and after digging beyond a depth of twenty feet the thermometer
gradually rises at the rate of one degree of Fahrenheit's scale for
every fifteen yards. The bad conducting power of the crust of the earth
must, therefore, be apparent, as it is easy, knowing the diameter of our
globe, to calculate that the increase of heat downwards amounts to 116°
for each mile, consequently at a depth of thirty and a half miles below
the surface, there will be a temperature most likely equal to 3500°, or
a heat that might easily melt cast-iron, and would help to account for
the earthquakes and eruptions of volcanoes, which still remind us by
their terrible warnings, that we live only on the bad conducting upper
crust of a globe, the inside of which is still, perhaps, in a liquid and
molten state. Monsieur Fourier has demonstrated the non-conducting power
of this shell by calculating that, supposing the globe was wholly
composed of cast-iron, the central heat would require myriads of years
to be transmitted to the surface from a depth of 150 miles; and by
inverting the process of reasoning, we may come to the conclusion that
the [Page 379] internal heat must be excessive, because it is confined
and shut out from those influences that would carry off and weaken the
intensity.
There are no two words, says Tyndal, with which we are more familiar
than _matter_ and _force_. The system of the universe embraces two
things, an object _acted upon_, and an agent _by which_ it is acted
upon; the object we call matter and the agent we call force. Matter, in
certain respects, may be regarded as the _vehicle_ of force; thus, the
luminiferous ether is the vehicle or medium by which the pulsations of
the sun are transmitted to our organs of vision. Or, to take a plainer
case, if we set a number of billiard balls in a row, and impart a shock
to one end of the series in the direction of its length, we know what
will take place; the _last ball_ will fly away, the _intervening_ balls
having served for the transmission of the shock from one end of the
series to the other. Or we might refer to the conduction of heat. If,
for example, it be required to transmit heat from the fire to a point at
some distance from the fire, this may be effected by means of a
conducting body--by a poker, for instance; thrusting one end of a poker
into the fire, it becomes heated, the heat makes its way through the
mass, and finally manifests itself at the other end. Let us endeavour to
get a distinct idea of what we here call heat; let us first picture it
to ourselves as an agent apart from the mass of the conductor, making
its way among the particles of the latter, jumping from atom to atom,
and thus converting them into a kind of _stepping stones_ to assist its
progress. It is a probable conclusion, even had we not a single
experiment to support it, that the mode of transmission must, in some
measure, depend upon the manner in which those little molecular stepping
stones are arranged. But we must not confine ourselves to the molecular
theory of heat. Assuming the hypothesis, which is now gaining ground,
that heat, instead of being an agent apart from ordinary matter,
consists _in a motion of the material particles_; the conclusion is
equally probable that the transmission of the motion must be influenced
by the manner in which the particles are arranged. Does experimental
science furnish us with any corroboration of this inference? It does.
More than twenty years ago MM. De la Rive and De Candolle proved that
heat is transmitted through wood with a velocity almost twice as great
along the fibre as across it. This result has been recently expanded,
and it has been proved that this substance possesses three axes of
calorific conduction; the first and greatest axis being parallel to the
fibre; the second axis perpendicular to the fibre and to the ligneous
layers; while the third axis, which marks the direction in which the
greatest resistance is offered to the passage of the heat, is
perpendicular to the fibre and parallel to the layers.
If many solids are bad conductors of heat, they are at all events
greatly surpassed by fluids, and especially by water. The conduction of
heat by that fluid is almost imperceptible, so much so, that it has even
been questioned whether liquids do really conduct heat downwards at all.
It has, however, been found that liquid mercury will conduct heat
downwards, and therefore by analogy it may be assumed that other liquids
must possess a conducting power, although it may be exceedingly
limited.
[Page 380]
In order to prove that water is an exceeding bad conductor of heat, a
tube with a large glass bulb blown at one end is partly filled with
tincture of litmus, until it will just sink below the surface of water
placed in a tall cylindrical or open jar. If a copper basin, containing
burning ether, is now floated on the top of the water, so as to leave
about a quarter of an inch between the top of the air thermometer--viz.,
the bulb containing the coloured liquid--and the bottom of the copper
pan, it will be noticed that whilst the water surrounding the latter
almost boils, not the slightest effect arising from the conduction of
heat can be perceived in a downward direction. After the ether has burnt
out of the copper vessel, it may be removed, and the boiling water
stirred down and around the air thermometer, when the air within it
expands, drives out the colouring liquid, and the bulb becoming
specifically lighter, rises to the top of the containing glass. (Fig.
365.)
[Illustration: Fig. 365. A A. Cylindrical glass full of water. B. The
glass air thermometer containing the coloured liquid just standing
upright, the mouth of the tube at C being open. D D is the copper basin
containing the burning ether. E shows how the glass bulb and tube rise
after the upper basin is removed, and the hot water comes in contact
with and expands the air, making the thermometer light, and causing it
to rise.]
Again, if the tube of an air thermometer is placed through a cork in the
neck of a gas jar, inverted and standing on a ring stand, and the [Page
381] jar is then filled with water, and boiled at the top with a red-hot
iron heater, the heat does not pass downwards and affect the
thermometer. By introducing a syphon the water surrounding the
thermometer at the bottom of the jar may be drawn off, until the hot
water is within a fraction of an inch of the air thermometer, and still
no heat is conducted, and the liquid in the latter remains stationary.
(Fig. 366.)
[Illustration: Fig. 366. A A A. Inverted gas jar supported by the ring
stand. B. The red-hot urn heater. C C. The air thermometer, with the
coloured liquid stationary at C. D. The syphon for drawing off the cold
water, and bringing the hot down close to the bulb of C C.]
The diffusion of heat through water does not take place like that of
solids, but is effected by the motion of the particles of the water.
When heat is applied to the bottom of a vessel containing water, such as
an inverted glass shade, the first effect is to expand the layer of
water which is first affected by the heat; this expanded layer being
specifically lighter than the cold water above, it rises to the upper
part of the glass shade, and its place is immediately taken by other,
colder and heavier, water, which in like manner moves upwards, and is
again succeeded by a fresh portion. Now, the first and succeeding
strata [Page 382] of water all carry off so much heat, and thus by the
convective or carrying power of the water the heat is diffused finally
in the most perfect manner through the whole bulk of fluid; and indeed,
the movement itself of the particles of water may easily be watched by
putting a little paper pulp at the bottom of the inverted glass shade
containing the water. (Fig. 367.)
[Illustration: Fig. 367. A. A. Inverted glass shade containing water and
some paper pulp. B. Burning spirit lamp placed under _one_ side of the
glass; the pulp shows the rising of the heated water and the sinking of
the cold, in the direction indicated by the arrows.]
This bad conducting power is not merely confined to water, but is
likewise apparent with oil and other fluids, and if some water is frozen
at the bottom of a long test-tube by means of a freezing mixture, oil
may then be poured upon it, and some alcohol above the latter. If the
flame of a spirit-lamp is now applied to the alcohol at the top of the
tube it may be entirely boiled away, and no heat will travel down the
oil and communicate with the ice, and even after the alcohol has been
evaporated away the tube can be filled up with water; this may also be
boiled, and whilst demonstrating the bad conducting power of the oil,
the curious anomaly is observed of a vessel or tube containing ice at
the bottom and boiling water at the top, and further showing the wisdom
of the Supreme Creator in preventing the freezing of the water of lakes,
rivers, and seas, by the exceptional law of the expansion of water by
cold. It is evident from what has been stated that liquids acquire and
lose their heat by means of those currents and movements of the
particles of water which have already been partly explained. Whatever
interferes with this movement must prevent the passage of heat, and
consequently thick viscous liquids are always difficult to boil, and in
consequence of their motion being impeded they rise to too high a
temperature and are burnt. This fact is remarkably apparent in the
manufacture of nice white lump sugar; as the syrup is evaporated it
becomes very thick, and if boiled over a fire might frequently be burnt,
but it is boiled by the heat of steam, and under a vacuum produced by an
air-pump, and thus the sugar-boiler is enabled to avert all danger from
burning.
[Page 383]
It is, then, by a continual and perpetual motion, involving circulation
of the particles, that heat travels through water; and the fact already
described is still further elucidated by one of Professor Griffith's
simple but telling experiments. A glass tube, about three feet in length
and half an inch in diameter, is bent as at A (Fig. 368), and then being
filled with water, is suspended by a string attached to any convenient
support inside a copper dish containing water, so that the straight end
is at the top of the water, and the curved end at the bottom. Just
before it is used some ink or other colouring matter is poured into the
copper pan of water; and it should not be added till the moment the
experiment is to begin, as any rise of temperature in the room promotes
circulation, and interferes with the colourlessness of the water in the
tube, which is compared with the inky fluid in the basin. Directly heat
is applied the hot water rises to the top of the copper vessel, and
thence gradually up the tube; and this movement is rendered visible by
the hot coloured liquid matter creeping slowly up the tube, and
displacing the colourless water, which falls gradually into the copper
pan. (Fig. 368.)
[Illustration: Fig. 368. A. The bent glass tube full of water. B B. The
copper pan containing coloured water. The arrows show the circulation of
the water.]
The principle of the circulation of the particles of water being once
understood, it is easy to comprehend how it is applied to the heating of
buildings by what is called the "Hot Water Apparatus." A coil of pipe is
enclosed in a proper furnace, and the bottom end communicates with a
pipe coming from a second tube or set of coils, placed above it in
another apartment, whilst the top of the latter coil communicates with
the top pipe of the first coil. When the fire is lighted, the
circulation through the first coil of pipe commences, and is
communicated to the second, and from that back again to the first; so
that the "hot water system" [Page 384] involves an endless chain of
pipes of water, provided with proper safety valves to allow for the
escape of any expanded air or steam; and serious accidents have occurred
in consequence of persons neglecting to look after the perfection of
this safety valve. The fearful accident which occurred to the hot water
casing around one of the funnels of the _Great Eastern_ offers a painful
but memorable example of the heating of water, and of the dangers that
must arise if the pipe, casing, or other vessel which contains it, is
not provided with an escape or safety valve, which must always be in
_good working order_.
Mr. Jacob Perkins, in 1824, made his name remarkable for experiments
with the circulation of water through tubes, and his account of the
invention and improvement of the "Steam Gun," in which the improvement
consists chiefly in the circulation of water through coils of pipe, is
so important that we give it verbatim, with a drawing of the steam gun;
and the author is enabled to vouch for the accuracy of the statements
made in the description of the apparatus, as he purchased one of the
improved steam guns, and exhibited it at the Polytechnic Institution,
where it discharged three hundred bullets per minute.
[Illustration: Fig. 369. The charging tube and gun-barrel of steam gun.]
"The expansive power of steam has often been proposed as a substitute
for gunpowder, for discharging balls and other projectiles; the great
danger, however, which was formerly thought to be inseparably connected
with the generation and use of steam, at so extraordinary a pressure as
appeared necessary to produce an effect approximating to that of
gunpowder, prevented scientific men from testing the power of this new
agent by experiment. It was also apparent that the apparatus which was
ordinarily used for generating steam for steam-engines was wholly
inadequate to sustain the necessary pressure, and that one [Page 385]
of a totally different character must be contrived before steam could be
sufficiently confined to come into competition with its powerful rival.
"In the year 1824, Mr. Jacob Perkins succeeded in constructing a
generator of such form and strength, as allowed him to carry on his
experiments with highly elastic steam without danger, although subjected
to a pressure of 100 atmospheres. The principle of its safety consisted
in subdividing the vessel containing the water and steam into chambers
or compartments, so small, that the bursting of one of them was
perfectly harmless in its effects, and only served as an outlet, or
safety valve, to relieve the rest.
"Although Mr. Perkins' generator was originally intended for working
steam engines (it having long been evident to him that highly elastic
steam used expansively would be attended with considerable economy), the
idea occurred to him, in the course of his experiments, that he had
already solved the problem of safely generating steam of sufficient
power for the purposes of _steam gunnery_; and that the steam which
daily worked his engine possessed an elastic force quite adequate to the
projection of musket balls. He therefore caused a gun to be immediately
constructed, and connected by a pipe to the generator, the first trial
of which fully realized his most sanguine anticipations. Its
performance, indeed, was so extraordinary and unexpected, that it gave
rise to a paradox, which was difficult of explanation--viz., that
_steam, at a pressure of only forty atmospheres, produced an effect
equal to gunpowder_; whereas it was known that the combustion of
gunpowder was attended with a pressure of from 500 to 1000 atmospheres.
"Mr. Perkins gives the following explanation of this apparent
discrepancy, by referring to the small effect produced by fulminating
powder, compared to gunpowder, although many times more powerful; he
supposes that the action of fulminating powder, however intense, does
not continue sufficiently long to impart to the ball its full power. The
explosion of gunpowder, although not so powerful at the _instant of
ignition_, is nevertheless, in the aggregate, productive of greater
effect than that of fulminating powder, because the _subsequent
expansion continues_ in action upon the ball (but with decreasing
effect), until it has left the barrel. The action of steam differs from
either of these agents, inasmuch as it _continues in full force until
the ball has left the barrel_; and to this is assigned the cause of its
superiority.
"In the year 1826, Mr. Perkins had so perfected the mechanism of the gun
and generator that, at an exhibition and trial of its power, in the
presence of the Duke of Wellington and other distinguished officers of
the Ordnance Department, balls of an ounce weight were propelled, at the
distance of thirty-five yards, through an iron plate one-fourth of an
inch in thickness; also, through eleven hard planks, one inch in
thickness, placed at distances of an inch from each other. Continuous
showers of balls were also projected with such rapidity, that when the
barrel of the gun was slowly swept round in a horizontal direction, a
plank, twelve feet in length, was so completely perforated, that the
line of holes nearly resembled a groove cut from one of its ends to the
other.
[Page 386]
[Illustration: Fig. 370. Perkins's steam gun.]
[Page 387]
"A is an _iron furnace_, containing a continuous coil of iron tubing, 80
feet in length, 1 inch of external and 5/8th inch of internal diameter,
within which the fire is made; the upper end of this tube, B, called the
flow-pipe, is extended any required distance to the top of the
generator.
"The furnace is provided with a very ingenious _heat governor or
regulator_, by which the intensity of the fire is always proportionate
to the temperature which it may be requisite to maintain in the tubes.
"H is an iron box, containing a series of levers, _b b b_; _c_, a nut
screwed upon the flow-pipe, and in contact with the short arm of the
lowest of the levers. E. A lever, from one end of which is suspended the
damper _f_, and from the other end the rod _g_, which rests upon the
long arm of the highest of the levers, _b b b_. When the apparatus has
arrived at the required temperature, the nut _c_ is screwed down until
it bears upon the lever. Any farther increase of temperature will expand
or lengthen the flow-pipe, and depress the short arm of the lever, which
is in contact with the nut. The combined and multiplied action of the
levers will then elevate the rod _g_, and the damper _f_ will descend to
check the draught. When the fire slackens, and the apparatus cools, the
action of the levers will be reversed, and the damper will open. The
space through which the damper moves, compared with the nut _c_, is as
200 to 1.
"C is the _generator_, composed of a strong iron tube, 3 inches diameter
and 6 feet in length, within which are eight smaller tubes, having their
ends welded to the ends of the larger tube. These small tubes
communicate at the top with the _flow-pipe_ B, and at the bottom with
the _return-pipe_ D, which is continued to the bottom of the
furnace-coil of tubing. The circulation in the tubes is occasioned by
the difference in the specific gravities of the water composing the
ascending and descending currents; the portion contained in the
flow-pipe and fire coil becoming expanded by the heat, ascends by its
superior levity; while that contained in the small tubes of the
generator, having given off its heat, acquires increased density, and
descends through the return-pipe D to the bottom of the furnace-coil, to
take the place of the ascending current. When the hot-water current has
arrived at a temperature of 212° and upwards, cold water is injected
into the generator, and becomes converted into steam by its contact with
the small tubes; the rapidity of evaporation and the pressure of the
steam depending, of course, upon the temperature of the hot-water
current, which at 500° will cause a pressure within the tubes of 50
atmospheres, or 750 lbs. upon the square inch. The whole apparatus is
proved to be capable of sustaining a pressure of 200 atmospheres, or
3000 lbs. upon the square inch.
"G. A force pump for injecting water into the generator.
"I. The indicator for exhibiting the pressure of the steam in the
generator, and of the water in the boiler; it may be connected with
either by means of the valves attached to the levers.
"J. Valve to regulate the pressure of water.
"J 1. Valve to regulate the pressure of steam.
"K. The steam pipe.
"L. The gun.
"M. The discharging lever acting upon the valve N.
"O. The discharging cock, by a simple adjustment in which balls are
transferred from the charging tube P to the gun barrel, _singly_ or in a
_continuous shower_.]
"As the perfection and introduction of the steam gun was not a field for
private enterprise, and the British Government having declined to
institute experiments at its own expense, Mr. Perkins was reluctantly
compelled to leave the project, and to engage in others of a more
lucrative, although, perhaps, of a less important nature. He did not
suspend his operations, however, until he had constructed for the French
Government _a piece of artillery which discharged balls weighing five
pounds at the rate of sixty per minute_.
"The gun and generator exhibited at the Polytechnic Institution during
the time that Mr. Pepper was the Resident Director were the production
of Mr. A. M. Perkins, of London, who has invented an entirely _new
method of generating steam_, which has been successfully applied to
steam engines, and is at once so simple, safe, and economical, as to
leave little doubt that, with its aid, the steam gun will ere long rank
amongst the first instruments of warfare.
[Page 388]
"The gun, except in a few minor mechanical details, does not differ from
that originally constructed by Mr. Jacob Perkins.
"The novelty which distinguishes the generator from all others, consists
in the manner of conveying the heat from the fire to the water, _without
exposing the generator to the action of the fire_. This is accomplished
by means of the circulation, in iron tubes, of a current of hot water,
which is entirely separate from, and independent of, that to be
evaporated in the generator.
"The following are the principal advantages which this generator
possesses over all others: _Freedom from all wear or deterioration
consequent upon exposure to the fire_, an important quality in a
generator that is to be subjected to great pressure, inasmuch as its
original strength remains unimpaired; _no accident can arise from want
of water in the generator_, and the precautions indispensably requisite
when a generator is in contact with the fire are quite unnecessary, as
the water may be drawn off with impunity without producing the least
injurious effect, and the grossest neglect is followed by no worse
consequences than an inefficient supply of steam; _an explosion of the
generator is impossible_, as the temperature of the furnace-coil always
exceeds that of any other part of the apparatus, and consequently, being
the weakest part, is invariably the first to yield when the pressure is
carried beyond the strength of the pipes; _economy of fuel is also
obtained, with a small amount of fire surface_. The circulation of the
water has likewise the effect of preserving the fire-coil from the decay
to which boilers are liable; many such coils, which have been in
constant use for eight years, being apparently as good as when first
erected.
"The whole apparatus is exceedingly simple, and will be readily
understood by reference to the accompanying diagram. (Fig. 370.)
"The steam has often been raised to a pressure of 700 lbs. on the square
inch, but _one-third_ of that pressure is sufficient to completely
_flatten the balls_ when discharged against an iron target one hundred
feet distant from the gun; and a pressure of 400 lbs. per square inch,
at the same distance, _shivers the ball to atoms_, with the production
in a dark room of a visible flash of light. Steam guns are generally
mounted upon a ball and socket joint, which allows the barrel to move
freely in every direction."
* * * * *
The conduction of heat through gases is also very slow when heat is
applied to the upper part of any stratum of air. Heat appears to be
diffused through air only by the circulation and rising of the heated
and lighter strata, and the sinking of the colder currents which take
their places; hence the danger of sitting in a room under an open
skylight. A current of cold air may descend upon the head of the
individual, whilst the warmer air takes some other opening to escape
from. No doubt the movement of heated volumes of air is subject to
definite laws, which apply themselves under every case, but are rather
difficult to grasp when the subject of ventilation is concerned. The
philosophical ventilator is often dreadfully teased by the inversion of
all that he had [Page 389] planned, or the total failure of his
apparatus. No specific mode of ventilation can be found to suit all
rooms and buildings; they are like the patients of a physician who
cannot be cured by one medicine only, but must have a treatment adapted
properly to each case. If the fires, candles, gas, or oil-lamps, doors,
windows, and chimneys, were always under the control of the scientific
ventilator, his task would be very simple, but it is well understood
that a ventilating system which answers well if certain doors
communicating with lobbies are closed, fails directly they are
accidentally opened. The watchful care of the ventilator must begin with
the lowest area door, and in his calculations he must study the effect
of every other door or window that may be opened, so that if a
scientific man undertakes to ventilate a house, he must have a
well-drawn plan hung up in the hall, and it must be clearly understood
by the inmates that any interference with that plan will prejudice the
whole.
There are a few common principles which will guide in ventilation, and
these are, first, the rise of hot and the fall of cold air; second, that
if an aperture is provided at the top of a room for the escape of hot
air, an equally large aperture must be left for the entry of cold air;
third, the aperture for the escape of hot air must be adapted in size to
the number of persons likely to enter the room, and the number of gas or
other lights burning in it. During the daytime, moderate apertures for
the exit and entrance of air may suffice, but these must be largely
increased at night, when the room is filled with people and lighted up.
Expanding and contracting openings are therefore desirable, and they are
to be regulated by rules stated on the plan of the ventilating system
(already alluded to as being hung up in the hall) of the house which has
submitted itself to a perfect system of ventilation, and no hall-keeper,
footman, or butler should be allowed to remain in his post unless he
undertakes to comprehend the system and work it properly by the written
rules.
Dr. Angus Smith, in a very able paper "On the Air of Towns," says--"One
of the conditions of health, and a most important, if not the most
important of all, is to be found in the state of the atmosphere. As to
the effect on the inhabitants, the question becomes exceedingly
complicated; but the Registrar-General's returns are an unanswerable
reply as to the results of the lethal influences of the district. Few
people seem clearly to picture to themselves the meaning of a decimal
plan in the percentage of death, and few clearly see that there are
districts of England where the deaths at least in some years, and when
no recognised epidemic occurs, are three times greater than in others.
When we hear of the annual deaths in some districts being 3.4 per cent.,
and in the whole of England 2.2, it is simply that 34 die instead of 22,
whilst even that is too slightly stated, as the whole of England would
show a lower death-rate if the towns were not used to swell it."
This quotation is given here to remind our readers of the important
question of a supply of pure air as well as pure water and pure food;
and if the agricultural labourer, with all his exposure to variable
[Page 390] weather, can take the first place in the scale of mortality,
and outlive the members of all other trades and professions, it is
evident that the importance of pure air is not overrated.
Every effort ought, therefore, to be made in large schools, hospitals,
and barracks, to enforce a rigid system of supply of fresh air, and a
sewage or removal of the impure; and in the use of a certain test
employed by Dr. Smith for the detection of organic matter in the air a
number of approximations were obtained, which clearly demonstrated that
1 grain of organic matter was detected in 72,000 cubic inches of air in
a room, and the same quantity in 8000 cubic inches taken from a
_crowded_ railway carriage.
[Illustration: Fig. 371. A B. The glass tube. C. The spirit lamp, with a
very large wick; if a little ether is mixed with the spirit in the lamp
it increases the length of the flame. D. The effect of the ascension of
air, increased by warming the top of the tube with the lamp D.]
To show the rising of heated air, a long glass tube, about
three-quarters of an inch in diameter, may be provided and held over the
flame of a spirit lamp at an angle of sixty degrees. As the tube warms,
the heated air rushes past the flame with great rapidity, and pulls it
out or elongates it so much, that the sharp point of the spirit-flame
[Page 391] will frequently be seen at the end of a tube ten feet six
inches in length. The flame is, as it were, the sign-post that indicates
the path or direction of the air. (Fig. 371.)
Upon the like principle, heated air may be dragged down the short arm of
a syphon, provided the other arm is sufficiently long to impart a strong
directive tendency to the upward current, and this mode of setting air
in motion has been frequently proposed in numerous schemes for
ventilation. In order to prove the fact that an inverted syphon will act
in this manner, an iron pipe of three inches diameter and six feet long
may be bent round during the construction into the form of a syphon, so
that the short length is about one foot long, and the long length the
remaining four feet, allowing one foot for the bend. If the interior of
the long arm is first warmed by burning in it a little spirits of wine
from a piece of cotton or tow wetted with the latter (which can be
easily done by dropping in such a wetted piece into the bend of tube, so
that it is just under the opening of the long part of the tube), the air
is soon set in motion up the long pipe, and as it must be supplied with
fresh volumes of air to take the place of that which rises, and as the
only entrance for the fresh air can be _down_ the short arm of the
syphon, the circulation soon commences, and it proceeds as long as the
upper arm is kept sufficiently warm. If a flame is held over the mouth
of the short arm, it is immediately dragged downward, whilst, if held at
the mouth of the long pipe, the motion of the air is seen by the
assistance of the flame to be in the contrary direction. (Fig. 372.)
[Illustration: Fig. 372. A B. Inverted sheet iron syphon. At C is seen
the piece of tow moistened with alcohol, which, being set on fire, warms
the tube B. D. A lighted torch of coloured spirit, the flame of which is
dragged down the tube at A by the descending current, and is impelled
upwards by the ascending current B.]
This plan of ventilation was proposed to be used in rooms in connexion
with the chimney and chimney-piece, and in order to give it an
ornamental appearance, the chimney-piece was supplied with two
ornamental hollow columns, the ends of which were open at the
mantel-shelf, and the tubes or columns were continued under the
hearthstone, proceeding up the back of the grate and entering the
chimney, in which there would be a constant current of heated air, and
it was expected that the [Page 392] syphon arrangement would keep a
current of air always in motion, and thus help to ventilate the room.
(Fig. 373.) This plan, however, does not appear to have been adopted,
and wisely so, because half the time the syphon arrangement might invert
itself, and vomit smoky air out of the chimney into the room; indeed it
is surprising what odd and contradictory freaks are performed by
currents of air. The author remembers a case where two rooms on the same
floor, the one a dining-room and the other a drawing-room, were always
exhibiting the most absurd phenomena of smoke. If the fire in one room
was lit, then the other, in a few moments, began to smell exactly like
the inside of a gas manufactory, and was, of course, more or less filled
with smoke, whilst the room in which the fire was actually burning
remained quite free from this annoyance. The smoke appeared to issue
from the wainscot or moulding which runs round at the bottom of the
wall, and was at first thought to be an escape from the chimney of the
kitchen beneath, the inside of which was duly examined and thoroughly
stopped with cement in every place likely to afford a channel to the
smoke, and [Page 393] the crevice whence the smoke issued was also
filled in neatly with cement. But it was all in vain; the smoke then
made its way out from another part of the cornice, and at last the rooms
exhibited a beautiful reciprocating action. If the drawing-room fire was
lighted the dining-room was full of smoke, and if the latter was lighted
the former had the agreeable visitation. At last the backs of the two
grates were examined, and in each was discovered a hole about one inch
in diameter; and it was also found that the spaces at the back of the
stoves had not been filled in properly, and, indeed, communicated with
the hollow space behind the cornice. When, therefore, the fire was
lighted, and coals heaped on just above the hole, the gas and smoke
distilled through the orifice and travelled on, where it found the most
convenient exit; and the fact is sadly at variance (_apparently_) with
theory, because it might be considered that cold air would rush towards
a fire, and that the draught ought to have been from the cornice to the
chimney instead of _vice versâ_. The fact seems to be that the coal in
all grates is, in the act of burning, distilling and giving off
inflammable gas; when the coal was, therefore, heaped above the orifice,
and was, possibly, caked hard at the top, the gas distilling from it
escaped more easily from the little orifice than elsewhere, and chance
determined that the channel or delivery pipe should be in the direction
of the drawing-room when the fire was burning in the dining-room, and in
the contrary direction when the fire was lighted in the latter chamber.
The nuisance was stopped by plugging the holes at the back of the grate
with clay, and putting a sheet of iron over the orifice.
[Illustration: Fig. 373. A B. Chimney-piece supported on two hollow
ornamental pillars corresponding with the short arm of a syphon. C C C.
The dotted line showing the pipes leading from each pillar under the
hearth, and terminating in a long pipe passing into the chimney. The
arrows show the path of the air descending from the chimney-piece and
ascending in the chimney.]
Before Dr. Faraday was appointed as a scientific counsellor to assist
the deliberations of the Trinity Board in connexion with lighthouses,
all the lamps were burnt in the lanterns with the smallest and most
imperfect arrangement for carrying off the heated air and products of
combustion; as a natural consequence, and particularly on cold nights,
the windows of the lantern of the lighthouse were covered with ice
derived from the condensation of the water produced by the combustion of
the hydrogen of the oil, whilst the carbon generated such quantities of
carbonic acid that the light-keepers were unable to stay in the lantern,
and if obliged to visit the latter (whilst looking to improving the
light of any single lamp that might be burning dimly), they were almost
overpowered with the excess of carbonic acid, and stated, in their
evidence, that it produced headache and sickness, and a tendency to
insensibility. Faraday immediately established a system of ventilation;
and by attaching a copper tube to the top of each lamp-chimney, and
centering them all in one large funnel passing to the top of the
lighthouse, the whole of the water which previously condensed on the
glass windows and impeded the light, besides injuring the brass and
copper fittings, was carried off, as also the poisonous carbonic acid
gas; and thus, as Dr. Faraday expressed himself, a complete system of
sewage was applied to the lamps of the lighthouses.
If any one of the numerous stories of ships saved by the Eddystone
Lighthouse could demonstrate more than another the value of this beacon
[Page 394] in mid ocean, it must be the graphic account in the _Times_
of the gallant conduct of the British Admiral with his fleet whilst
breasting the frightful storm of October, 1859, and endeavouring to
reach Plymouth Sound:--
"It was on Saturday, the 22nd October, that the _Hero_, the _Trafalgar_,
the _Algiers_, and the _Aboukir_, accompanied by the _Mersey_, the
_Emerald_, and the _Melpomene_, put to sea from Queenstown. Up to the
afternoon of Monday the squadron met with no remarkable adventure, but
about that time, just after the crews had been exercised at gunnery
practice, heavy storms of hail and sleet began to set in. Still there
was no immediate indication of the tempest at hand, and at sunset
topsails were double-reefed and courses reefed for the night, with no
particular character about the wind, except that of extreme variability.
As the morning broke on Tuesday--the day of the storm--the Land's-end
was sighted, and the rain and the wind continued to increase. About nine
A.M. the advent of the gale was no longer doubtful; topgallantyards were
sent on deck and topgallantmasts struck, and the signal was given from
the flagship, 'Form two columns; form line of battle; Admiral will
endeavour to go to Plymouth.' To Plymouth, accordingly, the course of
the fleet was shaped, but so terrifically had the wind increased that it
became very questionable whether the sternmost ships of the line could
possibly succeed in entering the Sound. Upon this the Admiral determined
to wear the fleet together, stand off, and face the storm, a manoeuvre
which, under circumstances of great difficulty, was most gallantly
executed. The ships were close upon the Eddystone Lighthouse, round
which they 'darted like dolphins' under the tremendous pressure of the
gale, the _Trafalgar_ stopping in the midst of the storm to pick up a
man who had fallen overboard. The whole squadron now stood off the land,
the _Mersey_ and _Melpomene_ furling their sails, and the former vessel
steaming along 'like an ocean giant.' Still the gale increased till
about three P.M., when there occurred that remarkable phenomenon by
which these rotatory tempests are characterized. The fleet had got into
the very centre of the storm, the 'eye' of the tornado, and, though the
sea towered up and broke in tremendous billows all around, the wind
suddenly ceased and the sun shone. When, however, the signal had been
given and obeyed for setting sail again, the ships soon encountered the
gale once more--not, as before, from the S.E., but the N.W.--and in
greater force than ever. It was now a perfect hurricane; and for three
hours the whole fury of the tempest was poured upon the squadron. When
it began, at length, to abate a little, the four line-of-battle ships
and one of the frigates were still in company, and all doing well. The
_Mersey_ and the _Emerald_ had steamed into Plymouth, but the five
remaining vessels kept in open order throughout that terrible night,
wore in succession by night signal at about one A.M., made the land at
daylight, formed line of battle, came grandly up Channel under sail at
the rate of eleven knots an hour, steamed into Portland, and 'took up
their anchorage without the loss of a sail, a spar, or a ropeyarn.'"
After making the important improvement in the ventilation of
lighthouses, many letters were addressed to the learned philosopher
by [Page 395] numerous light-keepers, one of which in plain but
striking language related that "_the enemy_ (alluding to the water and
carbonic acid) _was now driven out_."
[Illustration: The British fleet rounding the Eddystone Lighthouse
during the great storm of October, 1859. _p. 394_]
The ingenious invention alluded to was succeeded by another and equally
simple but philosophical arrangement, which Dr. Faraday presented to his
brother, and it was duly patented. It consisted of an arrangement for
ventilating gas burners, and it must be obvious that a necessity exists
for such ventilation, because every cubic foot of coal gas when burnt
produces a little more than a cubic foot of carbonic acid. A pound
weight of ordinary coal gas contains about 3/10ths of its weight of
hydrogen, which when burnt produces two pounds and 7/10ths of a pound of
water. A pound of ordinary coal gas also contains about 7/10ths of its
weight of charcoal, which produces when burnt rather more than two and a
half pounds of carbonic acid gas--viz., 2.56. In order to burn this
quantity of gas nineteen cubic feet and 3/10ths of a foot of atmospheric
air, containing 4.26 cubic feet of oxygen, are required.
[Illustration: Fig. 374. A B. Gas pipe and argand burner; the air
enters, as usual, up the centre of the argand. C C. The first glass
chimney open at the top. D D. The second glass chimney closed at the
top, with a disc of double talc, and fitting over C C, and leaving a
space between the two glasses, down which the air passes, and into the
ventilating tube, E E. H H. The ground-glass globe closed at the top,
and surrounding the whole.[I]]
[Footnote I: Mr. Faraday, of Wardour-street, supplies this ventilating
lamp.]
It is not therefore surprising that as common coal gas is sometimes
purified carelessly, and contains a minute trace of sulphuretted
hydrogen, with some bisulphide of carbon vapour, that it should produce
the most prejudicial effects in badly ventilated rooms, and especially
in some of those perched up glass boxes in large places of business,
where clerks are obliged to sit for many consecutive hours, lighted by
gas, and breathing their own breath and the products of combustion from
the gas light, thereby rendering themselves liable to diseases of the
lungs, and also to very troublesome throat attacks, when leaving their
close glass boxes, and passing into the cold night air. The dangerous
product of the combustion of ordinary coal gas is sulphurous acid--viz.,
[Page 396] the same gas as that generated when a sulphur match is
burnt; and if it will attack the bindings of books, and damage
furniture, goods in shops, curtains, &c., in consequence of the large
quantity of water with which it is accompanied, how much more is it not
likely to injure the delicate organism of the breathing apparatus of the
lungs? Dr. Faraday's lamp is therefore a great boon, but, like a great
many other clever things, it must be adapted to the currents of air and
draught from the room; and means must be taken to prevent the draught
becoming too powerful in Faraday's lamp, or else the illuminating power
is destroyed by the thorough combustion of the carbon of the coal gas,
and the heat generated is so intense that the glasses soon crack, and of
course become useless. The lamp will answer very well if (as has been
already stated) the draught in the ventilating pipe is not too great.
[Illustration: Fig. 375. Section showing the two air-shafts. A. The
downcast. B. The upcast. C C. One of the working galleries in connexion
with the upcast and downcast. D. The furnace at the bottom of the
_upcast_. In this sketch _one_ gallery only has been shown, to prevent
confusion and to show the principle.]
The system already explained and illustrated is likewise carried out on
a much larger scale in the ventilation of coal pits, where a shaft is
usually sunk into the ground for the admission of air, which, after
circulating through the intricate windings and mazes of the coal pit
workings, escapes at last from another shaft, at the bottom of which is
placed a powerful furnace, and this is kept burning night and day, so
[Page 397] that the movement of the air is maintained in one
direction--viz., from the outer air down the shaft called _the
downcast_, thence to the galleries, where the coal hewers are working,
to the second shaft, near which the furnace is placed, and up this
latter the air travels; the shaft, pit, or funnel being very
appropriately termed the _upcast_.
Should the furnace at the bottom of the upcast be neglected, the
ventilation may be just balanced, or set slightly towards the downcast;
under these circumstances the carbonic acid from the fire will begin to
circulate in the galleries, and poison those who are not aware of its
presence and take the proper means to escape. Such accidents, amongst
the host of others that occur in a coal pit, have actually been
recorded; and the firemen, whose duty it might be to attend to the
proper burning of the furnace, have had to pay the penalty of death for
their own carelessness in falling asleep and neglecting to maintain the
ventilation of the mine in one direction. (Fig. 375.)
These details are amply sufficient to demonstrate the manner in which
heat is diffused through air, whilst the rarefication of the air by heat
suggests the cause of those frightful storms of wind that rush from
other and colder parts of the surface of the globe, to supply the void
produced by the cooling and contraction of the enormous volumes of
gaseous matter.
_The Radiation of Heat._
When rays of heat are emitted from incandescent matter, they are not
necessarily visible, nay, they are generally invisible, and not
accompanied with a manifestation of light, and pass with great velocity
through a void or vacuum, also through air and certain other bodies.
From what has been stated respecting the manner in which air, by
continually moving, and by convection, carries off heat, it might be
thought that no proof existed that invisible rays of heat are really
thrown off from a ball filled with boiling water. But this question is
set at rest by the fact, that such a ball will cool rapidly when
suspended by a string inside the receiver of an air pump from which the
atmospheric air has been removed, so that no conduction of the particles
of air could possibly remove the heat.
In the year 1786, Colonel Sir B. Thompson examined the relative
conducting powers of air and a Torricellian vacuum--the latter being
used because, as the experimenter stated, it was impossible to obtain a
perfect vacuum, on account of the moist vapour which exhaled from the
wet leather and the oil used in the machine, for at that time carefully
_ground_ brass plates were not used in air-pumps, but plates only, with
a circular piece of wet leather upon them. In a paper which Colonel Sir
B. Thompson read before the Royal Society, he stated that "It appears
that the Torricellian vacuum, which affords so ready a passage to the
electric fluid, so far from being a good conductor of heat, is a much
worse one than common air, which of itself is reckoned among the worst;
for when the bulb of the thermometer was surrounded with air, and the
instrument was plunged into boiling water, the mercury rose from 18° to
27° [Page 398] in forty-five seconds; but in the former experiment,
when it was surrounded by a Torricellian vacuum, it required to remain
in the boiling water one minute thirty seconds to acquire that degree of
heat. In the vacuum it required five minutes to rise to 48°-2/10ths; but
in air it rose to that height in two minutes forty seconds; and the
proportion of the times in the other observation was nearly the same.
"It appears, from other experiments, that the conducting power of air to
that of the Torricellian vacuum, under the circumstances described, is
as 1000 to 702 nearly, for the quantities of heat communicated being
equal, the intensity of the communication is as the times inversely. By
others it appears that the conducting power of air is to that of the
Torricellian vacuum as 1000 to 603."
[Illustration: Fig. 376. The air-pump and receiver, containing at A the
electric light in the focus of a concave mirror, and at B a delicate
thermometer, also in the focus of a concave mirror.]
It is therefore very interesting to discover that the attention of
experimentalists was early directed to the fact that heat was
independent of the air, and passed either as waves of heat or molecules
of heat through space. The velocity with which heat moves through a
vacuum is very great, and in an experiment performed by M. Pictet, no
perceptible interval took place between the time at which caloric
quitted a heated body and its reception by a thermometer at a distance
of sixty-nine feet. It appears also, from the experiments of the same
philosopher, to be thrown off or radiated in every direction, and not to
be diverted (as shown at p. 369) by any strong current of air passing it
transversely. Sir Humphrey Davy ignited the charcoal points connected
with a battery in a vacuum, taking care to place the charcoal points at
the top of the jar, and a concave mirror, with a delicate thermometer in
its focus, at the bottom of the vessel placed upon the air-pump plate.
The effect of radiation was [Page 399] ascertained first when the
receiver was full of air, and next when it was exhausted to 1/120th
(_i.e._, 199 parts pumped out, leaving only one part of air in the
receiver). In the latter case, the effect of radiation was found to be
three times greater than in an atmosphere of the common density. The
greater rise of the thermometer _in vacuo_ than in air is to be ascribed
to the conducting power of the latter; for this conducting power, by
reducing the temperature of the heated body, has a constant tendency to
diminish the activity of radiation, which is always proportional to the
excess of the temperature of the heated body above that of the
surrounding medium. (Fig. 376.)
Count Rumford's experiments with a Torricellian vacuum gives the
proportion of five _in vacuo_ to three in air for the quantities of heat
lost by radiation, and by conduction or diffusion. It is not, perhaps,
departing very far from the truth, if it be stated that one half of the
heat lost by a heated body escapes by radiation, and that the rest is
carried off by the convective power of currents of air.
[Illustration: Fig. 377. Negretti and Zambra's terrestrial radiation
thermometer. The bulb of this instrument is transparent, and the
divisions engraved on its glass stem. In use it is placed with its bulb
fully exposed to the sky, resting on grass, with its stem supported by
little forks of wood, and protected from the wind.]
If the process of radiation was not constantly proceeding, it can easily
be imagined that the temperature of our globe would become so elevated
by the regular accession of heat from the sun's rays, that the
vegetation would be parched up and destroyed, and consequently all
animals and the human race must become extinct. The best time to notice
the radiation of heat from the earth is at night and after a hot
summer's day. If the sky is clear, it will be noticed (with the help of
a thermometer,) that the ground is several degrees colder than the air a
few feet above it. (Fig. 377.) It is this reduced temperature that
causes the deposition of dew, and produces the earth-cloud which so
nearly resembles a sheet of water as to have been occasionally mistaken
for an inundation, the occurrence of the previous night. Mr. Luke Howard
has called this cloud, which is the lowest form of these draperies of
the sky, "The Stratus," or evening mist; but when permanent, and
increased to a depth so as to rise above our heads, it is then called
the morning fog, so peculiarly agreeable in London when incorporated
with the black smoke, making a fine reddish-yellow ochreous mist. By
placing a thermometer, standing at the ordinary temperature of the air,
cased [Page 400] with a good radiating material, such as filaments of
cotton, in the focus of a concave mirror, and by turning this
arrangement towards a clear sky in the evening, it will be noticed that
the temperature falls several degrees. Good radiators of heat are black
and scratched surfaces, filaments of cotton, grass, twigs, boughs, and
certain leaves, especially those with a rough surface.
Bad radiators of heat are bright and polished metallic surfaces, white
woollen cloth or flannel, hard and dense substances, such as a gravel
path and stone, or those leaves which have a polished surface, such as
the common laurel. It is the frozen dew and mist which produce the
beautiful effect of hoar-frost and icicles on the trees and bushes, the
primary cause being the radiation of heat from the various objects on
the surface of the earth, as well as from the latter itself. When the
wind is high, dew does not deposit, as it is necessary that the air
should be calm, in order to receive the cooling impression of the cold
earth, and to deposit the moisture, which it holds in solution as
invisible steam. When the wind blows, it mixes all parts of the air
together, and prevents that difference of temperature which causes the
deposit of dew. Hence the evening mist will be more generally observed
in the bosom of a valley surrounded by hills and screened from the winds
that may blow from either quarter. The continual presence of moisture in
the air is well shown by the condensation of water on the outside of a
glass of cold spring water, or especially on the outside of a jug
containing iced water. The invisible steam is always ready to bathe the
tender plants with dew, which would otherwise perish and be burnt up
during a hot summer, if they did not radiate heat at night, and thus
condense water upon themselves. The presence of watery vapour in the air
becomes therefore a matter of great importance, and hence the
construction of hygrometers or measurers of the moisture in the air.
Regnault's condenser hygrometer consists of a tube made of silver, very
thin, and perfectly polished; the tube is larger at one end than the
other, the large part being 1.8 in depth by 8.10 in diameter. This is
fitted tightly to a brass stand, with a telescopic arrangement for
adjusting when making an observation. The tube has a small lateral
tubulure, to which is attached an India-rubber tube with ivory
mouthpiece; this tubulure enters at right angles near the top, and
traverses it to the bottom of largest part. A delicate thermometer is
inserted in through a cork, or India-rubber washer, at the open end of
the tube, the bulb of which descends to the centre of its largest part.
A thermometer is attached for taking the temperature of the air; also a
bottle for containing ether.
To use the condenser hygrometer, a sufficient quantity of sulphuric
ether is poured into the silver tube to cover the thermometer bulb. On
allowing air to pass bubble by bubble through the ether, by breathing in
the tube, an uniform temperature will be obtained; if the ether
continues to be agitated by breathing briskly through the tube, a rapid
reduction of temperature will be the result. At the moment the ether is
cooled down to the dew-point temperature, the external surface of that
portion [Page 401] of the silver tube containing the ether will become
covered with a coating of moisture, and the degree shown by the
thermometer at that instant will be the temperature of the dew-point.
The most simple form of the hygrometer was formerly a very favourite
indicator of the state of the weather, and usually consisted of the
figure of a monk with his hood, which is attached to a bit of catgut;
this covering of paper, painted to represent the hood, falls over the
head on the approach of damp weather, and inclines well back during the
period that the air is dry or contains less moisture; and simple as it
is, this hygrometer, in conjunction with the reading of the barometer,
may assist _Paterfamilias_ in deciding the fate of a pet bonnet or
velvet mantle, which is or is not to be worn on a doubtful day. (Fig.
378.)
[Illustration: Fig. 378. The monk hygroscope, in which the hood, A B,
covers the head to dotted line C in wet weather, and takes various
intermediate positions, being quite back and on the shoulders in dry
states of the air. A thermometer, D, is usually attached.]
A decision on the possible changes of the weather requires considerable
experience, and it has been said that one of the most celebrated
marshals of France owed his invariable success in military combinations
and attacks to his attention to the signs of the weather, as indicated
by the state of the air during the phases of the moon. Inexperienced
persons (and by that we mean young persons) may, however, take a certain
position in the rank of "weather prophets" by consulting the
weathercock, the barometer, and the hygrometer, before committing
themselves to an opinion, if asked to say what the weather will be.
The dry and wet bulb hygrometer (as represented in the next engraving)
consists of two parallel thermometers, as nearly identical as possible,
mounted on a wooden bracket, one marked _dry_, the other _wet_. The bulb
of the wet thermometer is covered with thin muslin, round the [Page
402] neck of which is twisted a conducting thread of lamp-wick, or
common darning-cotton; this passes into a vessel of water, placed at
such a distance as to allow a length of conducting thread of about three
inches; the cup or glass is placed on one side, and a little beneath, so
that the water within may not affect the reading of the _dry bulb
thermometer_. In observing, the eye should be placed on a level with the
top of the mercury in the tube, and the observer should refrain from
breathing whilst taking an observation. The temperature of the air and
of evaporation is given by the readings of the _two thermometers_, from
which can be calculated the dew-point, tables being furnished for that
purpose with the instrument. (Fig. 379.)
[Illustration: Fig. 379. The dry and wet bulb hygrometer.]
The colour of the sky at particular times affords the most excellent
guidance to doubting members of pic-nic or other out-of-door parties.
Not only does a rosy sunset presage fine weather, and a ruddy sunrise
bad weather, but there are other tints which speak with equal clearness
and accuracy. A bright yellow sky in the evening indicates wind; a pale
yellow, wet; a neutral grey colour constitutes a favourable sign in the
evening, an unfavourable one in the morning. The clouds, again, are full
of meaning in themselves. If their forms are soft, undefined, and
feathery, the weather will be fine; if their edges are hard, sharp, and
defined, it will be foul. Generally speaking, any deep, unusual hues
betoken wind or rain, while the more quiet and delicate tints bespeak
fine weather.
The principle of radiation of heat is employed by the Indian natives in
the neighbourhood of Calcutta for the purpose of obtaining small [Page
403] quantities of ice. In that climate, the thermometer during the
coldest nights does not indicate a lower temperature than about 40°
Fahr. The sky, however, is perfectly cloudless, and as heat radiates
with great rapidity from the surface of the ground, the Indian natives
ingeniously place very shallow earthenware pans on straw, which is a bad
conductor of heat, and hence insulates the pans from communication with
the parched earth. In a few hours, the water in the pans is covered with
a thin sheet of ice, and there can be no doubt of its production by an
absolute loss of heat by radiation, because the plan does not succeed on
a windy night, and succeeds best even when the pans are sunk in trenches
dug in the earth. A windy night prevents that difference of temperature
between one portion of the surface of the earth and another, which is so
essential to a steady and uniform loss of heat, as it must be evident
that the continual mixture of warmer portions of air with that which is
colder would tend to prevent the desired lowness of temperature being
attained.
The manner in which heat is observed to be radiated has suggested
another theory to the fertile brain of philosophical observers, and it
has been supposed that the conduction of heat may be nothing more than a
radiation from one particle of matter to another, as through a bar of
copper, in which the particles, though packed closely together, are not
supposed to be in actual contact, so that it is possible to conceive
each separate atom of copper receiving and radiating its heat to the
neighbouring particle, and so on throughout the length and breadth of
the metal. By this theory the radiation of heat through a vacuum is
brought into close connexion with that of the radiation of heat through
the air and other solid and liquid bodies.
Some of the most interesting phenomena of heat are those discovered by
Leslie, who has proved in a very satisfactory manner that the rapidity
with which a body cools, depends (like the reflection of light) more on
the condition of the surface than on the nature of the material of which
the surface is composed. With a globular and bright tin vessel it was
observed that water of a certain heat contained in it, required 156
minutes to cool; but when the latter vessel was covered with a thin
coating of lamp-black and size, the water fell to the same degree as
that noticed in the first experiment in the space of eighty-one minutes.
By very careful observations made with a differential air thermometer,
Leslie determined that the power of radiating heat in various substances
was as follows:--
Lamp-black 100
Writing paper 98
Sealing wax 95
Crown glass 90
Plumbago 75
Tarnished lead 45
Clean lead 19
Iron, polished 15
Tin plate 12
Gold 12
Silver 12
Copper 12
[Page 404]
As in the reflection of light, it was noticed that a piece of charcoal
covered with gold leaf, partook of the nature of the precious metal so
far as its power of throwing off or scattering the rays of light was
concerned, so a piece of glass covered with gold-leaf appears to possess
the same power of radiating heat as that of any brilliant metal.
Radiant heat, like light, can be propagated through a great variety of
substances, but is stopped by the larger number; and it can be
reflected, refracted, polarized, absorbed, or it may undergo a secondary
radiation.
The intensity of radiant heat follows the same law as that of light, and
decreases as the square of the distance from its source. The same law
that governs the reflection of light, also prevails with that of heat;
and it may be found by experiment that the angle of incidence is equal
to the angle of reflection, so that the heat is disposed of in the same
manner as light when it falls upon bright polished planes, convex and
concave surfaces; hence the use of bright tin meat screens and Dutch
ovens, and of all those simple pieces of culinary furniture which are
employed in the kitchen for the purpose of arresting the cold currents
of air that set towards burning matter, as also to reflect the heat upon
whatever viands may be cooking before the fire. A bright silver teapot
retains its heat better than a dirty one, and the fact is determined
very readily by pouring boiling water into two teapots, the one being
made of bright tin and the other of black japanned tin. A thermometer
inserted into each vessel will soon show that the latter radiates, and
therefore loses its heat quicker than the former; the relative radiating
powers of bright and blackened tin being as 15 to 100. Pipes for the
conveyance of hot water or steam should be kept bright, if possible,
although this trouble is avoided usually by packing them in bad
conductors of heat, whilst the polish of the cylinder of a steam-engine
is of great importance as a means of economizing heat.
[Illustration: Fig. 380. A B. The cone of paper, gilt inside. C. The
red-hot ball. D. Stand with wood supporting a slice of phosphorus, which
is brought into the focus of the rays of heat reflected through the
cone.]
When the finger is approached within an inch or so of a red-hot ball,
the heat radiated from the latter is so intense that it cannot be held
there [Page 405] for more than a few seconds. If, however, the finger
is coated with gold leaf it may be kept near the iron ball for some
considerable time, because the radiant heat is reflected from the
surface of the gold. If the word heat is written upon a sheet of paper
and the letters afterwards gilt, the whole of the white surface is
rapidly toasted and scorched when held before a fire, whilst the surface
of the paper under the gold leaf remains perfectly white, which can be
ascertained by turning the paper round and observing the other side. A
sheet of paper gilt inside and turned round as a cone, being left open
at both ends, may be employed as a reflecting surface; and if a bit of
phosphorus, placed on paper, is held, say at two feet from a red-hot
ball of about two inches diameter, the radial heat from the latter has
not sufficient intensity at that distance to set it on fire quickly; if,
however, the cone of gilt paper is used between the two, and the
phosphorus brought into the focus of the rays of radial heat, it very
quickly takes fire. (Fig. 380.)
Dr. Bache has determined by experiments that the radiation of heat from
a body is not affected by colour, so that in winter all coloured clothes
are alike in that respect, and radiate heat without any appreciable
difference. The power of _absorbing_ heat, however, is greatly dependent
on colour; and as a general rule, good radiators of heat (such as a
black cloth, or indeed any surface covered with lamp-black), are also
excellent absorbents of heat. Dr. Hooke and Dr. Franklin placed pieces
of cloth of similar texture and size on snow, allowing the sun's rays to
fall equally upon them. The dark specimen always absorbed more heat than
the light ones, and the snow beneath them melted to a greater extent
than under the others; and they both remarked that the effect was nearly
in proportion to the depth of the shade, as in the following
order:--After black, the maximum absorbent quality was possessed by,
first, blue; second, green; third, purple; fourth, red; fifth, yellow.
The minimum absorbent power was observed to belong to white.
When radiant heat is allowed to pass through glass, the latter substance
is not found to be transparent to heat rays as it is to those of light,
but a considerable proportion of heat is arrested and stopped;
consequently glass fire-screens are to be found in the mansions of the
wealthy, because they obstruct the heat but do not exclude the cheerful
light and blaze of the fireside.
Melloni's researches on the nature of the rays of heat, and also on the
media which affect them, would demand and merit a chapter to themselves;
want of space, however, obliges us to omit the consideration of
thermo-electricity, and the refined and beautiful experiments of
Melloni, whose labours are a model for the imitation of all original
seekers after truth.
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