G. P. (George Payn) Quackenbos.

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

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temperature prevails at no very great depth. At the surface this internal
heat is not perceptible, because the outer crust of the earth is a bad conductor.

, 478. CHEMICAL ACTION, A SOURCE OF HEAT. "When, by
combining two or more substances, we produce a new sub-
stance totally different in its properties from either, we say
that Chemical Action has taken place. Such action is al-
ways accompanied with an increase of temperature. If,
for instance, we mix equal quantities of sulphuric acid and
water, chemical action takes place, a new substance is
formed, and heat is given out. The heat produced by chem-
ical action is sometimes sufficient to ignite inflammable sub-
stances. Thus a drop of sulphuric acid will set fire to a
mixture of sugar and chlorate of potassa.

479. Combustion. One of the commonest processes in
which chemical action is exhibited, is Combustion, or Burn-
ing. This is the great source of artificial heat, as the sun
is of natural heat.

Combustion is nothing more than a chemical union of the oxygen of the
air with the combustible body or some of its elements. Latent heat is given
out, by which the gases or vapors produced are rendered luminous ; and
hence what we call Flame. The rise of temperature is proportioned to the
rapidity with which the chemical union takes place ; and this depends in a
great measure on the amount of oxygen supplied.

If we wish to make a fire hotter, we have only to bring more air in con-
tact with the fuel. This may be done with a bellows, or in the case of grates
with a blower. To fill the vacuum produced by the ascent of the heated air
through the chimney, cold air must enter ; by putting on the blower, we pre-



distance to -which the sun's heat penetrates? Beyond this depth, whence is the heat
derived? Descending below the earth's surface, at what rate does the temperatura
increase ? At what depth would water boil ? How great would the temperature bo
at a depth of 125 miles ? In what state is the interior of the earth supposed to be ?
What phenomena support this opinion ? 478. When does Chemical Action take place ?
With what is chemical action always accompanied ? Give an example. 479. In what
common process is chemical action exhibited ? What is Combustion ? What is the
cause of flame? To what is tho rise of temperature proportioned ? "What must be



196 P YEONOMICS.

vent it from entering anywhere except at the bottom of the grate, and cause
what does enter to pass through the ignited coals, thus increasing their sup-
ply of oxygen.

480. Animal Heat. To Chemical Action is attributable
Animal or Vital Heat, that is, the heat 'generated in all
organic beings that possess life.

Different living creatures have different degrees of ani-
mal heat. Birds have the most ; beasts come next ; then
fish and insects. In the same class of animals, however, the
amount of vital heat is nearly uniform ; and under ordinary
circumstances it remains the same, whether the surround-
ing medium be warm or cold. Other things being equal,
the heat of the human body is as great in winter as in sum-
mer, in the frigid as in the torrid zone. We do not feel
equally hot, to be sure ; but, as already explained, we must
not judge of temperature by our feelings.

481. Animal heat is produced by a process similar to combustion. When
we breathe, air is taken into the lungs, where it comes in contact with par-
ticles of carbon contained in the blood. This carbon unites chemically with
the oxygen of the air inhaled, and, as in the case of combustion, latent heat
is evolved. The heat is less than that produced by combustion, because the
particles of carbon are extremely small.

As in combustion, whatever increases the supply of oxygen increases the
animal heat. Running or bodily exertion of any kind makes us hotter, be-
cause it quickens the circulation of the blood, obliges us to breathe faster,
and thus brings more air (and consequently more oxygen) into the lungs.

482. The carbon consumed comes from the food we eat. Greasy food gen-
erates it most plentifully. In winter, therefore, when we need an abundance
of carbon, we eat meat more freely than in summer, when we seek to reduce
our vital heat as much as possible. So, the inhabitants of cold regions con-
sume more greasy food than those of warmer climates. The Esquimaux
thrive on fish-oil and seals' fat, which to the people of the tropics would bo
neither palatable nor wholesome.

483. MECHANICAL ACTION, A SOURCE OF HEAT. Mechan-

done, if we wish to make a fire hotter? 4SO. What is Animal or Vital heat? To
what is it attributable ? What is said of animal heat in different living creatures ?
In the same class of animals ? Does it differ in different seasons ? 481. How is ani-
mal heal produced ? Why is it less than the heat produced by combustion ? How is
animal heat increased? Give examples. 482. How is the carbon consumed, pro-
duced ? What sort of food generates carbon most plentifully ? What follows, with re-
spect to our diet at different seasons ? IIow does the diet of the inhabitants of cold
regions compare with that of tropical nations ? 483. What is the third source of heat !



SOURCES OF HEAT. 197

ical Action is a familiar source of heat. Under this head
are embraced Friction or Rubbing, and Percussion or Strik-
ing. By compressing the particles of a body, mechanical
action forces out its latent heat and makes it sensible.

484. Heat from Friction. Touch a row-lock, in which
an oar has been rapidly plying, or a gimlet that has just
been vigorously worked, and you will feel the heat pro-
duced by friction. Rub a metallic button to and fro on a
dry board, and you will soon make it so hot that you can
not bear your finger on it. By drawing a match across a
rough surface, you develop heat enough to ignite it. By
rubbing two pieces of ice together, in a freezing tempera-
ture, latent heat is liberated in sufficient quantities to melt
them.

Machinery has been ignited by the rubbing of its parts on each other.
Savages kindle a fire by rubbing two dry sticks violently together. In bor-
ing a brass cannon, immersed in water by way of experiment, sufficient heat
has been generated to boil the water in two hours and a half. The fric-
tion of two large iron plates has even been employed as a practical source
of heat,

It is to be observed that in all the above cases heat is produced by the
friction of solids. The friction of fluids is insufficient to generate heat.

485. Heat from Percussion. By striking flint and steel
together, we develop sufficient heat to ignite the minute
fragments broken off, and produce sparks. In like manner,
the hammer of a gun, descending on a percussion-cap, sets
fire to the fulminating mixture of which the cap is made.

A nail may be made red-hot by hammering it rapidly on an anvil. Be-
fore lucifer matches were invented, blacksmiths used to ignite sulphur
matches and kindle their forge-fires with a nail hammered to a red heat.

By violent and quick compression, enough heat can be set free from air
to ignite tinder. This is done with the Fire Syringe (see Fig. 210). In the
extremity of the piston is a small cavity, in which some tinder is placed.
When the piston is driven rapidly down, the air in the barrel is compressed,

"\Vhatare included under this head? How is it that mechanical action produces heat?
484. State some familiar cases in which heat is produced by friction. What is som-
times the effect of friction on machinery ? How do eavages kindle their fires ? llovr
great a heat has been produced by boring a brass cannon ? How has friction been
turned to practical use ? What is said of the friction of fluids ? 485. Give some
familiar examples of the production of heat by percussion. How did blacksmiths
formerly kindle their forgc-flrcs ? Describe the Fire-syringe, and the experiment



198



PYEONOMICS.



Fig. 210. latent heat is evolved, and on withdrawing the piston the tin-
der will be found ignited.

If a body is compressed by violent percussion more than
once, the heat produced is less each time, until at last all the
latent beat is forced out, and it may be struck or hammered
without any material increase of temperature. Iron, when
thus deprived of its latent heat, becomes stiff and brittle. The
metals generally lose their ductility, and can not be drawn out
into wire till their latent heat is restored by subjecting them
to the action of fire.

486. ELECTRICITY, A SOUECE OF HEAT. The
passage of electricity is sometimes attended with
intense heat. Lightning, for instance, sets fire
to trees and houses, and melts metallic bodies
that it strikes. The heat produced by the gal-
vanic battery ignites or fuses every known sub-
stance.



IMffnsioEi of Meat.

487. Heat tends to diffuse itself equally among bodies
of different temperature. So strong is this tendency, that,
unless fresh supplies are received, the hottest body soon be-
comes cool, in consequence of parting with its heat to sur-
rounding objects cooler than. itself.

488. Heat is diffused in three ways :

1. By CONDUCTION, when it passes from one particle of

a body to another in contact with it. If one end
of a poker is placed in a fire, the other becomes
heated by Conduction.

2. By CONVECTION, when it is conveyed by the actual

motion of some of the particles of a body. When
a pot of water is placed over a fire-, the particles
at the bottom are first heated, and ascend, carry-
ing heat with them and diffusing it by Convec-
tion.



performed with it. What is found, when a body is violently struck more than once?
What change is produced in iron thus treated ? In the metals generally ? 486. What
is the fourth source of heat ? Give examples. 48T. What is the tendency of heat ?
'1SS. In how many ways is heat diffused ? Name, describe, and give an example of



DIFFUSION OP HEAT. 199

3. By RADIATION, when it passes from one body to an-
other not in contact with it, leaping over the in-
tervening space. A joint of meat, placed before
the fire, is roasted by Radiated Heat.

489. CONDUCTION. Some substances allow heat to pass
freely through their particles ; others do not. The former
are called Conductors of heat ; the latter, Bad Conductors,
or Non-conductors.

As a general rule, dense solids are conductors of heat ;
porous and fibrous solids, as well as liquid?, gases, and va-
pors, are bad conductors.

490. The Conduct om<,ter. The metals rig. 211.
are all good conductors of heat, but some

are better than others. This is shown by
the Conductometer, jepresented in Fig.
211.

The couductometcr consists of a circular plate of
brass, in the outer edge of which are inserted rods of
different metals, of the same size and length, each hav-
ing a small cavity in its extremity for holding a piece
of phosphorus. When the plate is brought over the

flame of a lamp, the heat passes along the different rods TnE CONmjCTOMETEK .
and ignites the pieces of phosphorus, but not all at the

same time. It first reaches the end of the rod that is the best conductor ;
and thus the order in which the pieces of phosphorus take fire indicates the
order in which the metals that the rods are made of rank as conductors of
heat.

491. Conducting Power of different Substances. Gold
is the best conductor among the metals. The conducting
power of gold being set down at 1^000, that of some other
common substances compares with it as follows :




Platinum 981

Silver 973

Copper - 898

Platinum and silver, it will be seen, are nearly as good conductors as gold.



Iron 374 I Lead ISO

Zinc 363 | Marble 24

Tin 304 I Clay 11



aach. 489. What are Conductors of heat ? What are Bad Conductors, or Non-con-
ductors ? As a general rule, what substances are good conductors of heat, and what
not? 490. How do the inetals rank in conducting power? Describe the Con-
ductometer, and its mode of operation. 491. Among the metals, what is the best
conductor? The next? The next? Which is the better, iron or lead? How may



200 PYRONOMICS.

A silver spoon containing water, with a piece of muslin wrapped smoothly
around it, may be held in the flame of a lamp till the water boils without the
muslin's burning, so rapidly does the metal carry off the heat.

492. Wood is a bad conductor of heat. A log blazing at one end may be
handled at the other without inconvenience. Hence metallic tea-pots, sauce-
pans, &c., are often provided with wooden handles. Dense wood and coal
are better conductors than porous wood. This is one reason why they are
harder to kindle ; they conduct the heat away before a sufficient amount is
collected in them to produce combustion. Earthen-ware of all kinds ranks
far below the metals in conducting power.

' 493. Fibrous substances, like wool, hair, and fur, are bad conductors.
The finer and closer their fibres, the less their conducting power. Thus we
see why Providence has clothed the animals of cold climates with a shaggy
covering, from which those of the tropics are free ; and why the coats of
many animals in temperate regions change with the seasons, being closer and
longer in winter, thinner and shorter in summer.

494. The best non-conductors among solids are straw, saw-dust, pow-
dered charcoal, and plaster of paris. Recourse is had to these articles when
it is desired to protect an object from extremes of temperature. Straw is
bound round tender plants in winter, to prevent their warmth from being
drawn off. It is also used for thatching the roofs of houses, preventing the
external heat from entering in summer, and the heat within from being with-
drawn in winter. Ice shipped to warm climates is packed in saw-dust, to
keep out the heat of the atmosphere. For the same reason, the hollow apart-
ments that constitute the sides of refrigerators are filled with powdered char-
coal. Plaster of paris is used for filling in the sides of fire-proof safes. So
impervious to heat does it render them that they may be exposed to flames
for hours without injury to the papers within.

495. If we bare our feet, and place one of them on a
carpet and the other on oil-cloth, the latter feels much
colder than the former. This is not because the oil-cloth
is colder than the carpet, for being in the same room their
temperature must be the same ; but oil-cloth is a good con-
ductor, whereas carpet is not. A good conductor, brought
in contact with the body, carries off our animal heat and
makes us feel cold. A bad conductor, on the other hand,
prevents our animal heat from escaping. Hence the differ-

the conducting power of silver be proved ? 492. Why are metallic tea-pots often pro-
vided with wooden handles ? Why is dense wood hard to kindle ? How does earthen-
ware rank in conducting power? 493. How do fibrous substances rank? As re-
gards the coats of animals, liow is the goodness of Providence shown? 494. What
are the best solid non-conductors ? For what are these substances severally used, and
what is the effect in each case? 495. If we bare our feet, and place one on a carpet
and the other on oil-cloth, what do we feel ? Explain the reason of this. Of the



DIFFUSION OF HEAT.



201



ence of warmth, in different kinds of clothing. That fabric
feels the warmest, which is the worst conductor.

Of the materials used for clothing, wool is the worst conductor and linen
the best ; cotton and silk rank between the two. Linen is therefore the most
comfortable fabric for summer clothing, and woollen for winter. A linen
under-garment is cooler than a silk or muslin one, and these in turn are
much cooler than flannel.

496. The heat of our bodies is generally greater than that of the atmos-
phere surrounding them. If we were placed in an atmosphere warmer than
our bodies, woollen would be the coolest dress that could be worn, because,
being a bad conductor, it would not transmit the external heat. Hence fire-
men and others exposed to a high degree of heat, always wear flannel.
Hence, also, a blanket is wrapped round ice, to keep it from melting.

497. Conducting Power of Liquids. Liquids (except
mercury, which is a metal) are very bad conductors of heat.
This may be shown by several experiments.

Freeze some water in the bottom of a tube, FJO-. 212.

and on the ice pour some more water. Inclining
the tube, apply the flame of a lamp to the liquid
till it boils. The ice remains for a long time un-
melted. If mercury is used instead of water, the
ice begins to melt almost immediately on the ap-
plication of heat.

Again, in a funnel-shaped glass vessel (repre-
sented in Fig. 212) fix a thermometer, or instru-
ment for measuring heat, with its bulb uppermost.
Cover the bulb with water to the depth of half an
inch ; then pour on some ether, and set fire to it.
The burning of the ether generates a great heat ;
yet the thermometer, only half an inch below it,
indicates little or no increase of temperature.

498. Conducting Power of Gases
and Vapors. Gases and vapors are

still worse conductors of heat than liquids. The less their
specific gravity, the less appears to be their conducting
power.

499. Air is one of the worst conductors known. If we




materials xised for clothing, which is the worst conductor? Which, the. best ? How
do cotton and silk rank ? What fabric, then, is the most appropriate for summer
wear, and what for winter? 496. Why do firemen wear flannel ? Why is a blanket
wrapped round ice ? 497. How do liquids rank in conducting power ? Prove that
water is a bad conductor. Prove it by an experiment with the apparatus represent-
ed in Fig. 212. 498. How do gases and vapors rank in conducting power? 499. What

9*



202 PYKONOMICS.

could keep a body of air perfectly still, it would take a long
time for heat applied to one portion of it to be transmitted
throughout the whole.

In summer, when there is no breeze, we feel oppressively warm, because
the air does not carry off the heat generated within us. Fanning cools us,
because it drives off the air heated by contact with our bodies and brings up
a fresh supply, which, after withdrawing more or less heat, is in turn driven
away. In this case it will be observed that the heat is carried off by convec-
tion, and not by conduction. If air were a good conductor, it would soon
take so much heat from animals and plants that their vital action could not
make up the deficiency, and they would be chilled to death.

Closed cellars are cooler than the surrounding air in summer, and wann-
er in winter. If air were a good conductor, this would not be the case. As
it is, the doors being kept closed, currents of air are excluded ; and, since
heat passes very slowly from particle to particle, extremes of temperature
without are not felt within.

It is the air in fibrous and porous solids that makes them bad conductors.
Drive out this air by compression, and you increase their conducting power.
Let wool, or cotton, for instance, be twisted into rolls, and it will carry off
heat faster than it did when loose. Accordingly, clothing that allows some
air to remain in contact with the body is warmer than that which fits very
tight. So, double sashes and double doors, confining a body of non-con-
ducting air, protect apartments from extremes of heat and cold.

500. The uses of air as a non-conductor are seen in the operations of na-
ture. Filling the pores and interstices in the bark of plants, it protects the
tender parts within from sudden falls of temperature. In cold climates, vege-
tation is further protected by snow, which, owing to the air imprisoned
among its particles, is a very bad conductor. A mantle of sno'v on a field
has very much the same effect that a covering of wool would have. Hence
we are told in Scripture that God " giveth snow like wool". The Esquimaux
shield themselves from the excessive cold of their climate in huts of snow.

501. CONVECTION. Fluids, as we have just seen, are
bad conductors, but they are readily heated by convection.
Heat being applied beneath, the lower particles become
expanded and rarefied. They therefore ascend, carrying
up their heat, while cooler and heavier particles from above

is said of the conducting power of air? Why do we feel oppressively warm in sum-
mer, when there is no breeze? What is the effect of fanning ? If air were a good
conductor, what would be the consequence to animals and plants ? Why are closed
cellars exempt from extremes of temperature ? What makes fibrous and porous sol-
ids bad conductors ? Prove this. Compare the warmth of loose clothing with that
which fits very tight. On what principle do double sashes operate ? 500. Show the
uses of air as a non-conductor in the economy of nature. What is the effect of snow ?
What use is made of it by tho Esquimaux ? 501. How are fluids readily heated ?



DIFFUSION OF HEAT. 203

take their place. This process is repeated till heat is dif-
fused throughout the whole, not conducted from one sta-
tionary particle to another, but actually conveyed by the
particles receiving it.

The process of convection is exhibited when water is set over a fire to
boil. The particles soon begin to move, as may be shown by throwing in
some powdered amber, which is seen to rise and descend, more and more
rapidly as the temperature increases. Heat is thus diffused throughout the
whole body of liquid, till ebullition, or boiling, commences.

502. In cooling, this process is reversed. The particles at the top yield
their heat to the air in contact with them. Being thus made heavier, they
descend, while warmer and lighter particles take their place. The greater
the surface exposed to the air, the sooner the liquid loses its heat ; hence we
pour our tea into a saucer, to cool it.

503. To heat a body of liquid by convection, the fire must be applied be-
neath. A pot of water can not be made to boil by a fire kindled on its lid.
The particles at the top may be heated, but they will remain there on ac-
count of their superior lightness, and there will be no diffusion of heat.

504. Thin liquids, like water, are heated and cooled more quickly than
thick ones, like tar, because their particles move more freely among them-
selves, and thus diffuse heat more readily.

505. Heat is diffused through gases and vapors, as
through liquids, by convection. Heated air, like heated
water, ascends, carrying its heat with it. Consequently, to
make the temperature of a room uniform, a fire-place should
be set as near the floor as possible. With the same tem-
perature, we feel colder on a windy day than on a still one ;
because the heat is more rapidly withdrawn from our bodies
by the fresh currents of air constantly brought in contact
with them.

506. Solids can not be heated by convection, because
their particles cohere.

507. RADIATION. A body not in contact with the source
of heat can not be heated by conduction or convection. If
it receives heat, it is by a third process, called Radiation.

Describe the operation. In what familiar process is convection exhibited ? Describe
the process of boiling. 502. Describe the process of cooling. 503. To heat a liquid,
where must, the fire be .applied ? Why can not a pot of water be made to boil by a fire
kindled on its lid ? 504. What kind of liquids are heated and cooled most quickly?
Why? 505. What, besides liquids, are heated by convection ? Where should a fire-
place be set, and why? Why do we feel colder on a windy day than on a still one ?
506. Can solids be heatod by convection? Why not? 507. What bodies are heated



201 PYRONOMIC3.

If we place our hands under a fire in a grate, we at once feel a sensation
of heat. This heat can not reach our hands by conduction, for air is a bad
conductor, nor by convection, for heated currents ascend. It is transmitted
in rays sent forth from the fire through the intervening space. Heat thus
diffused is called Radiant Heat. All the heat that we receive from the sun,
and much of that from fire, is radiant heat.

508. All substances radiate heat, but not equally well.
Much depends on the character of the surface. Rough and
dull surfaces radiate better than smooth and bright ones.

Lamp-black is the best radiator known. Rating its ra-
diating power at 100, that of crown-glass is 90 ; black lead,
75 ; tarnished lead, 45 ; clean lead, 19 ; bright metals gen-
erally, 12. The radiating power of metals is increased by
scratching their surface, or letting them become tarnished.

509. A heated body confined in a covered vessel parts with its heat more
or less rapidly according to the radiating power of the vessel containing it.
For tea-pots, therefore, bright silver is preferable to earthen-ware, because it
is a worse radiator and keeps the tea warm for a longer time. Stoves, on



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 19 of 42)