G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

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the contrary, should be made of a good radiator, so that the heat of the fire
may be freely diffused. Cast-iron is better for this purpose than sheet-iron,
because its surface is rough ; the radiating power of both is increased by
rubbing in black lead. When heat is to be conveyed from one room to an-
other, a pipe should be used of bright tin, which is a bad radiator and pre-
vents the escape of heat by the way.

The atmosphere receives its heat, not directly from the sun, but by radia-
tion from the earth ; hence, as we ascend from the earth's surface, the heat
diminishes.

510. Law of Radiant Heat. Radiant heat diminishes
in intensity as the square of the distance from the radiating
body increases.

A body 10 feet from a fire will receive from it only Vioo of the heat that a
body 1 foot from it receives.

511. Radiant heat, striking different bodies, is reflected



by radiation? What is heat diffused by radiation called? Give a familiar example
of radiant heat. 508. By what is a body's radiating power affected ? What surfaces
radiate heat the best? What is the best known radiator? Rating the radiating
power of lamp-black at 100, what is that of crown-glass ? Black lead? Tarnished
lead ? Clean lead ? Bright metals generally ? How may the radiating power of the
metals be increased ? 509. Why is bright silver preferable to earthen-ware for tea-
pots ? Of what should stoves be made ? When heat is to be conveyed from one room
to another, what should be employed ? Why ? How does the atmosphere receive*
fts heat' What follows? 510. State the law of radiant heat. Give an example.



DIFFUSION OF HEAT. 205

by some, absorbed by others, and transmitted by a third
class.

512. Reflection of Radiant Heat. Radiant heat is re-
flected by polished and light-colored surfaces. Polished
gold reflects about three-fourths of the radiant heat it re-
ceives, and looking-glass about one-fifth ; whereas metallic
surfaces blackened reflect only one-twentieth.

513. White and light-colored clothes are worn in summer, because they
reflect heat. For the same reason, it is harder to heat water in a new tin ves-
sel than in one that has been blackened over the fire.

514. The reflection of radiant heat may be illustrated with the apparatus
represented in Fig. 213. A and B are concave metallic mirrors, highly pol-

Fig. 213.




ished. In the focus of A is placed a red-hot ball C. This ball radiates hea?
in all directions, and some of its rays strike the mirror A, from which they
are reflected in parallel lines to B. By B they are again reflected and brought
to a focus at D, where a thermometer indicates a rise of temperature. Suffi-
cient heat may thus be concentrated at D to set fire to phosphorus or gun-
powder.

515. When radiant heat is reflected by a plane surface,
the angle of reflection (see 96) is always equal to the an-
gle of incidence. If it strikes the surface perpendicularly,
it is reflected perpendicularly, back to the radiating body.
If the line in which it approaches the surface forms an angle

511. "When radiaut heat strikes different bodies, what becomes of it? 512. By what
surfaces is radiant heat reflected ? What portion does polished gold reflect ? Look-
ing-glass ? Metallic surfaces blackened ? 513. Why are light-colored clothes worn in
summer? In what sort of a vessel is it hardest to heat water? 514 Illustrate the
reflection of radiant heat with Fig. 213. How much heat may be concentrated with
this apparatus ? 515, When radiant heat is reflected, to what is the angle of reflection



206 PYHONOMICS.

with the perpendicular, it glances off at an equal angle on
the other side.

516. Absorption of Radiant Heat. Radiant heat is
absorbed by dull and dark-colored surfaces. Good reflec-
tors are bad absorbents and radiators ; bad reflectors are
good absorbents and radiators.

Of the colors, black is the best absorbent of heat, and
violet the next best ; white is the worst, and yellow next to
the worst.

Lay two pieces of cloth, one white and the other black, on a snow-bank,
in the sunshine. Under the black piece, which absorbs the heat that strikes
it, the snow melts rapidly ; not so under the white cloth, for by it the heat is
reflected. Dark-colored clothing is therefore best adapted to winter.

Dark mould absorbs the sun's heat; hence one cause of its fertility.
White sand reflects the hot rays ; hence it burns our faces when we walk
over it in summer. Hoar-frost remains longer in the morning on light than
dark substances : this is because light colors reflect the sun's heat, while
dark colors absorb it, and thus melt the hoar-frost, which is nothing more
than frozen dew.

517. Transmission of Radiant Heat. Transparent sub-
stances, or such as allow light to pass through them, for
the most part transmit heat also. The sun's rays, for in-
stance, falling on the atmosphere of the earth, which is a
transparent medium, are transmitted through it to objects
on the surface. More or less heat is absorbed in the act
of transmission.

518. Substances that transmit heat freely are called Di-
a-ther'-ma T nous. Those that absorb the greater part and
transmit little or none are called A-ther '-ma-no us.

519. All transparent substances are not diathermanous. Water, for ex-
ample, which offers but little obstruction to rays of light, intercepts nearly
all the heat that strikes it. Alum is another instance in point.



equal ? 516. By what surfaces is radiant heat absorbed ? What is said of good reflect-
ors? What, of bad reflectors? What color is the best absorbent of heat? "What, tho
next best? What color is tho worst absorbent? What, the next worst? Prove by
nn experiment tho difference in absorbing power between white and black. Why is
dark-colored clothing best adapted to winter ? What is the difference between dark
mould and white sand in absorbing power? Why does hoar-frost remain longer in
the morning on light than dark substances ? 517. What substances, for the most part,
transmit heat? Give an example. 518. What are Diathermanous substances? What
are Athermanous substances? 519. Name a transparent substance that is not dia-



EFFECTS OF HEAT. 207

All diathermanous substances are not transparent. Quartz, though it may
Intercept light almost entirely, transmits heat quite freely.

As a general rule, the rarer transparent substances, such as gases and
vapors, transmit heat the best ; the denser ones, such as rock-crystal, trans-
mit it the least freely. The farther the rays have to pass through a given
substance, the more heat is intercepted.

Efect of Meat.

520. The effects of heat are five in number: Expansion,
which changes the size of bodies ; Liquefaction and Vapor-
ization, which change their form ; Incandescence, which
changes their color ; and Combustion, which changes their
nature.

521. EXPANSION. Heat expands bodies.

Insinuating itself between the particles of bodies, it forces them asunder,
and thus makes them occupy a greater space. Heat, therefore, opposes co-
hesion. Solids, in which cohesion is strongest, expand the least under the
influence of heat ; liquids, having less cohesion, expand more ; gases and
vapors, in which cohesion is entirely wanting, expand the most. Heat con-
verts solids into liquids, and liquids into gases and vapors, by weakening
their cohesion. It turns ice, for example, into water, and water into steam.

522. Expansion of Solids. All solids except clay are
expanded by heat ; but not equally. Of the metals, tin is
among those that expand most. Clay is contracted by bak-
ing, and ever afterwards remains so ; this is supposed to be
owing to a chemical change produced in it by heat.

The expansion of solids is illustrated with the apparatus represented in
Fig. 214. A brass ball is suspended from a pillar, to which is also at-
tached a ring just large enough to let the ball pass through it at ordinary
temperatures. Heat the. ball with a lamp placed beneath, and it will ex-
pand to such a degree that it can not pass through the ring. Let it cool, and
it will go through as before.

523. A sheet-iron stove in which a hot fire is quickly kindled or put
out, sometimes makes a cracking noise, in consequence of the rapid ex-

thermanons. Name a diathermanous substance that is not transparent. As a gen-
eral rule, what transparent substances transmit heat the best, and what the worst?
520. State the effects of heat. 521. What is the first of these ? How is it that heat
expands bodies ? What force does it oppose? Which expand the most under the
influence of heat, solids, liquids, or gases, and why ? Into what does heat convert
solids ? Into what, liquids ? 522. What solids are expanded by heat ? What metal
is expanded more than most of the others ? What is the effect of heat on clay ? Il-
lustrate the expansion of solids with the apparatus represented in Fig. 214. 523. Why



208



PYKONOMICS.




Fig. 214. pansion or contraction of the metal. A blower

placed on or taken from a hot fire produces a sim-
ilar noise for the same reason. New furniture
standing in the sun or near a fire is apt to warp
and crack in consequence of the expansive effects
of heat.

When boiling water is poured into china cups
and glass vessels, they often crack. This is be-
cause the inner surface is expanded by heat
while the outer is not, china-ware and glass be-
ing bad conductors. The unequal expansion
cracks the vessel. Cold water poured on a hot
glass or stove produces the same effect. On the
same principle, glass chimneys are apt to crack,
when brought too suddenly over the flame of a
lamp or gas-burner. A cut made in the bottom
with a diamond allows an opportunity for expan-
sion, and prevents the chimney from breaking.

When a glass stopper becomes fastened in a bottle, it may often be with-
drawn by placing the neck of the bottle in warm water. The neck is ex-
panded before the heat reaches the stopper.

524. The force with which a body expands when heat-
ed and contracts when cooling, is very great. In iron
bridges, therefore, and other structures in which long bars
of metal are employed, there is danger of the parts' sep-
arating, unless provision is made for the expansion caused
by a rise of temperature. The middle arch of an iron
bridge has been known to rise an inch in the summer of a
temperate climate. So, when great lengths of iron pipe
are laid for conveying steam or hot water, sliding joints
must be used, or the apparatus will burst in consequence
of the expansion of the metal.

525. The fact that heat expands bodies and cold contracts them, is often
turned to practical account. Coopers, for instance, heat their iron hoops,
and while they are thus expanded put them on casks which they just
fit. As they cool, they contract and bind the staves tightly together. The



do a sheet-iron stove and a blower sometimes make a cracking noise? What causes
new furniture to warp ? What makes glass vessels crack when boiling water is poured
into them? When are glass chimneys apt to crack? IIow may their cracking be
prevented ? When a glass stopper becomes fastened in a bottle, how may it be with-
drawn ? 524 What is said of the force with which bodies expand and contract ?
What precautions must be taken in consequence ? 525. What practical use is made
f the fact that heat expands bodies and cold contracts them ? What ingenious appU-



EXPANSION. 209

wheel-wright fastens the tire, or outer rim of iron, on his wheel in the
same way.

The contraction of iron, when cooling, has been ingeniously used for
drawing together the walls of buildings that have bulged out and threaten
to fall. Several holes are made opposite to each other in the walls, into
which are introduced stout bars of iron, projecting on both sides and termi-
nating at each end in a screw. To each screw a nut is fitted. The bars are
then heated by lamps placed beneath, and when they have expanded the
nuts are screwed up close to the walls. As the bars cool, they gradually con-
tract, and with such force as to bring the walls back to a perpendicular po-
sition.

526. Expansion of Liquids. Liquids, when heated,
expand much more than solids, but not all alike. Thus
water, raised from its freezing-point to the temperature at
which it boils, has its bulk increased one-twenty-second ;
alcohol, between the same limits, increases one-ninth.

The higher the temperature, the greater the rate at
which liquids expand.

527. In proportion as heat expands liquids, it rarefies
them, the same quantity of matter being made to occupy
a larger space. This fact is shown in the process of boil-
ing, described in 501.

528. Water at certain temperatures forms a remarkable
exception to the general law that liquids are expanded by
heat and contracted by cold. As it cools down from the
boiling-point, it contracts, and consequently increases in
density, till it reaches 39 degrees, or 7 degrees above its
freezing-point. Below this temperature, it expands.

The expansion of water in freezing is proved every winter by the burst-
ing of pipes, pitchers, &c., containing it. The force with which it expands
is tremendous. An iron plug weighing three pounds and closing a bomb-
shell filled with water, has been thrown 15 feet by the freezing and expansion
of the liquid within. Immense masses of rock are sometimes split off by the
freezing of water which has insinuated itself into minute fissures.

The expansion and consequent rarefaction of water in freezing, afford a

cation has been made of the contraction of iron when cooling? Give an account of
the process. 526. How does the expansion of heated liquids compare with that of
solids? Compare the expansion of water with that of alcohol. On what does the rate
at which liquids expand depend ? 527. Besides expanding liquids, what does heat do
to them? 52S. What exception is there to the law that liquids are contracted by
cold ? How is the expansion of water in freezing proved ? What cases are cited, to
how the great force with which water expands in freezing ? How does the expansion



210 PYRONOMICS.

striking proof of the goodness of Providence. The great body of a large
mass of water never becomes cold enough to freeze ; it freezes only oil the
.top, where it comes in contact with very cold air. As it is, the ice formed
on the surface remains there on account of its superior rarity, and protects
the water below and the fish that inhabit it from further cold. If water con-
tinued to contract and increase in density as it approached the freezing-point,
the ice first formed would sink ; the fresh surface exposed to the air would
in its turn freeze, and another layer of ice would sink ; and this would go on
till even in a mild winter every body of water would be converted into a solid
mass, and all living things therein destroyed.

529. Iron, zinc, and several other metals, when cooling down from a melt-
ed to a solid state, expand like freezing water. This is because the particles
assume a crystalline arrangement, by which greater interstices are left be-
tween them.

530. Expansion of Gases and Vapors. Aeriform bodies
expand equally under a given increase of temperature. At
the boiling-point of water, their bulk is one-third greater
than at the freezing-point.

531. Fill a bladder with air, tie its neck, and place it before a fire ; the
heat will soon expand the confined air to such a degree as to burst the
bladder.

The popping of grains of corn, the bursting open of chestnuts when
roasting, and the crackling of burning wood, are caused by the expansion
of the air within them. Porter-bottles have to be kept in a cool place in
summer, lest the heat expand the carbonic acid in the porter and break the
bottles.

532. LIQUEFACTION. Heat melts solids. This process
is called Liquefaction.

Some solids, such as wax and butter, require but little heat to melt them.
Others, like metals and stones, melt only at the highest temperatures that
can be produced. Such substances are called refractory.

Even substances that are liquid at ordinary temperatures may be looked
upon as melted solids, for they can be reduced by cold to the solid state.

533. When a solid is converted into a liquid, sensible
heat is absorbed. When a liquid is converted into a solid,

of water in freezing exhibit the goodness of Providence ? 529. How do we account
for the expansion of several of the metals, when cooling down from a melted state ?
530. "What is said of the expansion of aeriform bodies ? How great is their expansion,
when they are raised from the freezing-point to the boilinq-point of water ? 531. HOMT
may we illustrate the expansion of air by heat with a bladder ? "What familiar exam-
ples are given of the expansion of air by heat? 532. What is Liquefaction ? What
difference is there in solids, as regards their capability of being melted ? How may
substances that are liquid at ordinary temperatures be looked upon? 533. By what



VAPORIZATION. 211

latent heat is given out. This is another merciful provision,
for thus extremes of temperature and their effects are mod-
ified.

"When a solid is rapidly melted, so much heat is absorbed by the liquid
that intense cold is produced. This is the principle on which freezing mix-
tures operate. Ice cream, for instance, is frozen with a mixture of salt and
snow or pounded ice ; the latter is rapidly melted, and so much heat is ab-
sorbed in the process that the cream is brought to a solid form.

534. VAPORIZATION. Heat converts liquids into vapors.
This process is called Vaporization.

Heat, applied to a solid, first expands it, then melts it, and finally turns it
into vapor. Some solids pass at once into vapor, without becoming liquids.

535. A great degree of heat is not essential to vapori-
zation. At ordinary temperatures, wherever a surface of
water is in contact with the air, vapor is formed. This pro-
cess is known as Spontaneous Evaporation. By its means
the atmosphere becomes charged with moisture, and clouds
and dew are formed. The drier the air, and the more it is
agitated, so as to bring fresh currents in contact with the
liquid, the more rapidly does evaporation take place.

536. A drop of water let fall on a cold iron moistens its surface; let fall
on a very hot iron, it hisses and runs off without leaving any trace of moist-
ure. In the latter case, the water does not touch the iron at all, but is sep-
arated from it by a thin layer of vapor into which part of the drop is con-
verted by the heat radiated from the iron. Laundresses try their irons in
this way, to see if they are hot enough for use. On the same principle, jug-
glers plunge their hands into melted metal with impunity, by first wetting
them. The moisture on their hands is converted into vapor, which keeps the
seething metal from their skin.

537. "When vapor is formed, sensible heat is absorbed,
and cold is produced.

Hence when the skin is moistened with a volatile liquid (that is, one that
readily passes into vapor) like alcohol, a sensation of cold is soon expe-
rienced. So, a shower or water sprinkled on the floor cools the air in sum-
merciful provisions are extremes of temperature modified ? On what principle do
freezing mixtures operate ? 534. What is Vaporization ? What are the successive
effects of heat on solids ? 535. What is Spontaneous Evaporation ? What are the
effects of evaporation on the earth's surface ? To what is the rapidity of evaporation
proportioned? 536. Explain the principle on which laundresses try their irons.
What use do jugglers make of this principle ? 537. With what phenomena is the-
formation of vapor accompanied ? Give some example, of cold produced by the for-



212



PYKONOMICS.



mer. Green wood does not make so hot a fire as dry, because, when the
moisture it contains is converted into vapor, a large amount of sensible heat
is absorbed and carried off.

538. CONDENSATION. The turning of vapor back into a
liquid state is called Condensation.

539. Distillation. Some substances are converted into
vapor at lower temperatures than others. This fact is
taken advantage of in Distillation.

Distillation is the process of separating one substance
from another by evaporating and then condensing it. It
was known to the Arabians at an early date. Fig. 215
represents a Still, or apparatus for distilling.

Fig. 215.




540. A is a boiler, resting on a furnace. In its Ttead, B, is inserted a pipe,
b c, which enters the worm-tub, R, and there terminates in a worm, represented
by the dotted lines. The substance to be distilled having been placed in the
boiler and a fire kindled beneath, vapor soon rises. Passing through the
pipe o c, it enters the worm, in which it is to be condensed. The worm is
surrounded with cold water, with which the vat is filled, and the vapor is
soon cooled down into a liquid form, and issues from the lower extremity of



mation of vapor. Which makes tho hotter fire, green wood or dry, and why?
53S. What is meant by the Condensation of vapor ? 539. What is Distillation ? On
what fact is the process based ? To whom was distillation early known ? What is
an apparatus for distilling called ? 540. With the aid of Fig. 215, describe the still.



INCANDESCENCE. 213

the worm, falling into a vessel prepared to receive it. To condense the va-
por, the water in the vat must be kept cold. For this purpose, a stream is
kept flowing into it through the pipe pp, while a similar stream of water
partially warmed by the hot vapor as constantly escapes at q. By this pro-
cess water may be obtained perfectly pure, as the earthy matter dissolved in
it is not converted into vapor, but remains behind in the boiler. With a
similar apparatus, spirituous liquors are distilled from grain.

541. INCANDESCENCE. When a body is raised to a cer-
tain very high temperature, it begins to emit light as well
as heat. This state is called Incandescence, or Glowing
Heat.

An incandescent body becomes successively dull red,
bright red, yellow, and white. All solids and liquids, not
previously converted into vapor by heat, become incan-
descent. The temperature at which incandescence com-
mences is the same for all bodies, and may be set down at
977 degrees of Fahrenheit's Thermometer (see 544).

Instruments for measuring Meat.

542. The expansion of bodies by heat furnishes us the
means of measuring changes of temperature. Liquids,
which are easily affected, are used for measuring variations
in moderate temperatures. Solids, which require a higher
degree of heat to expand them perceptibly, are used for
measuring variations in elevated temperatures. Hence we
have two instruments, the Thermometer and the Pyrom-
eter.

543. THE THERMOMETER. The Thermometer is an in-
strument in which a liquid, usually mercury, is employed
for measuring variations that occur in moderate tempera-
tures.

The thermometer (see Fig. 216) consists of a tube closed at one end and
terminating in a bulb at the other. The bulb and part of the tube contain
mercury, above which is a vacuum, all air having been excluded before the
top of the tube was closed. Expanded by heat, the mercury rises in the

and its mode of operation. 541. What is Incandescence ? What colors mark tho
successive stages of incandescence? What substances become incandescent? At
what temperature does incandescence commence? 542. What means have we of
measuring changes of temperature ? In what cases are liquids used ? In what, sol-
ids? Name the instruments used for measuring changes of temperature. 543. What



214



PYEONOMICS.



tube ; when the temperature falls, the mercury, contracting,
falls also. The tube is fixed in a stand or case, and has a
graduated scale beside it for measuring the rise and fall of the
mercury. This scale is formed in the following way : The ther-
mometer is brought into contact with melting ice, and the point
at which the mercury stands is marked. It is next plunged
in boiling water, and the point to which the mercury rises is
also marked. The interval is then divided into a number of
equal spaces, called degrees.

544. As the thermometer does not indicate
the amount of heat in a body, but merely its
changes of temperature, the number of degrees
into which the interval between the freezing and
the boiling mark is divided is arbitrary. Three
different divisions are in use : Fahrenheit's, in
the United States, Great Britain, and Holland ;
Reaumur's [ro'-murz], in Spain and parts of Ger-
many; and the Centigrade, the most convenient
of the three, in France, Sweden, &c.

In Fahrenheit's scale the freezing-point is called 32, the
boiling-point, 212 ; when, therefore, the mercury stands at 0,



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 20 of 42)