept at a high temperature, and the other
as cold as possible, when the air is brought to the hot end it becomes
heated, and has its pressure increased; and when it is brought to the
cold end, its heat and pressure are diminished. Now, as the interior
vessels necessarily move in opposite directions, it follows that the
pressure of the enclosed air in the one vessel is increased, while that
of the other is diminished. A difference of pressure is thus produced
upon the opposite sides of the piston, which is thereby made to move
from the one end of the cylinder to the other, and by continually
reversing the motion of the suspended bodies or plungers, the greater
pressure is successively thrown upon a different side, and a
reciprocating motion of [Page 368] the piston is kept up. The piston is
connected with a fly-wheel in any of the usual modes; and the plungers,
by whose motion the air is heated and cooled, are moved in the same
manner, and nearly at the same relative time, with the valves of a steam
engine.
The pressure is greatly increased and made more economical by using
somewhat highly-compressed air, which is at first introduced, and is
afterwards maintained, by the continued action of an air-pump. The pump
is also employed in filling a separate magazine with compressed air,
from which the engine can be at once charged to the working pressure.
Mr. Stirling's chief improvement consists _in saving all or nearly all
the heat of the expanded air after it has done its work_, by passing it
from the hot to the cold end of the air vessel through a multitude of
narrow passages, whose temperature is at the beginning of the tubes
nearly as great as that of the hot air, but gradually declines till it
becomes nearly as low as the coldest part of the air vessel. The heat is
therefore retained by these passages, so that when the mechanism is
reversed, the cold air returns again through these hot pipes, and is
thus made nearly hot enough by the time it reaches the heating vessel to
do its work. Thus, instead of being obliged to supply at every stroke of
the engine as much heat as would be sufficient to raise the air from its
lowest to its highest temperature, it is necessary to furnish only as
much as will heat it the same number of degrees by which the hottest
part of the air vessel exceeds the hottest part of the intermediate
passages. This portion of the engine may be called the _economical
process_, and represents the foundation of all the success to which it
has attained in producing power with a small expenditure of fuel. No
boiler being required, of course the danger of explosions is much
lessened. The higher the pressure under which the engine was worked the
greater was the effect produced. A small engine on this principle was
worked to a pressure of 360 pounds on the square inch; and perhaps the
best popular notion of the novelty in the arrangement is that suggested
by Mr. George Lowe, who compared the economical part of the machine to a
"Jeffrey's Respirator" used by consumptive patients. The heat from the
air _expired_ being retained by the laminæ, and again used when cold air
is inspired or drawn into the lungs. Mr. Stirling states that the
consumption of fuel as compared to the steam engine which the air engine
had replaced was as 6 to 26; the same amount of work being now performed
by about six cwt. of coals which had formerly required about twenty-six
cwt., though he ought to have stated that the steam engine removed was
not of the best construction, nor had the boiler any close covering.
(Fig. 354.)
_Conduction of Heat._
This property of heat with reference to matter, and the consideration of
the curious manner in which it creeps, as it were, through solid
substances, brings the thoughtful mind at once to the bold question of
What is heat? Is it to be regarded as something real or material? or
[Page 369] must it be considered only as a property or state of matter?
These questions are not to be solved easily, and they demand a
considerable amount of experiment and reasoning even to appreciate their
meaning.
If a red-hot ball is placed in the focus of a concave metallic speculum,
it gives out certain emanations that are quite invisible, but which are
reflected from the surface of the mirror in the same manner as visible
rays of light, and may be collected in the focus of another and second
concave speculum, when they can be concentrated on to a bit of
phosphorus, and will cause the combustion of that substance. If the air
from a pair of bellows is blown forcibly across the rays of heat as they
are being concentrated upon the phosphorus, the rays are not moved from
their course, they are no more blown away than a sunbeam darting through
an aperture in a cloud on a stormy, windy day. The heat has, therefore
nothing to do with the air, and is wholly independent of that medium in
its passage from one mirror to the other. Such an experiment as that
described would at once suggest the idea that heat is a matter _sui
generis_, a component part of all bodies, and given off from
incandescent matter, the sun, &c., and that it may be propagated through
space much in the same manner as light. (Fig. 355.) The mechanism may be
very much like the corpuscular movement of light as defined by Sir Isaac
Newton, and already explained in another portion of this book. Hence it
has been supposed that heat is propagated through the air, water, and
solid substances by a direct emission of material particles from the
heat-giving agent, and that these molecules of heat force their way
into, or along, or through them, according to circumstances.
[Illustration: Fig. 355. Heat reflected by mirror, but not blown away by
air from bellows.]
Certain bodies are almost transparent to heat rays, such as air, whilst
others take an intermedial position, and only stop a certain quantity of
the heat molecules, such as rock crystals, mirror glass, and alum. A
third class of bodies absorbs the heat plentifully, such as charcoal,
black cloth, &c.; and a fourth, when polished and placed at the proper
angle, reflects or throws off the heat, as in the case of polished
mirrors. The transparency or opacity of substances (so far as light is
concerned) [Page 370] does not affect the transmission of heat. Light of
every colour and from all sources is equally transmitted by all
transparent bodies in the liquid or solid form, but this is not the case
with heat.
The rays of heat emitted by the sun and other luminous bodies have
properties quite different to the rays of light with which they are
accompanied. From these statements it will be evident that the _material
theory_ of heat is surrounded with difficulties and anomalies that
cannot be reconciled the one with the other, or neatly adapted, fitted
in, and dovetailed with all the puzzling phenomena that arise. Our
knowledge of the theory of heat has been greatly assisted by the
researches of Melloni, who has demonstrated that different _species_ of
rays of heat are given off by the same body at different temperatures,
which may be distinctly sifted and separated from each other. Long
before the experiments of Melloni philosophers had endeavoured to weigh
heat; trains of the most delicate levers were exposed, without effect,
to the action of heat rays; and all attempts, experimental as well as
theoretical, to define heat by the _material_ theory, are imperfect,
crude, and unsatisfactory. We are perforce obliged to adopt another
theory, and the one that obtains the greatest favour, as offering the
best definition of heat, is the _dynamical_ theory, which is more or
less analogous to the undulatory theory of light. At pages 262, 328,
335, this theory has been partly explained, and in speaking of it again,
great care must be taken not to confuse the undulations of heat with
those of light. The sun and the stars swim in a molecular medium, and
39,180 vibrations or waves must occur in one inch to produce the
sensation of red light, and 57,490 undulations in the space of one inch
to produce a violet light. As vibrations of the ethereal molecules
affect the eye, so there may be other nerves in our bodies which are
peculiarly sensitive to the waves of heat. It requires eight vibrations
of the air to occur in a second to produce an audible sound; whilst if
the vibrations of the air amount to 25,000 per second they cannot be
appreciated by the human ear, although it is possible to conceive that
the ears of certain animals may be so susceptible of rapid vibrations
that they may be able, for certain wise purposes of the Creator, to
appreciate sounds which are inaudible to human ears.
Melloni exhibited a spectrum to a number of persons, and found that
there was more light apparent to some eyes than to others. Lubeck put a
scarlet cloth on a donkey, and found that the two were frequently
confounded together by the eyes of many spectators. These facts indicate
that there may be vibrations of molecules that produce the sensation of
heat, but which do not affect the nerves that are sensitive to the
action of light waves, and vice versâ; and it is also probable that all
these different undulations, some affording heat and some light, may be
generated and propagated through space, as from the sun; or through
shorter distances, as from burning lamps and fires, without in any way
interfering with or impeding each other's progress.
The dynamical theory seems to offer the best idea of the transmission
[Page 371] of heat which is carried, conducted, or propagated through
solids with variable rapidity, either by the vibration of the
constituent molecules of the body itself, or by the undulation of a rare
subtle fluid which pervades them. If a copper and iron wire of the same
length and diameter are bound together and heated at the point of union,
the waves of heat travel faster through the copper than the iron, and
the former is said to be the best conductor of heat; and the fact itself
is demonstrated by placing a bit of phosphorus at the end of each
metallic wire, and it will be found by experiment that the combustible
substance melts first and takes fire on the copper, and that a
considerable interval of time elapses before the phosphorus ignites on
the iron.
[Illustration: Fig. 356. C. Copper wire bound at A to I, an iron wire.
After the heat of the lamp has been applied for about five minutes the
heat travels to C first, and ignites the bit of phosphorus placed there.
After some time has elapsed the phosphorus at I also ignites.]
The same fact is exhibited in a most striking manner by inserting a
series of rods of equal lengths and thicknesses in the side of a
rectangular box, allowing them to pass across the interior to the
opposite side. The rods are composed of wood, porcelain, glass, lead,
iron, zinc, copper, and silver, and have attached to each of their
extremities, by wax or tallow, a clay marble. When the water placed in
the box is made to boil, the heat passes along the different rods, and
melting the wax or tallow, allows the marble to drop off. Consequently
the first marble would drop from the silver rod, the next from the
copper, the third from the iron, the fourth from the zinc, the fifth
from the lead, whilst the porcelain, glass, and wooden rods would hardly
conduct (in several hours) sufficient heat to melt the wax or tallow,
and discharge the marbles.
_Conduction of Metals._
Gold 1000
Silver 973
Copper 898.2
Iron 374.3
Zinc 363
Lead 179.6
[Page 372]
The experiment is made more striking if the marbles are allowed to fall
on a lever connected with the detent of a clock alarum, which rings
every time a marble falls from one of the rods. (Fig. 357.)
During a cold frosty day, if the hand is placed in contact with various
substances, some appear to be colder than others, although all may be
precisely the same temperature; this circumstance is due to their
conducting power: and a piece of slate seems colder than a bit of chalk,
because the former is a much better conductor than the latter, and
carries away the heat from the body with greater rapidity, and diffuses
it through its own substance.
[Illustration: Fig. 357. A B. Trough containing boiling water, heated by
gas jets below. C. The eight rods and marbles attached, one of which has
fallen. D. The tray to receive the marbles.]
The gradual passage of heat along a bar of iron as compared with one of
copper, is well illustrated by supporting the ends of the two bars on
the top of the chimney of an argand lamp, whilst the other extremities
are held in a horizontal position by little blocks of wood. If marbles
are attached by wax to the under side, they fall off as the heat travels
along the metallic bars, and more rapidly from the copper than the iron,
because the former is a better conductor of heat than the latter. (Fig.
358.)
[Illustration: Fig. 358. A. Section of an argand gas lamp, with a copper
chimney supporting the ends of the bars of copper and iron marked C and
I. The balls have fallen from C, the copper bar.]
From the experiments of Mayer, of Erlangen ("Ann. de Ch.," XXX.), it
would appear that the conducting powers of different woods are to a
certain extent to be regarded as in the inverse proportion to their
specific gravities--_i.e._, the greater the density of the wood the less
conducting power, and the contrary.
If a cylindrical bar or thick tube of brass, six inches long, and about
two inches in diameter, is attached to a wooden cylinder of the same
size, the conducting powers of the two substances are well displayed by
first straining a sheet of white paper over the brass, and then holding
it in the flame of a spirit lamp. The heat being conducted rapidly away
by the metal will not scorch the paper, until the whole arrives at a
uniform high temperature; whereas the paper is rapidly burnt when [Page
373] strained over the wooden cylinder, because the heat of the flame of
the lamp is concentrated upon one point, and is not diffused through the
mass of the wood. (Fig. 359.)
In the course of the highly philosophical experiments of Sir H. Davy,
which led him gradually to the discovery of the construction of the
safety lamp, he connected together, by a copper tube of a small bore,
two vessels, each containing an explosive mixture composed of fire damp
and air. When the mixture was fired in one vessel he found that the
flame did not appear to be able to travel, as it were, across the
bridge--viz., the copper tube--and communicate with the other magazine,
because it was deprived of its heat whilst passing through the tube, and
was no longer flame, but simply gaseous matter at too low a temperature
to effect the inflammation of the mixture in the second box.
[Illustration: Fig. 359. Cylinder, half brass and half wood. The paper
strained over the wood is taking fire. The other extremity, shaded, is
the brass portion.]
A mass of cold metal may be suddenly applied to a small flame, such as
that of a night light, and depriving it rapidly of heat (like the case
of the unfortunate Russian described at page 354), it is almost
immediately extinguished (fig. 360), not by the mere exclusion of the
oxygen of the air, but on account of the withdrawal of the heat
necessary for the maintenance of the combustion.
[Illustration: Fig. 360. A. Small flame from night light. B C. Large
mass of cold copper wire open at both ends to place over flame, and by
conduction of the heat to extinguish it.]
Sir H. Davy first thought of making his safety lamp with small tubes,
which would supply fresh air, and carry off the burnt or foul air, at
the [Page 374] same time they were to be so narrow that no flame could
pass out of his lamp to communicate with an outer explosive atmosphere;
and in speaking of his lamp with tubes he says:--"I soon discovered that
a _few apertures_, even of very small diameter, were not safe unless
their _sides_ were very _deep_; that a single tube of one-twenty-eighth
of an inch in diameter, and two inches long, suffered the explosion to
pass through it; and that a _great number_ of small tubes, or of
apertures, stopped explosion, even when the depths of their sides was
only equal to their diameters. And at last I arrived at the conclusion
that a _metallic tissue_, however thin and fine, of which the apertures
filled more space than the cooling surface, so as to be permeable to air
and light, offered a _perfect barrier_ to _explosion_, from the force
being divided _between_, and the heat communicated to an _immense number
of surfaces_. I made several attempts to construct safety lamps which
should give light in all explosive mixtures of fire damp, and after
complicated combinations, I at length arrived at one evidently the most
simple, that of _surrounding the light entirely by wire gauze, and
making the same tissue feed the flame with air and emit light_."
If a number of square metallic tubes of a fine bore are placed upright
side by side, and a section cut off horizontally, it would represent the
wire gauze which possesses such marvellous powers of sifting away the
heat from a flame, so that it is destroyed in its attempted passage
through the metallic meshes; and of this fact a number of proofs may be
adduced.
A gas jet delivering coal gas may be placed under a sheet of wire gauze,
the gas permeates the gauze, and may be set on fire at the upper side,
but the flame is cut off from the mouth of the jet by the cooling action
of the wire gauze. The same experiment reversed, by holding the gauze
over the gas burning from the jet, shows still more decidedly that flame
will not pass through the metallic tissue. (Fig. 361.)
[Illustration: Fig. 361. A A. A number of square tubes placed upright.
The arrow shows the direction of the section to obtain a figure like
wire gauze.]
Sir H. Davy again says: "Though all the specimens of fire damp which I
had examined consisted of carburetted hydrogen mixed with different
small proportions of carbonic acid and common air, yet some phenomena I
observed in the combustion of a _blower_ induced me to believe that
small quantities of olefiant gas may be sometimes evolved in coal mines
with the carburetted hydrogen. I therefore resolved to make all lamps
safe to the test of the _gas produced by the distillation of coal_,
which, when it has not been exposed to water, always contains olefiant
gas. I placed my lighted lamps in a large glass receiver through which
there was a current of atmospherical air, and by means of a [Page 375]
gasometer filled with coal gas, I made the current of air which passed
into the lamp more or less explosive, and caused it to change rapidly or
slowly at pleasure, so as to produce all possible varieties of
inflammable and explosive mixtures, and I found that iron gauze wire
composed of wires from one-fortieth to one-sixtieth of an inch in
diameter, and containing twenty-eight wires or seven hundred and
eighty-four apertures to the inch, _was safe under all circumstances in
atmospheres of this kind_; and I consequently adopted this material in
guarding lamps for the coal mines, when in January, 1816, they were
immediately adopted, and have long been in general use."
The remarkable conducting power of wire gauze is further shown by
placing some lumps of camphor on a piece of this material, and when the
heat of a spirit-lamp is applied on the under side of the gauze, the
camphor volatilizes, and as the vapour is remarkably heavy, it falls
through the meshes of the gauze, and takes fire; but the most curious
and further illustration of the conducting power of the wire meshes is
shown in the fact that the fire does not communicate through the thin
film of gauze to the lumps of camphor placed upon it.
The camphor may be ignited by applying flame to the upper side of the
gauze, showing that, although this substance is so exceedingly
combustible, it will not take fire even if placed at no greater distance
from flame than the thickness of the wire gauze, provided the latter
material is interposed between it and the flame.
A square box made of wire gauze, with a hole at the bottom to admit a
candle or spirit-lamp, may have a considerable jet of coal gas forced
upon it from the outside, or a large jug of ether vapour poured upon it;
and although the box may be full of flame, arising from the combustion
of the gas or ether, the fire does not come out of the wire box or
communicate with the jet or the ether vapour as it is poured from the
jug. (Fig. 362.)
[Illustration: Fig. 362. A box made of wire gauze, with a hole in the
bottom to admit a spirit lamp lighted. A hot jug full of the vapour of
ether may be poured on to the flame, but it only burns inside the box,
and does not communicate with that in the jug.]
Sir Humphrey Davy's safety lamp consists of a common oil-lamp, _f_, with
a wire through the cistern for the purpose of raising or depressing the
cotton wick without unscrewing the wire gauze; _b_ is the male screw
fitting the screw attached to the cylinder of wire gauze, which is made
double at the top. The entire lamp is shown at A, whilst the platinum
coil which Sir H. Davy recommends should be wound round the wick is
shown at _h_. The small [Page 376] cage of platinum consists of wire of
one-seventieth to one-eightieth of an inch in thickness, fastened to the
wire for raising or depressing the cotton wick, and should the lamp be
extinguished in an explosive mixture, the little coil of platinum begins
to glow, and will afford sufficient light to guide the miner to a safe
part of the mine. With respect to this platinum coil, Sir H. Davy gives
a careful charge, and says:--"The greatest care must be taken that no
filament or wire of platinum protrudes on the exterior of the lamp, _for
this would fire externally an explosive mixture_."
[Illustration: Fig. 363. Sir Humphrey Davy's safety lamp.]
Since the invention of the Davy lamp, a great number of modifications
have been brought forward, some of which for a short time have occupied
the public attention, but whether from increased cost or a sort of
inertia that arrests improvement, it is certain that the lamp originally
devised by Sir Humphrey Davy is still the favourite. It was perhaps
unfortunate that the lamp was called the _safety_ lamp, because it is
not so under every circumstance that may arise, unless it happens to be
in the hands of persons who have taken the trouble to study it and
understand how to correct the faults. The lamp might have escaped the
incessant attacks that have been made upon its just merits, if the name
had simply been that of its illustrious inventor--"a Davy lamp." No one
could carp at that, whilst "safety" was held to mean perfect immunity
from every possible and probable danger that might arise in the
coal-pits. The lamps are now usually placed under the charge of one man,
who trims them and ascertains that the wire gauze is in perfect order;
this latter is usually locked upon the lamp, and as it is a penal
offence, and punishable by a heavy fine and imprisonment, to remove the
wire gauze from safety lamps in dangerous parts of the mine, of course
the miners are being gradually brought to a sense of the obligations
they owe themselves and their brother-miners, and the rash, ignorant,
and foolhardy offences of breaking open safety lamps for more
illumination, or to light pipes, are becoming much less frequent than
formerly. One of the most ingenious "detector lamps" is that of Mr.
Symons, of Birmingham. (Fig. 364.) It consisted of the old-fashioned
Davy, but [Page 317] inside the rim of the wire gauze is placed a small
extinguisher and spring, which does not move so long as the gauze is
screwed _on_ to the lamp, but directly the gauze is unscrewed, the
reversed movement releases the detent, and the extinguisher falls upon
the light. In spite of the manifest ingenuity of this lamp, it is not
adopted, because it costs a trifle more than the ordinary "Davy." To
show the remarkable perfection of the wire gauze principle, some
turpentine may be poured upon a lighted safety lamp, when a great smoke
is produced by the evaporation of the spirit, but no flame passes
through to the outside, although the turpentine burns inside the lamp.
If some coarse gunpowder is laid upon two thicknesses of fine wire
gauze, it may be heated from below with the flame of the spirit lamp,
and the sulphur will gradually volatilize without setting fire to the
mass of powder. To show the security of the Davy lamp, it may be lighted
and hung in a large box with glass sides, open at the top, and a jet of
coal gas supplied at the bottom; as this rises and diffuses in the air,
the mixture becomes explosive, and the fact is at once evident by the
alteration in the appearance of the flame of the lamp, which enlarges,
flickers, and frequently goes out, in consequence of the suddenness with
which the explosion of the mixture takes place inside the lamp,
producing a concussion that extinguishes the flame. In this case the
utility of the platinum coil is very apparent, and it continues to glow
with a red heat until the explosive character of the air in the box is
changed.
[Illustration: Fig. 364. Symons' self-extinguishing Davy lamp.]
If a large washhand-basin is first warmed by some boiling water, which
is then poured away, and a drachm of ether thrown in, a
highly-combustible atmosphere is obtained, and when a lighted Davy lamp
is placed into the basin so prepared, the flame inside the lamp
immediately enlarges and flickers, but is not extinguished, and does not
communicate to the combustible vapour outside. The contrast between the
safety lamp and an unprotected flame is very striking; if a lighted
taper is thrust into the basin, the ether catches fire, and burns with a
very large flame. The solid conductors of heat, which are said to enjoy
this property in the highest degree, are the metals, marble, stone,
slate, and [Page 378] other dense and compact solid substances; whilst
the opposite quality of being non-conductors, or nearly so, is possessed
by fur, wood, silk, cotton, wool, eider and swansdown, paper, sand,
charcoal, and every substance which is of a light or porous nature. The
practical application of this knowledge is very apparent in the affairs
of every-day life. Thus we rise in the morning, and immediately after
the necessary ablutions, if it is winter time, proceed to encase the
body in non-conductors, such as flannel and wool. When we sit down to
the breakfast table to make tea, we may notice the contrivances for
preventing the handle of the top of the urn, or that of the teapot, from
becoming too hot for the fingers, by the interposition of ivory or wood.
If asked to place water in the teapot from the kettle, we instinctively
seek for the well-worn kettle-holder made of Berlin wool, and therefore
a bad conductor. As we cut our meat or fish at the same meal, we may
shiver with cold, but our fingers are not quite frozen by contact with
the steel knives, as we hold them by ivory handles; and we are agreeably
reminded that some metals are good conductors of heat, by the pleasant
warmth of the silver teaspoons, as we stir our tea or coffee.
Even the polish of the well-rubbed mahogany is protected from the heat
of the dishes by non-conducting mats, and plates are handed about, if
"nice and hot," with a carefully-wrapped non-conducting linen napkin.
Supposing we prefer a bit of fresh-made toast, the fork is provided with
a non-conducting handle; and should we peep out of window some wintry
morn whilst the baker delivers his early work in the shape of hot rolls,
we notice they come out of nicely-wrapped flannel or baize, which being
a bad conductor is employed to retain their heat. We read, occasionally,
in the military intelligence, statements respecting some
newly-constructed shells which are to burst and scatter melted iron
(!!); and of course the idea of the interposition of a good
non-conductor of heat between the bursting charge and the molten metal
must be realized in their construction.