Changes in bodies caused by Heat. 395 distribution of heat in liquids, the waters of the ocean do not indi cate, like the solid strata of the earth, any increase of temperature in proportion to depth, there can be no doubt that, at no great distance below the bottom of the deepest sea, subterranean heat exists. The numerous volcanoes which rise out of the bed of the sea, either active or extinct, in all parts of the globe, from Hecla and Jan Mayen in the north, to Mounts Erebus and Terror in the south, furnish sufficient evidence of the correctness of this inference. 579. The question here arises, What is the source of the heat thus proved to exist in the interior of our planet? No mode of motion, nor any conceivable cause of motion, has been suggested to account for it. The hypothesis of falling meteorites fails-of friction, or percussion, or of chemical combustion there is no evidence, and we can only fall back on the supposition that it is due to the retention of a portion of the primordial heat to which the earth has been subjected, as shown by the abundance and diffusion of igneous rocks over its surface. "The change of the quantity of heat in bodies is conveniently estimated by the concomitant change of their bulk, any substance so circumstanced as to allow this to be accurately measured, serving as a measurer of heat.” 580. If we heat a wire it is lengthened; if we heat water in a full vessel, a part runs over; if we heat air in a bladder, the bladder is distended in a word, if we heat any substance, its volume increases in some proportion to the increase of temperature—and this increase of volume admits of measurement. For the measurements of heat generally, a mercurial thermometer is commonly preferred to others. This depends for its action on the expansion of mercury in a closed vacuum tube. A mercurial thermometer is a small bulb of glass filled with mercury, and having a long narrow stalk or neck, in which the mercury rises when expanded by heat, and falls when heat is withdrawn. The stalk between the points at which the mercury stands when the bulb is placed first in freezing and then in boiling water, is divided into a convenient number of parts called degrees, which division appearing on a scale applied to the stalk, is continued similarly above and below these points. (See Art. Ther. mometer.) Heat, by entering bodies, expands them through a range 396 Physical conditions depending on Heat. grand antagonist and modifier of the effects of that attrac- 581. The solid, liquid, and gaseous states, are physical condi tions of matter depending on the amount of heat which penetrates the substance. By heating a solid we may cause it to pass through the liquid and the vaporous or gaseous conditions, and by withdrawing heat from the vapour or liquid, we may cause it to repass into the state of solid. Thus a body expanded by heat returns to its original condition on cooling. If an experimenter take a body which is as free from heat as human art can obtain it-a bar of solid mercury, for instance, as it may be produced by a bath of solid carbonic acid in ether-and if he then gradually heat such body, it will acquire an increase of bulk with every increase of temperature. At first there will be simple enlargement or expansion in every direction; then the mass will, in addition, be softened; then it will be melted or fused, that is to say, in the case supposed, the solid bar will be reduced to the state of liquid mercury, with the cohesive attraction of the atoms nearly overcome. If the mass be still farther heated, it will continue to gain bulk until, at a certain point, some of the atoms will be suddenly repelled from the mass and from one another to much greater distances, constituting then an elastic fluid called vapour or gas, many hundred times more bulky than the same matter in the solid or liquid state, and capable of forcibly distending an enclosing vessel, just as common air distends a bladder; susceptible, moreover, of dilating indefinitely farther, by farther additions of heat, or by diminution of the atmospheric or other pressurc, against which it had to expand during its formation. A subsequent removal of the heat from the gaseous fluid, will occasion a progress of contraction corresponding to the previous progress of expansion, and the various conditions or forms of the substance above enumerated, will be reproduced in a reverse order, until the mercury is again converted into a solid mass, as at first. What is thus true of mercury, is proved by modern chemical art to be true also of all the ponderable elements of our globe, and of many of the combinations of these elements-as water, for instance, which is familiarly known to us in its three forms of ice, water, and steam. Compound substances generally are decomposed into their con. Illustrative Examples. 397 stituent elements by great changes of temperature. Thus Mr. Justice Grove has proved that when a platinum ball, made white hot, is introduced into water, the elements, oxygen and hydrogen, are liberated as gases, and may be collected and re-converted into water by the application of a red heat. In the chemical decomposition of water, hydrogen alone is liberated, the oxygen entering into a new combination. This effect on water shows that, besides altering its physical condition, heat can exert a decomposing and a recomposing power on this compound. 582. In the above paragraph, mercury has been taken as an illustration; but this metal, which in this climate is naturally liquid, requires to be cooled to 72° below the freezing point of water before it will assume the solid state, and it must be heated to the very high temperature of 650° before it is converted into an elastic vapour. Hence the changes produced in mercury by heat could be made evident only under very exceptional circumstances. If we take a substance like camphor, which is already a solid, we may easily demonstrate the physical changes which are produced in it by heat and by the withdrawal of heat. A lump of camphor, placed in a retort and heated to 347°, melts or passes into the liquid state. If the temperature is raised to about 400°, it is rapidly converted into a transparent vapour or gas, which, on coming into the air, is deposited in white flocculent masses like snow upon all cool surfaces. Thus distilled from a short and wide-necked retort into a tall jar placed upright, it furnishes a beautiful illustration of the conversion of a solid into a liquid, this into a vapour, and the solidification of the vapour by mere cooling. Benzole, which is ordinarily seen as a liquid, becomes a solid at 32°, and a vapour at 177°. Ether, which is also a liquid at 60°, is converted into gas or vapour at 96o. Purc alcohol is a liquid below 174°, but an elastic vapour above this temperaturc. It has never yet been brought to the solid state, and thus it is most useful for measuring low temperatures. At 166° below freezing water, it acquires an oily consistency, and its indications of temperature below this, are uncertain. Sulphurous acid, which at ordinary temperatures is a gas, passes into the liquid state when cooled to 14°; but in order to maintain this condition, it must be kept in hermetically sealed glass tubes. If one of these be broken at the common temperature of the air, under a jar of mercury, it is instantly converted into a large volume of gas. On the other hand, liquid ether is converted into vapour or gas by passing a small quantity under a jar filled with water at or above 100°, and inverted in a basin of 398 Gases and Vapours. water at the same temperature. A teaspoonful of liquid thus forms a large quantity of vapour of ether, which is highly inflammable, and burns like coal gas. If removed to a basin containing water at 60°, the water rises and fills the vessel by condensing the vaporous ether to the liquid state. These facts shew that the solid, liquid, and aëriform or gaseous states of bodies depend essentially on the presence or absence of a definite amount of heat. 583. Gases and Vapours.-The difference between a gas and a vapour is, that a gas is permanently, what a vapour is temporarily. If we take two bladders provided with stop-cocks, and fill one with ordinary coal gas, while into the other, we pour about half an ounce of liquid ether, shut the stop-cock, and then immerse the bladder in water at 170°, the ether will be converted into vapour and expand the bladder just as if it had been filled with coal-gas. A small quantity of the contents may be burnt out of each bladder by opening the stop-cocks. The flames will appear precisely similar. If now the two bladders are placed in a dish, and cold water poured over them, the one with the coal gas will remain unchanged, while that which held the ether-vapour will collapse from its immediate condensation. In a vapour, as represented by ether, we have the gaseous condition of a liquid, the boiling point of which (96°) is above the ordinary temperature, while, in a gas represented by sulphurous acid, we have the vapour of a liquid, the boiling point of which (140) is below the ordinary temperature of the atmosphere. As these conditions depend on heat, so they vary with climate. In Siberia, during the winter season, sulphurous acid, unless artificially heated, could exist only as a liquid; while in Egypt and in Central Africa or India, ether could not exist as a liquid, except under pressure in closely secured bottles artificially cooled. The question then arises,―Are not the bodies which are called gases merely the vapours of liquids uncondensed? Faraday's researches have furnished an answer to this question. They have shown that with a few exceptions, gases are really the vapours of highly volatile liquids, the boiling points of which are far below the freezing or solidifying point of mercury. By compressing gases in strong tubes of glass, and at the same time cooling them to a very low degree of cold, he was able to bring a large number into a liquid state, and in some cases even into the state of solids. He found that many of them, like sulphurous acid, were liquefied by mere cooling. 584. For this purpose a bath of solid carbonic acid and ether was employed. It was found that a cold of — 106° F. was produced from Liquefaction of Gases. 399 these materials under exposure to air, and at this temperature eight gases, among which were chlorine and ammonia, passed at once into the liquid state.* By employing this bath in the vacuum of an air-pump, a more intense degree of cold, —166° F. or 198° below the freezing point of water, was produced. There were six gases which still remained unchanged at this low temperature, and among these were the important elements, oxygen, hydrogen, and nitrogen. Even when this degree of cold was conjoined with great pressure, these gases still resisted liquefaction. Since these experiments were performed, a lower degree of cold has been produced by Natterer, in employing in vacuo a bath of liquid nitrous oxide and sulphide of carbon. It was estimated in this case at 220° F., or 252° below the freezing point of water. When exposed to this degree of cold, the gases above-mentioned did not change their state—they were not liquefied. A student might at first have difficulty in believing that the beautiful variety of solid, liquid, and gaseous conditions found among natural bodies, could depend upon the quantities of heat in them, because these forms are all seen existing at the same common temperature; but he will soon learn that each substance has its peculiar relation or affinity to heat, and that hence, while, at the medium temperature of the earth, some bodies contain so little as to have a solid form-like the metals and earths; others have enough to be liquids-as mercury, water, and oils; and others have enough to be gases—as oxygen, nitrogen, and hydrogen. *The intensity of the cold produced by this bath may be estimated from the following experiment performed by Faraday in the presence of the writer. On a warm summer's day, after a lecture at the Royal Institution, a small quantity of mercury was poured into a groove or trough roughly made by folding brown paper. It thus formed a silvery stream of about one quarter of an inch wide and eight inches long. The surface of the liquid mercury was covered with solid carbonic acid, and a small quantity of ether was then poured over it. The carbonic acid was at once liquefied, and the mercury, by cooling, was converted into a pliable metallic bar. It could be touched only, like the solid carbonic acid, with wooden forceps A cold knife cut through it as a very hot knife would cut through wax or butter. A portion of the silvery bar was thrown into a glass of water. The mercury was instantly liquefied and the water frozen to solid ice. Heat was thus transferred from the water to the mercury. The solid metal became a liquid, and the liquid water became a hard solid by the exchange. |