"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 s.p.a.ce 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 G.o.d's devising. If the operations of heat are beneficent to man, it is because G.o.d 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 _G.o.d_ 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 G.o.ds; G.o.d freely gave it. Heat is indeed a gift from heaven."

CHAPTER V.

CONVEYANCE AND VARIETIES OF HEAT.

"To-day we review the modes in which heat pa.s.ses or is conveyed from place to place. It is evident that if heat were confined to the very place or point where it is generated, it could subserve none of those uses to which it is now applied in the economy of Nature or in the works and arts of man. But heat pa.s.ses from place to place with great facility, and by one method, with the speed of light, it tends to diffuse itself evenly through all; it seeks an equilibrium. The modes of its diffusion, or conveyance, are three in number. Ansel may name them."

"Heat pa.s.ses from place to place and from body to body by 'conduction,' by 'radiation,' and by 'convection.'"

"What is meant, Ansel, by the 'conduction' of heat?"

"The pa.s.sing of heat from atom to atom and from particle to particle through a body is called conduction."

"That is right. I will call upon Peter to give some ill.u.s.trations of the conduction of heat."

"The examples are so many," Peter answered, "that I hardly know what to mention first. If I hold a pin in the flame of a lamp, the part of the pin that touches the flame is first heated, but soon the heat runs along the whole length of the pin and burns my fingers. The parts of a stove which touch the fire are first heated, and from them the heat spreads through the whole stove. A pine-wood shaving, kindled at one end, is heated by conduction, but the heat pa.s.ses through it very little faster than the flame follows. Heat escapes from our bodies by being slowly conducted through our clothing. There is no end to the examples of conduction which one might give."

"We must not think of the conduction of heat," said Mr. Wilton, "as if it were a fluid slowly absorbed by a porous body, as water poured upon the ground soaks into it, or as water percolates through a lump of sugar and moistens the whole of it. We must remember that the transfer of heat is not a transfer of any substance, but a transfer of motion. One atom is set in motion, and strikes against another atom and sets that in motion, and thus motion is communicated from atom to atom and from molecule to molecule through the whole ma.s.s of matter till every atom is agitated with the heat vibrations. Do all bodies conduct heat with equal rapidity?"

"No, sir," replied Ansel; "there is the greatest possible difference. Some substances are called good conductors, because heat permeates them so readily and rapidly; others conduct heat very slowly, and are called poor conductors or bad conductors."

"That is right. Every child soon learns by experience to make a practical distinction of this kind. He very soon understands that he can hold a stick of wood without burning his hand, even though it be blazing at the other end, but that when a piece of iron is red hot at one end he must not take hold of it at the other. The child very soon learns to know the different feeling of a cotton night-gown from one of flannel, and the difference in apparent warmth between a linen pillow-case and a woolen blanket. After a room has been heated for a considerable time the various objects in it all become of the same temperature, and the same is true in a cold room; but how great the difference in the sensations produced by touching the oil-cloth and a woolen carpet in a cold room! Good conductors of heat, if hot, feel very hot; or if cold, feel very cold; while poor conductors make a much less decided impression. Why is this, Samuel?"

"The good conductors receive heat or part with it very readily. If the good conductor be hotter than our bodies, it imparts its heat rapidly to our hand, and because we receive heat rapidly from it, it feels to us very hot. Or if it be colder than our bodies, it takes heat from our hands very rapidly, and gives the impression of being very cold. Poor conductors impart heat to the skin or take it away more slowly, and hence feel as if their temperature were more nearly like that of the body."

"The conducting qualities of bodies," said Mr. Wilton, "seem to depend chiefly upon their structure or the arrangement of their atoms. Bodies which are compact and solid in their structure convey heat more rapidly than those which are loose and porous. Hence solids are better conductors than fluids, and fluids are better conductors than gases, and among solids the metals are better conductors than organized bodies, like wood or flesh, and better than the loose and porous minerals. In bodies of loose, porous, or fibrous texture, the continuity of the conductory substance is constantly broken. The particles in a ma.s.s of sawdust touch only at a few points, leaving frequent s.p.a.ces. In woolen and cotton fabrics the points of junction of the fibres are very few, comparatively. For this reason the motion is not readily communicated from atom to atom.

"The crystalline arrangement of atoms has an influence upon conduction of heat. Heat is conducted more rapidly in a direction parallel with the axis of crystallization than across that axis. Wood conducts heat more rapidly in the direction of the grain. This arrangement seems to be well adapted for keeping trees warm in winter. Their roots reach down into the earth, which remains warm in the coldest weather. This heat of the earth travels along the fibres up through the tree, while the heat conducted across the fibres escapes much more slowly into the open air. The bark also, being a very bad conductor, hinders the escape of heat. Of metals, silver is the best conductor. I will give you a brief table which will show the great difference in the conducting qualities of some of the metals. Counting the conducting qualities of silver as 100, the table is: 'Silver, 100; Gold, 53; Copper, 74; Iron, 12; Platinum, 8; German Silver, 6; Bis.m.u.th, 2.'--_Youmans._

"What is the second method by which heat pa.s.ses from place to place?"

"It is radiated," replied Ansel.

"And what is radiation?"

"It is motion in straight lines or rays diverging from a centre. From a hot body heat is pa.s.sing off in straight lines in every direction. As a lamp radiates light, so does a hot body radiate heat."

"Radiant heat," said Mr. Wilton, "moves with the same velocity as light, that is, one hundred and ninety-two thousand miles per second. It also follows the same general principles as light in all its motions. It is absorbed, reflected, or transmitted in the same manner as light. And this is true of either luminous heat--that is, heat radiated from a body which is red hot--or obscure, or dark heat.

"As there are good and poor conductors, so there are good and bad radiators of heat. The radiation of heat depends upon three conditions:

"1. Upon the temperature of the body. The higher the temperature, the more rapid and energetic is its radiation.

"2. Upon the surface of the radiating body. A dull, rough surface radiates heat more rapidly than a surface bright and polished.

"3. Upon the substance of the radiating surface. With surfaces equally smooth and bright, some substances radiate heat much better than others. A surface of varnish radiates heat much more powerfully than a surface of gold or silver.

"Ansel, you may, if you can, explain the radiation of heat."

"I can give no other explanation than that radiation is conduction through that subtle ether which is supposed to pervade all s.p.a.ce."

"Very well; perhaps that is as good an explanation as can be given. But it seems rather like the propagation of an impulse than the spreading of atomic vibrations in every direction. The motion is propagated in straight lines. If it be conduction, it must be carried on by different vibrations from those of ponderable substances. Heat, light, and electricity are supposed to be all propagated through the same theoretical ether. Sir Isaac Newton estimated the density of the ether as seventy thousand times less than the density of our atmosphere, and its elasticity in proportion to its density as four hundred and ninety millions times greater. But the very existence of this universally-diffused ether is a supposition made to account for the phenomena of light, heat, and electricity; and, of course, all its qualities must be theoretical also.

Radiation is believed to be the propagation of a motion or impulse through an inconceivably rare and elastic ether.

"Peter, what is the third method by which heat pa.s.ses from place to place?"

"Convection," was his reply.

"What is meant by convection of heat?"