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[pg 84] 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[pg 85] 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.”

[pg 86]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 [pg 87]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[pg 88] 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[pg 89] 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.

[pg 90]“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[pg 91] 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[pg 92] 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,[pg 93] 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[pg 94] 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[pg 95] 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[pg 96] 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[pg 97] 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[pg 98] 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;[pg 99] 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.”