299. Davy's Safety-Lamp.—One of the most beautiful applications of the conduction of heat we have in the Safety-Lamp of Sir Humphrey Davy, an invention which has been the means of saving the lives of multitudes of miners. It is represented in Fig. 213. With this lamp one can go into the midst of the most explosive gases with impunity. Now all that prevents the flame within from setting on fire the gases without is a covering of wire-gauze. This, being a good conductor, conducts off the heat of the flame within so rapidly that it can not go through the openings as flame, and so does not set fire to the gas without. The fact upon which the construction of this lamp was based was discovered by trying many experiments. Among them were the following: A piece of wire-gauze was held over a candle so that its flame struck against it. The smoke issued above, but no flame. Then a stream of gas was allowed to pass through the gauze, as seen in Fig. 214, and was set on fire above. It burned without inflaming the gas below.[4]

300. Relation of Density to Conduction.—Generally the more dense a substance is the better is its conduction of heat. Thus the metals are better conductors than wood, marble than brick, the solids than liquids, and liquids than aeriform substances. We have frequently a good illustration of the difference between stone and brick as conductors in the melting of snow on sidewalks. If a light snow fall in the spring, after the earth has become somewhat warm, you will see it melted from the stone walks much before it is from the brick ones. This will be especially the case if the snow be melted mostly by the warmth of the earth without the agency of the sun. The explanation is obvious. The stone is a better conductor than the brick, and therefore the heat of the earth comes up through the former more rapidly than through the latter.

301. Conduction in Liquids.—That liquids are poor conductors of heat may be shown by an experiment or two. If a thin glass tube closed at one end be filled with water, and the heat of a spirit lamp be applied to its upper portion, as seen in Fig. 215, though the water at this portion may be made to boil, there will not be the least movement in the lower part. This will be very obvious if you have some amber-dust in the water. Again, let a little water be frozen in the lower part of the tube by placing it in a freezing mixture, and introduce a little oil, and then over that some alcohol. Hold now the tube over the chimney of a lamp, as represented in Fig. 216, until the alcohol boils. The ice in the bottom of the tube will not be in the least affected, and the oil will be but slightly heated. If the heat were to be applied in either of the above cases at the lower part of the tube the result would be different, because convection would then operate in the diffusion of the heat.

302. Air as a Non-Conductor.—Heat is rapidly diffused in air by convection; but it is only when the air is free that this can be done. When the air is confined in spaces or pores, or among fibres, heat makes its way through it very slowly, for it can be diffused through it then only by conduction. The variety of ways in which air is of service to us as a non-conductor is almost endless. I will notice some of them.

303. Double Windows.—The efficacy of double windows depends upon the confined air between them. In the case of the single window a great deal of the heat inside is lost in this way: The warm air of the room which comes in contact with the window imparts to it some of its heat, and, being thus cooled and therefore condensed, passes downward. As this process goes on continually this downward current by the window is constant. The current outside is in the opposite direction. The heat imparted to the window is taken up by the cold air, and as it thus becomes warmer it passes upward. And this upward current outside is as constant as the downward current inside. Now nearly all this is prevented by the non-conducting quality of confined air in the case of double windows. If a pane were taken out from the upper part of the inner window, and another from its lower part, the inner window would be of little use, for then the heat of the air in the room would be continually diminished by convection, as when the window is single. The warm air would pass in at the upper opening, and, being cooled, pass down through the lower one.[5]

304. Air as a Non-Conductor in the Walls of Buildings.—The spaces included between the outer wall of a building and the plastering inside being filled with confined air, prevent the heat of the air in the apartments from passing off readily through the wall. A house built of brick or stone, with the plastering placed directly upon the inside of the wall, would be kept warm with difficulty in winter, because the solid wall would so readily conduct off the heat to the external air. So, also, such a house would be very warm in summer, because the heat of the sun and of the external air would be so rapidly communicated to the air of the house. In this connection I will mention a contrivance to prevent the spreading of fires in blocks of buildings, which, though very effectual, is seldom made use of, partly because it occasions some trouble and expense, and partly because it takes up a little room. It is this: A small space is left in the division wall between each two houses from top to bottom, containing, of course, a body of confined air, that is, if the space be entirely shut in, which is as essential here as in the case of the double windows. With such an arrangement the interior of one house may be entirely consumed without communicating sufficient heat through the confined air to set on fire the other.

305. Fur, Hair, and Feathers.—Animals that live in cold climates are provided with suitable coverings for their protection. Quadrupeds, for example, are covered with fur, and birds have an abundance of downy feathers. These coverings have no warmth in themselves, though in common language we speak of them as being warm. They are simply non-conductors, and so prevent the heat which is made in the body of the animal from escaping as fast as it otherwise would. But why are they non-conductors? It is not because the substance of which they are made is a non-conductor, but because among their numberless fibres is partially confined that great non-conductor, air. Let the fur or down be condensed into a thin hard plate upon the animal, and it would prove of little service as a protection against cold. Down is much more abundant on the birds of cold climates than on those in warmer regions, because more air can be confined among the fibres of down than among those of common feathers. Quadrupeds that are natives of warm climates generally have hair instead of fur. When therefore the horse is taken to a cold climate he requires in winter the defense of a blanket; and the ox needs under the same circumstances to be better housed than he ordinarily is. As the elephant is a native of a climate positively hot, his hairs are scanty and coarse. Formerly there were elephants in the cold regions of Siberia, as has been ascertained by remains found there. But the elephant of Siberia had under its hair, close to the skin, a fine wool to protect it against the cold. Animals that live in cold climates have their coverings become finer in fibre in the cold season of the year, to give them the additional protection which they then need. And animals with a furry covering, if they are carried into a warm climate, have their fur become coarse, and approximate the condition of hair.

306. Clothing.—Man has no covering to guard him against cold, because he is capable of contriving clothing suitable to the various degrees of temperature to which he may be exposed. The object of clothing is not to make the body warm, but to keep it so. The heat of the body is generated continually within itself, and under all circumstances this heat is maintained quite uniformly at 98°. This, you see, is a much higher degree than the atmosphere ordinarily has. We are all the time, then, giving off heat to the air around us, except when the air gets up to 98°. We are comfortable only when we are giving off heat to a considerable amount, for the point of temperature which is most agreeable to us when we are at rest is 70° or a little less, that is nearly thirty degrees below the temperature of our bodies. When the temperature is below this we need extra clothing. In making choice of clothing for various degrees of temperature we practically apply the principles which I have developed. Those articles of clothing which can confine or entangle, as we may say, the largest quantity of air among their fibres are the best non-conductors, or, in common language, are the warmest. So, too, loose clothing is warmer than tight, on account of the amount of air between the clothing and the body. Thus a loose glove is much warmer than a tight one. The same general fact is exemplified in the coatings of straw which we put upon tender trees and shrubs in winter. It is the air that is confined in the tubes of the straw which makes these coverings so effective a defense. It is probably the air in the pores of the brick which makes it a poorer conductor than stone, as illustrated by the fact stated in § 300.

307. Cocoons.—Many insects pass through their pupa or transition state in cocoons. When this is done during warm weather, as in the case of the silk-worm, the cocoon is simple. But when the pupa state lasts through the winter special provisions are made in the arrangements of the cocoon to guard the insect against the cold. I will cite as an example the cocoon of one of our largest moths, the Cecropia. This cocoon, fastened to some shrub, keeps its inmate secure from the rigors of the winter by a very beautiful arrangement. The real cocoon is similar to that of the silk-worm; but it has a very dense air-tight outer covering, and the space between these two coverings of the pupa is filled with a loose substance, which has air, of course, mingled with its fibres, and acts, therefore, the part of a blanket for the insect.

308. Buds of Plants in Winter.—In the latter part of summer buds are formed on trees and shrubs, and these contain the germs of the branches, leaves, and flowers which are to come out the next year. These of course must be guarded against the cold of winter, and it is done very much as the pupa is guarded in the cocoon. Each bud you can see has a covering of scales which is air-tight, and inside of this there is a soft downy substance, the blanketing of the bud. In these coverings, which have been called by some one the "winter-cradles" of the buds, the infant vegetation of another year rocks back and forth in the wintry winds secure from the cold, till the warm sun of spring wakes its hidden life into activity.

309. Snow a Protection to Plants.—Snow is a good blanket to the earth, keeping its warmth from escaping into the cold air. This is because it contains mingled with its feathery crystals such a quantity of air. If snow come early, before the ground and the plants in it have become frozen, it will keep them from freezing through the winter, if it remain during all that time. It is curious to observe the peculiar arrangement of the snow in the arctic regions for the preservation of vegetation. First in the autumn come soft light snows covering up the grasses, and heaths, and willows. Then as winter advances there are laid on top of these the denser snows, making a compact, stout roof over the lighter snows in which the scanty but precious vegetation of those regions is imbedded. On top of this roof are deposited the snows of spring. As these melt the water runs off from the icy roof down the slopes, leaving untouched the plants underneath, which lie there alike secure from the rush of waters and from the nightly frosts until the season is sufficiently advanced to bring them out with safety from their concealment. Then the icy roof melts, and with it the light snows that have so long encircled the plants, and the sun wakes them from their long sleep to a new life.

310. Influence of the Conduction of Heat on Sensation.—If you place your hand upon fur hanging at the door of a fur-store it does not feel as cold as the wood from which it hangs, and the wood does not feel so cold as the iron bar of the shutter close by. Why is this, when these substances are exposed to the same atmosphere, and really have the same temperature? It is because the iron conducts the heat from your hand more readily than the wood, and the wood more readily than the fur. So the iron handle of a wooden pump feels colder than the pump, and the pump colder than the snow around it. For the same reason, in a cold room the rug or the carpet will not feel as cold as the poker and the hearth. If water has stood long enough in a room to be of the same temperature with the air of the room your hand will feel colder in the water than in the air, because the water is the better conductor. So much for the sensation of cold. On the other hand, when substances are so heated as to give us the sensation of heat, the conductors do this more than the non-conductors. As they receive heat readily they also readily impart it. For this reason, with a brisk fire the hearth-stone feels very hot, while the rug before the fire does not.

311. Radiation of Heat.—Every substance sends heat into space constantly in straight lines in every direction. These lines are radii, and hence the term radiation is applied to heat diffused in this way. It is very obvious in regard to the sun that it radiates heat in all directions. The same can be seen in the case of a heated iron ball. In whatever direction you hold your hand, above, below, or laterally, you feel the heat. And it makes no difference whether the ball be red-hot or not. That is, heat is radiated either with or without light. When a room is warmed by a furnace it is warmed altogether by convection; but when it is warmed by a fire, either in a fire-place or a stove, we have both convection and radiation. The heat which we receive from the sun comes altogether by radiation.

312. Connection between Heat and Light.—The heat and light of the sun pass together through transparent substances, as air, glass, water, etc., without heating them to any extent. Thus, when the heat is transmitted through a lens, § 272, the lens is little heated, that is, it lets almost all the heat pass through it. The air is heated by the sun, but not directly to any amount. It is heated indirectly in this way: the rays of the sun passing through the air heat the earth, and then the air receives a part of this heat from the earth, which is diffused through it by convection.

It is otherwise with heat that comes from a common fire. It does not seem to be so thoroughly united with the light, and therefore readily parts company with it, as we may say. While the heat and light of the sun go together through all transparent bodies the heat of a fire will not go with its light through all of them. So while the heat of the sun does not warm the glass through which it passes the heat of a fire will warm it, and therefore glass is an effectual screen against it. In some operations in the arts a mask of glass is sometimes worn to ward off the heat. The connection of light and heat will be farther noticed when I come to treat of light.

313. Relation between Radiation and Absorption.—All surfaces that radiate will absorb also equally well the heat that is radiated upon them. All rough and dark surfaces both absorb and radiate freely; but all light-colored and polished surfaces do both slowly. For this reason the black, rough tea-kettle is well fitted to heat water in; but it is not fitted to retain the heat in the water. On the other hand, the bright, polished tea-pot absorbs heat poorly, but retains it well.

314. Reflection of Heat.—Radiated heat is reflected; and here, as in the case of motion, § 206, and of sound, § 260, the angles of incidence and reflection are equal. Some interesting experiments in relation to the reflection of heat can be tried with concave metallic mirrors. Thus, if we take two such mirrors, as represented in Fig. 217, and place in the focus of one a thermometer, and in the focus of the other a small flask of hot water, or a heated iron ball, the mercury in the thermometer will rise, although the mirrors may be many feet apart. Observe how the effect is produced. Rays of heat go from the flask directly toward the thermometer, as represented by the lines in the figure; but that the effect does not come from these can be proved by removing the mirrors, leaving the flask and thermometer just as they are. When the experiment is tried in this way no effect is produced on the thermometer, because it is too far from the source of heat, the flask, to receive any perceptible influence in this way. The effect comes from the rays of heat which go to the mirror near the flask, and are reflected to the other mirror, and then are reflected upon the thermometer, all of which is represented by the dotted lines. There is another way, besides that already mentioned, of showing that it is not the direct rays that produce the effect. After arranging the apparatus put a screen between the thermometer and the mirror near it, and the effect will be prevented because the reflection is cut off. If a piece of ice be substituted for the flask of hot water the thermometer will fall—an effect opposite to that produced in the previous experiment. This would seem to show that cold is radiated, but as there is no such thing really as cold, § 270, the effect must be attributed to the radiation of heat from the thermometer to the ice. If a hot ball be placed in the focus of one mirror and a piece of phosphorus in that of the other, the phosphorus will be set on fire, though the mirrors may be twenty or more feet apart.

The reflection of heat may be exhibited very prettily with the experiment represented in Fig. 218. A sheet of bright gilt paper is rolled up in the shape of a funnel, with the metallic side inward. Holding the larger end toward a fire, the rays of heat coming from the fire into the funnel are reflected toward a central line, and so pass out of the smaller end of the funnel concentrated. If, now, a bit of phosphorus or a lucifer match be held a little distance from this end of the funnel it will be set on fire.

315. Formation of Dew.—It is by the radiation of heat that dew is formed. The earth is constantly radiating heat into space as well as the sun. In the daytime it receives a great deal more than it radiates. But at night this is reversed, and the earth is cooled. The cooled earth condenses the moisture in the air which is in contact with it, and so the moisture is deposited. If the weather be very cold this is frozen, and then we have frost instead of dew. You observe that the dew does not fall, though this is the ordinary expression. Its formation is analogous to the deposit of moisture which we so often witness in a hot day in summer on the outside of a tumbler containing cold water. As the cold tumbler condenses the moisture in the air, so does the earth at night, it being cooled by radiation, condense the moisture which has accumulated in the air by evaporation during the heat of the day.

Dew is deposited in different amounts on different substances. This is owing to a difference in radiation. Grass and leaves radiate heat better than earth, and earth better than stone; and therefore while stones and gravel-walks may be dry or nearly so, the loose earth may be moist and the grass and leaves thoroughly wet. So you see that not even the dew, plentiful as it is, is wasted by the Creator, but is deposited just where it is wanted to refresh the parched earth and its vegetation.

316. Gideon's Fleece.—If you lay a fleece of wool upon the ground, it is so poor a radiator of heat that no dew will be deposited upon it, although there may be an abundance of it on the grass and leaves in its neighborhood. But this was reversed in the case of Gideon's fleece. The laws of nature were set aside, and the fleece was wet with dew while all around was dry.

317. Dew-Point.—What is called the dew-point of the air is that degree of temperature to which any substance must be brought down in order to have dew deposited upon it. This depends upon the amount of water there is in the atmosphere. The more there is the higher is the dew-point. When water condenses on a cold tumbler in a hot day there is much more water in the air, and the dew-point is higher, than when no moisture is condensed upon the tumbler. So after a very hot clear day the earth needs not to be much cooled to produce a deposit of dew, because the air has become so highly charged with moisture through the evaporation of the earth under the hot sun. We can very readily at any time ascertain the dew-point. Take a glass of water, and, having a thermometer in it, drop into it some pieces of ice, and watch the outside of the glass. As soon as it begins to be dimmed with moisture look at the thermometer, and you have the dew-point.

318. Freezing Mercury.—Mercury can be frozen by radiation when the cold is excessively severe, although the thermometer may indicate a temperature considerably above -39°, the degree at which mercury freezes. Suppose that in a clear, still night the temperature of the air is at -20°. In order to freeze the mercury it must be cooled down 19 degrees below this. Now this can be done by surrounding the mercury with some good non-conductor, as charcoal. This cuts off the supply of heat to the mercury, while it is all the while giving off heat into space by radiation. In like manner can ice be formed in an atmosphere that is above the freezing point, and this is often done in warm climates.

319. Latent Heat.—You have seen, § 270, that our sensations do not inform us accurately of the amount of heat in any substance. The same is also true of the thermometer. This only indicates the sensible or free heat. There may be a great deal of heat locked up, as we may say, in the substance, which can be brought out or made free by some change in the substance. This heat thus locked up is called latent heat.

320. Capacity for Heat.—The more heat a substance can take in and render latent the greater is its capacity for heat, as it is expressed. Thus water has a much greater capacity for heat than mercury. This can be proved by various experiments. Thus, if we take two vessels just alike, and having, the one a certain quantity of water in it, and the other the same quantity of mercury, and expose them to the same degree of heat, it will take much longer to raise the water to any specified temperature than the mercury. Why is this, when they are both receiving the same amount of heat? It is because the water renders a much larger portion of the heat latent than the mercury does. We can reverse this experiment. Take these same vessels with their contents raised to the same temperature, as indicated by the thermometer, and allow them to cool in the air side by side. The mercury will cool much faster than the water, because it has much less of latent heat to part with. The difference in capacity for heat between water, oil, and mercury may be shown by the experiment represented in Fig. 219. A pound of water is put into one Florence flask, a pound of oil into another, and a pound of mercury into a third. They are all heated to 212°, and are then placed in funnels filled with pounded ice, the funnels resting in glass jars of the same size. Now in cooling these fluids down to a certain point, say 32°, different amounts of the ice will be melted, in the proportions of 100 and 50 and 3. This shows the proportions of latent heat in them which become sensible or free as their temperatures are lowered.

321. Relation of Latent Heat to Density.—The more dense a substance becomes the less is its capacity for heat. The heat produced by hammering iron is the latent heat rendered free by condensation, this lessening the capacity of the iron for heat. The same thing can be better illustrated in the condensation of a very compressible substance, as air. In Fig. 220 you have represented a glass syringe with a closed end. If there be placed in this end a little bit of cotton wool moistened with ether, and the piston be forced downward very quickly, the ether will be set on fire. This is because the compression of the air lessens so much its capacity for heat that a great deal of its latent heat is made sensible or free. The heat which is concealed in it in its ordinary state is, as we may say, fairly squeezed out, as you would squeeze out the water that is concealed in the interstices of a sponge.

322. Coldness of Air at Great Heights.—You learned in § 152 that the atmosphere is thinner the farther you go from the earth. It is very thin, therefore, on the summits of high mountains. This is the chief reason why it is so cold there, for the rarer the air is the greater is its capacity for heat, and the more of sensible or free heat therefore can it render latent.

323. Relation of Latent Heat to the Forms of Substances.—Whether a substance shall be in the form of a solid, liquid, or gas depends upon the amount of heat which is latent in it. If you take a piece of ice and melt it in a vessel, the ice and the water in the vessel that comes from the melting of the ice are both at 32° until the ice is all melted. But all this time heat is being communicated to the ice and water. What becomes of it? It is all taken up by the ice as it changes from its solid to its fluid state, and becomes latent in it. In fact every particle of ice must have just so much of latent heat in order to become fluid. So, also, if water be heated to the boiling point, 212°, and be kept boiling, the water will remain at that point till it is all vaporized. All this time the water is receiving heat, which, instead of raising its temperature, is becoming latent in the particles as they change from their liquid to their vaporous state. As I said of the change from the solid to the liquid state, so here, every particle of the liquid must have just so much of latent heat in order to become aeriform. Whenever therefore any solid substance becomes liquid, or liquid becomes aeriform, heat is absorbed and becomes latent. So, on the other hand, whenever any aeriform substance becomes liquid, or liquid becomes solid, latent heat is given out, and becomes free and sensible. The freezing of water, then, is a source of warmth to the air in its neighborhood—a fact which is practically made use of when tubs or pails of water are placed in conservatories to keep plants from freezing; and the thawing of snow and ice is a source of cold, as is exemplified by the chilliness of the air occasioned by this process.

324. Clouds and Latent Heat.—The water of which clouds are composed is heavier than air. Why, then, does it remain suspended? Why is it necessary that it should be collected into drops in order to have it fall? This question can be answered by looking at the manner in which clouds are formed. A cloud, I have stated in § 288, is made up of minute vesicles or bubbles containing air. Now the air in these bubbles is lighter than the air that is around the cloud, because it is warmer. But how does it get its heat? In order to understand this observe what the bubble is made from. It is made from the water which was in the air in a state of vapor, or in its aeriform state, for this is the state of water that is evaporated and dissolved in the atmosphere. But when it forms the vesicle it goes out of this state and becomes a liquid, for the wall of the vesicle is a liquid wall, just as the wall of a soap-bubble is. Now in passing out of the aeriform into the liquid state some latent heat must be made sensible. What becomes of this sensible heat? It just heats the air in the vesicle, and so makes it like a heated air-balloon. So all clouds are collections of innumerable heated air-balloons, and the reason that some clouds are higher up than others perhaps is that their balloons have warmer and therefore lighter air in them.

325. Freezing Mixtures.—The intense cold produced by these mixtures is the result of the change of free or sensible heat into latent. For example, when salt and snow are mingled together a melting of the two is quickly produced. In this sudden change of a solid into a fluid a great quantity of heat must be rendered latent, and therefore there will be a great loss of sensible heat by whatever the freezing mixture comes in contact with. The process here, you see, is the opposite of solidification in relation to latent heat. A portion of the snow, after melting with the salt, becomes solid ice. Why is this? It is because it gives up its sensible or free heat to portions of the snow that are in the process of melting and are therefore making heat latent.

326. Cold from Evaporation.—If you pour a little ether into the palm of your hand it will rapidly disappear in vapor, producing a very cold sensation. This sensation occurs because, in the change of the liquid into the vaporous or aeriform state, some of the sensible heat of your hand is abstracted to become latent in the vapor. The evaporation of water also produces cold, though not as decidedly as ether, because its change into vapor is not so rapid at ordinary temperatures. We make a practical use of the evaporation of water in many different ways. Thus we sprinkle water in a hot day upon the floors of piazzas, steps, etc., that the evaporation may make much of the sensible heat about our houses latent. For the same purpose, in hot climates, apartments are often separated from each other by mere curtains, which are occasionally sprinkled with water. So the inhabitants of such climates often cool their beverages by keeping a wet cloth for some time wrapped around the vessels that contain them. Evaporation is an important remedy for many cases of disease. For example, if the head be hot, a steady application of a wet cloth to the forehead, though a simple remedy, is generally effectual, and sometimes is very important. Most people make the application in a wrong manner. They put on several thicknesses of cloth, when a single thickness is the best, because it will best secure the evaporation, which is the cause of the relief afforded.

327. Freezing in the Midst of Boiling.—It is from the quantity of heat rendered latent by evaporation that water can be frozen in the midst of boiling ether; and, paradoxical as it may seem, the boiling of the ether is the cause of the freezing. The experiment is performed in this way: Place a test-tube or a little thin vial with water in it in the midst of some ether in a shallow vessel under the receiver of an air-pump. On exhausting the air the ether will boil, evaporation taking place rapidly because the pressure of the air is taken off from the ether. Now as the ether passes into vapor it extracts so much free heat from the vial of water that the water is cooled down to the freezing point, and so becomes solid. Water can be frozen even by its own evaporation. It is done in this way: Let a shallow vessel, b, Fig. 221, contain a little water, and the vessel c oil of vitriol or sulphuric acid. When the air is exhausted, the pressure of air being taken off from the water, vapor rises from it freely. As the sulphuric acid has a great attraction for water it absorbs this vapor, and so vapor continually rises from the water the more rapidly because what is formed is absorbed, instead of remaining to make pressure on the water. The result is that this rapid formation of vapor, requiring that a great quantity of heat should be made latent, at length abstracts so much heat from the water that remains that this becomes solid.

328. Degree of Heat Endurable by Man.—It was formerly believed that the human body could not endure with impunity, even for a short time, a much higher degree of temperature than that which is met with in hot climates. But in the year 1760 it was accidentally discovered that a much higher temperature than this could be endured. An insect was destroying at that time the grain gathered in some parts of France, and it was found that if the grain was subjected to a certain high degree of temperature the insect was killed, and yet the grain was not injured. In trying some experiments in regard to this matter the experimenters wished to know the point at which the thermometer stood in a large oven. A girl attending on the oven offered to go in and mark the thermometer. She did so, remaining two or three minutes, and the thermometer was at 260°, that is, 48° above the boiling point of water. As she experienced no great inconvenience from the heat she remained ten minutes longer, when the thermometer rose to 76° above that point. These facts were published, and prompted scientific men to try other experiments. In England, Dr. Fordyce, Sir Charles Blagden, and others, went into rooms heated even to 240° and 260°, and remained long enough to cook eggs and steaks, and yet themselves suffered little inconvenience. The pulse was quickened, the perspiration was very profuse, but the heat of the body, as ascertained by putting the thermometer under the tongue the moment they came out, was scarcely raised at all. The air in which they were roasted eggs quite hard in twenty minutes, and when it was applied by a pair of bellows to a steak it cooked it in thirteen minutes. The question arises, how is it that this high degree of heat did not produce more effect upon the body? One reason is that the heat of the air in the immediate neighborhood of the body was continually reduced by the evaporation of the free perspiration, sensible heat being thus converted into latent. Another reason is that the air is not a good conductor, and therefore did not communicate its heat readily to the body. Dr. Fordyce and his friends found that they could not touch with impunity any good conductor, as the metals, and they were obliged to wear upon their feet some non-conducting substance.

329. Formation of Ice.—Before dismissing the subject of heat I must notice the grand exception which we have to some of the operations of heat in the formation of ice. Heat generally produces expansion. But in the case of water this law of expansion is set aside, and the reverse is established. This is done, however, only within a small range of temperature, viz., from the freezing point up the scale about seven degrees. In all degrees above that the usual expansion by heat takes place. The exception occurs at this part of the scale for a special purpose, viz., that water, in distinction from other substances, shall become more bulky, and therefore lighter, as it takes the solid form.

330. Description of the Process of Freezing.—In order to make the process of freezing clear to you I will describe it as it ordinarily occurs, that is, from the action of cold air upon the surface of water. The uppermost layer of the water imparts some of its heat to the air in contact with it. This air rises and colder air takes its place, which being warmed in its turn rises to make way for more of the cold air. You have therefore a constant current of warmed air upward from the water. In the mean time there is a current of a different character in the water—a downward one. As fast as the water at the surface parts with heat to the air it falls, other warm water taking its place, to cool in its turn and go down. This falling of the cooled water goes on regularly until a portion of water becomes cooled down to 39°, that is, 7° above the freezing point. This layer does not sink, but remains at the surface, for it is lighter than the warmer water below. This is because the law that heat expands matter is now reversed. Beyond this point of the thermometer the colder the water is the lighter it is. As the cooling now goes on from the air coming, as before, in successive layers to the water, the cooled water at the surface continually increases. At first it is a mere single layer of particles, but after a while it is quite a body of cold water lying on the warmer water below. At length some of it is cooled down to 32°, the freezing point, and a thin film of ice now forms. The state of things just at this stage of the process may be represented by a simple diagram, Fig. 222. Let the line a represent the film of ice. The space between a and b is the portion of water cooled down below 39°. The space below b is occupied by the water which is above this temperature. In the space between a and b the cooler the water is the nearer it is to the surface. That is, from the line b, where the water is exactly at 39°, as you go upward, the water lessens in temperature, it being successively 38°, 37°, 36°, etc., till, just in contact with the film of ice, a, it is at 32°. The ice goes on to thicken gradually by additions below. But it is to be remembered that ice is a good non-conductor, so that the very first layer of ice makes the cooling of the water proceed more slowly than before. And the thicker the ice becomes the slower is the cooling. This secures against too great a formation of ice.

331. Why the Above Exception to Expansion by Heat Exists.—That we may see the reasons in part for the grand exception to the general law of expansion by heat which I have illustrated, let us see what would be some of the results if the exception did not exist. In that case the process of freezing would be as follows: The water would communicate its heat from the surface to the air, as before described, and there would be a constant downward current of the cooled water. When any portion of the water became cooled by the air down to 32°, it would become ice, and would sink to the bottom. And after the process of freezing had once begun, there would be a continual accumulation of ice at the bottom so long as the air remained cold enough to cool the water with which it comes in contact down to 32°.

The result may be stated in the general thus: Freezing would not begin so quickly as it now does; but when once begun it would prove very destructive. It would not begin as soon, because the whole of any body of water must be cooled down to just this side of 32° before it could begin. This would not take long where the water is shallow, but it would where it is deep. All shallow bodies of water, then, would be frozen up quite early in the winter; and as water is a poor conductor, and thawing must go from above downward, some of them would not be thawed out again fully till quite into the next summer, if even then. And where the water is quite deep ice would at length begin to form, and when formed it would be exceedingly slow in thawing. In some cases it would never be thawed with such a body of non-conducting water to guard it against the warmth above. It is easy to see that the heat of spring and summer would not thaw out any thing like the quantity of ice that it now does. The reign of ice and snow on our earth would therefore be vastly more extensive than now, and what is worse, it would be extended more and more every year. Under such circumstances there would be great destruction of both animal and vegetable life. I will mention, however, but a single item, as it would occupy too much space to go into this subject with any fullness. In the water under the ice, which is always above 39°, except that which is close to the ice when freezing is going on, there is a vast amount of busy life which would be destroyed if ice were formed at the bottom, chilling all the water above.

332. Why the Freezing Point is at 32°.—If the freezing point of water were higher than 32°, freezing would occur so early in the autumn, and the ice and snow would last so late in the spring, that the season would be too short to raise our supplies of fruits and grains. If, on the other hand, it were at a lower point, the earth would not have the protection of its light coat of snow, but instead would be chilled by rains so cold that barrenness would be the result. The multitudes of animals, too, that now live so securely in the water, some of them even with the ice above them, would all perish with the cold.

333. Force of Expansion in Ice.—As ice occupies one seventh more room than the water from which it is formed, it exerts in its formation an expansive force which under various circumstances produces varied and often remarkable results. Of the many experiments which have been tried to show the force of this expansion I will mention but one. A bomb-shell was filled with water, at Montreal, and closed with an iron plug which was driven in with great force. The plug was thrown a distance of 400 feet by the expansion when the water froze. This expansion is sometimes an inconvenience to us, as in bursting water-pipes; but besides the great service which it does in the earth, already noticed, it is of service also in loosening the soil, and in supplying it with requisite ingredients from the rocks by breaking them up and pulverizing them in small quantities from year to year.