HEAT A GIFT OF GOD.
The class is again promptly in place and ready for work.
"As I announced a week ago," said Mr. Wilton, "we will to-day take a rapid
review of the effects and laws of heat. Will you tell us, Peter, the first
and chief of these effects?"
"Yes, sir: combustion."
"What is combustion?"
"Commonly the rapid union of oxygen with some combustible substance,
attended with the evolution of heat."
"Was your answer correct, then?"
"No, sir," said Peter, blushing; "I spoke before I thought."
"Will you correct your answer?"
"The first and chief effect of heat is expansion."
"That is right. Our sensation of heat is of course only a
_sensation_--merely the _feeling_ which results from the effects of heat
upon our nerves--but the chief physical effect of heat is the expansion of
bodies. The chemical qualities of bodies are not changed: they are not
made either heavier or lighter. A sufficiently high temperature renders
bodies luminous, and then we call them red hot or white hot. Solid bodies
begin to be luminous at a temperature of about one thousand degrees. But
the one invariable effect of heat, with two or three apparent exceptions,
is expansion. You may mention, Samuel, some familiar illustrations of the
effect of heat in expanding bodies."
"The blacksmith heats the wagon-tire in order that it may easily slip over
the wheel. If a kettle be filled with cold water, by heating it the water
is expanded and runs over. I have noticed that the spaces between the ends
of the successive iron rails upon the railroad are larger in winter than
in summer, showing that the rails are shorter in winter than in summer.
While skating during the cold winter evenings upon the mill-pond, I have
seen cracks in the thick ice start and run across the mill-pond with a
roar almost like thunder. The ice was contracted by the cold till it
could no longer fill the whole space between the banks, and being frozen
fast to the banks, it was torn asunder. The mercury in the tube of a
thermometer is constantly expanding or contracting by every change of
temperature."
"Yes, those are all good illustrations, and we might go on to mention
others equally good by the score. In cold countries, during the intense
cold of winter, the surface of the earth cracks by shrinkage, just as you
have seen the ice upon the mill-pond torn in two. The Britannia iron
tubular bridge over the St. Lawrence at Montreal rises and falls two and
one-half inches on account of greater expansion of the upper surface when
exposed to the heat of the sun, while a loaded freight train causes a
depression of but one-fourth of an inch. A few years since, in order to
make some philosophical experiments connected with the rotation of the
earth upon its axis, a ball was suspended by a wire in the interior of
Bunker Hill monument. By this means it was accidentally discovered that
the heat of the sun, expanding the sides of the monument exposed to its
rays, caused the whole monument to sway back and forth daily."
Here Ansel raised his hand.
"What is it, Ansel?"
"I was going to mention the belief of geologists that the mountain ranges
were thrown up by the contracting of the earth's crust on account of
cooling."
"That is an illustration of contraction by loss of heat on an enormous
scale. The materials which form our globe may have existed in the
beginning in a nebulous or gaseous state. There is certainly very good
reason for believing that the earth was once in a fluid state, the whole
of its substance molten by intense heat. It is certain that the interior
is now hot, and portions of it molten. It is by very many believed that
the whole interior is molten. The crust of the earth may have been formed
by cooling. If after an outer crust had been formed, and its temperature
had fallen so low as to become nearly stationary, the interior mass
continued to cool, the molten mass would tend to sink away from the crust
and the crust would sink in upon it by wrinkling. Thus mountains may have
been formed. Along the line of fracture the easiest vents would be formed
for volcanoes. But this carries us somewhat aside from our subject, and
as the expansion of bodies by heat has been sufficiently illustrated, we
will leave it. Will some one now state the manner in which the dynamic
theory of heat explains this expansion?"
Samuel answered: "I think you have already given us the explanation."
"I have briefly referred to it, but you may give it again."
"The atomic motion which is supposed to constitute what we call heat,
whatever that motion be, whether a vibration or rotation or revolution,
requires that the atoms of bodies shall not be packed in absolute contact,
and the more intense the agitation or the wider the swing of the vibration
or revolution, the greater must be their separation. Hence heat expands
bodies by thrusting their atoms farther apart."
"That will do," said Mr. Wilton. "Let us look now at some of the secondary
effects of heat. You may mention some of them, Ansel."
"Heat relaxes or overpowers the cohesive attraction of bodies."
"What is cohesive attraction?"
"It is that force which binds together the atoms of matter in simple
substances, that is, bodies like iron or copper or silver, composed of but
one kind of substance, or in compound bodies it is the force which unites
the compound molecules of matter."
"Give us now some illustrations of the effect of heat in overcoming
cohesive attraction."
"The blacksmith heats his iron in order to overcome its cohesive
attraction and render it soft, that he may easily hammer it. The founder
heats his metal till its cohesion is so far destroyed that it becomes
fluid and can be poured into the mould. Heat relaxes the cohesive force of
ice and changes it to water, and by farther heating its cohesion is
entirely overcome and the water is changed to a gas."
"We use heat also in cooking our food," spoke up Peter: "is it not because
heat destroys the cohesive attraction, and thus softens it?"
"If that were the only effect of heat upon food," said Mr. Wilton, "we
should be obliged to eat our food hot, for as soon as it cooled the
cohesion would return and the food would be raw again. The operation of
heat in cooking is various, and part of the effect is commonly to be
ascribed to the water in which the food is cooked or to that which is
contained in it. By the combined agency of heat and water starch swells to
twenty or thirty times its original bulk and the minute starch grains
burst open. In cooking potatoes the starch of the potato absorbs a portion
of the water that is in it, and thus renders it dry and mealy. The action
of heat and water upon rice, wheat, and other grains is similar to their
operation upon starch. In the baking of bread the starch is converted into
gum. In boiling flesh the effect is partly due to the solvent powers of
water: the juices of the flesh are extracted, the gelatin is dissolved,
the fat is liquefied, and the cells in which the fatty matter is held more
or less burst, the albumen is solidified, and by long boiling the texture
and fibre of the flesh are destroyed. The albumen of an egg, that is, the
white, coagulates by heat. But in most of these processes the action of
heat cannot be separated from that of water.
"But there is another effect of heat very important both in nature and in
the arts. What is that?"
"The quickening of chemical affinity," answered Samuel.
"That is right: heat is necessary for the operation of chemical affinity.
Perhaps this is only a weakening of the cohesive force, thus allowing the
chemical attractions to assert their strength. But the fact is that, while
in many cases the chemical affinities act with great energy at ordinary
temperatures, in other cases they slumber, however closely the substances
are brought into contact, till their temperature is raised. Samuel, you
may mention some illustrations of this principle."
"A few months ago I visited Hazard's powder mills, in Enfield,
Connecticut, and there learned how gunpowder is made. The charcoal, the
sulphur, and the nitre are first finely pulverized, then ground together
for hours till thoroughly mixed, and afterward pressed together. This mass
is then broken into grains and the grains polished. But though these
elements are brought into so close contact, yet they do not combine and
explode till heat is applied. The same is true of the combustion of wood
and coal. The carbon and the hydrogen of the fuel are constantly
surrounded with the oxygen of the air, but they do not take fire and burn,
that is, they do not combine with the oxygen, till they are raised to a
red heat, or perhaps even to a higher temperature. If a stove filled with
burning coal be cooled down to a low temperature by applying ice, the
combustion will cease, the fire will go out. Our teacher at the academy on
one occasion heated a steel watch-spring red hot and plunged it into a jar
of oxygen, and the steel spring began quickly to burn with great fury."
"You have given us good illustrations, Samuel, and that which is true of
carbon and hydrogen and oxygen is true of substances in general. The
effect of heat in producing chemical changes is very important everywhere.
It is seen not only in the chemist's laboratory and in the artisan's shop,
but also in the laboratory of Nature. Plant a grain of corn in midwinter:
why does it not germinate and grow? Nothing is needed but the requisite
heat to quicken the chemical affinities into action. Earth and air furnish
the needed material for the growth of forest trees in winter as well as in
summer, but the cold holds in check the chemical forces and prevents the
requisite chemical combinations. No sooner does the sun quicken that
atomic vibration or revolution which we call heat than vegetable growth
begins. Heat is necessary for those chemical changes by which food is
digested in the stomach and the processes of nutrition carried on in every
part of the body. If a man finish his dinner with ice cream or ice water,
the process of digestion is delayed till the contents of the stomach
recover their proper temperature. This is one chief reason why warm,
comfortable clothing is so very important, especially for children. All
the vital processes are chemical processes: they are carried on through
chemical affinities. Unless the body be kept at a suitable temperature,
these processes are feeble and imperfect, nutrition and vital combustion
are hindered, and diseases are engendered.
"These, then, are the chief effects of heat. It expands bodies, weakens
cohesive attraction, and quickens the chemical affinities into activity."
Ansel again raised his hand.
"What do you wish?"
"Will you please tell us, Mr. Wilton, how this weakening of cohesive
attraction is explained upon the dynamic theory of heat?"
"I will do so with pleasure. The increased atomic motion in the heated
body throws the atoms farther apart, as we have already learned, and by
this increase of distance their attraction is diminished. If the earth
were twice its present distance from the sun, their attraction for each
other would be four times less than it now is; if its distance were three
times as great, their attraction for each other would be nine times less.
The attraction of gravitation diminishes in proportion as the square of
the distance through which it must act increases. Perhaps cohesive
attraction diminishes according to the same law, though the spaces are so
small that this cannot be demonstrated, but it is certainly weakened by
the expansion of bodies through the agency of heat."
Here Peter raised his hand.
"What will you say, Peter?"
"Do not men heat and burn bricks, not to soften them, but to harden them?"
"That is true," said Mr. Wilton; "but in this there is a process of drying
as well as of heating, and the hardening is due chiefly to the complete
drying by the intense heat. Too great heat will melt bricks while in the
process of burning. I once heard a brick-burner say that he could melt the
brick around the arches in his kiln in half an hour, if he pleased to put
in fuel and let the fire burn. Indeed, almost every known solid substance
has been fused by heat. Whether carbon has ever been melted is an
unsettled question."
"I would like to inquire," said Samuel, "why water will not burn. Is it
because it evaporates before it reaches a sufficiently high temperature?"
"This is a little aside from our subject, but the incombustibility of
water is a provision of the Creator so very important that we will stop to
notice it. I think, however, that by a little thought you yourself can
answer the question. Tell me again what combustion is."
"Combustion is commonly the combining of oxygen with some other substance
called a combustible. The rusting of iron and the decay of organic bodies
are forms of slow combustion."
"Now tell us the composition of water."
"Water is composed of oxygen and hydrogen--eight parts of oxygen to one of
hydrogen, by weight, or two parts of hydrogen to one of oxygen, by
measure."
"How is water formed from these two gases? Are they mixed together as
oxygen and nitrogen are mingled in the air, or are they chemically
united?"
"They are chemically united: they are burned together. When hydrogen
burns, the product is water."
"Water is then a _product_ of _combustion_. Can you not now tell why water
is incombustible?"
"I think I now see the reason. The oxygen, being itself the supporter of
combustion, will not burn, and the hydrogen has been already once burned
in the formation of water."
"And that which is true of water is true, in a greater or less degree, of
other products of combustion. The burning of charcoal produces carbonic
acid, and carbonic acid will not burn because it is the production of
combustion. A candle is extinguished by it as quickly as by water. By a
recent invention carbonic acid is used to extinguish conflagrations. The
carbon has once united with oxygen, and a second combination with an
additional amount, or, as a chemist would say, with another equivalent, of
oxygen is much more difficult."
"I think," said Samuel, "I now understand why water will not burn, but
will you please also to tell us why water puts out fire better than
almost anything else?"
"In order to extinguish fire one of two things must be done: either the
supply of oxygen must be cut off or the combustible must be cooled down to
a temperature below the burning point, when the combustion will cease of
itself. When we shut the draught of an air-tight stove, we check the
combustion by shutting off the full supply of oxygen. If we could wholly
prevent the access of oxygen to the fuel, the fire would at once be
extinguished. If oxygen should then be admitted again before the fuel had
cooled down below the burning point, combustion would at once begin again.
A blazing brand is extinguished by being thrust into ashes, because it is
shut away from oxygen. In the same way we extinguish the flame of a candle
with a tin extinguisher. On the other hand, fires often go out because the
necessary temperature is not maintained. Water puts out fire in both these
ways, but especially by the second. Water poured in torrents from a fire
engine upon a fire forms a film of water, and the burning material shuts
out the oxygen. But the water acts chiefly by lowering the temperature.
No other known substance except hydrogen gas requires so much heat to
raise it through a given number of degrees of temperature as water. As
much heat is required to heat one pound of water as thirty pounds of
mercury. Hence, water poured upon burning timber cools it to so low a
temperature that it ceases to burn.
"In addition to this, we may notice that wood saturated with water cannot
be heated above the boiling point of water till the water is evaporated.
As fast as the wood and the water rise or tend to rise above two hundred
and twelve degrees, the water changes into steam and carries away the
additional heat. The consumption of heat in the formation of vapor we must
look at more carefully in a future lesson. We will suppose that a house is
in flames. A fire engine throws a stream of cold water into the midst of
the conflagration. The cold water, dashing against the burning wood, cools
the heated surface; it is absorbed into the pores of the wood and hinders
its rapid heating; a portion of the water, being changed into steam,
carries off the heat; the steam, mingling with the flame, lowers the
temperature of the burning gas, and in proportion as steam fills the
surrounding space oxygen is driven away. A burning coal mine in England
was once extinguished by forcing steam into it, thus driving out the air
which supported the combustion and cooling down the burning coal.
"The advantages which men receive from these agencies of heat are so
manifest that we cannot help noticing them. I do not refer to the comfort
of a pleasant temperature, nor the impossibility of living in a
temperature extremely low, but to all those processes by which man subdues
nature, provides for himself food, clothing, and dwelling-places, and
builds up civilization. Heat is that force which enables man to accomplish
his ends. Heat brings the iron from the native ore, and heat renders it
malleable and plastic to be shaped for man's uses. Heat quickens the
chemical affinities and renders the arts of civilized life a possibility.
Heat brings together oxygen and carbon in ten thousand furnaces, and the
heat engendered by the combustion, changed to force, drives the ponderous
or nimble machinery which carries on the work of the world. Heat quickens
the chemical affinities and causes the wheat to grow; heat prepares the
wheat for man's food; and by the aid of heat that food is changed in
man's body, nutrition goes on, the body is built up, waste matter is
removed, and all the vital processes are supported. Without these agencies
of heat--softening and subduing stubborn matter on the one side, and
quickening its forces on the other--man could not exist.
"Let me remind you that these agencies of heat are of God's devising. If
the operations of heat are beneficent to man, it is because God wished to
bless his creatures. I am not much given to moralizing, but when I see how
completely these simple effects of heat meet man's wants, I cannot help
remembering and admiring the wisdom of the great Designer. It is _God_ and
not blind, unconscious Nature that is working."
"This reminds me," said Samuel, "of the tradition in Greek mythology that
Prometheus stole fire from Jupiter and brought it down to man in a reed as
a precious treasure. It seems to me like a gift from heaven."
"This mythological tradition has, however, one falsehood: there was no
need that men should steal fire from the gods; God freely gave it. Heat is
indeed a gift from heaven."