400 Effect of Heat on Solids. Fusion. The degrees in a general scale of temperature at which the sub. stances most important to man change their states from solid to liquid, or from liquid into gas, will be noted in a future page. 585. Good conducting solids, in melting, melt as soon in the centre of the solid as on the surface, such as lead and bismuth. Those which are bad conductors, on receiving heat become very soft before they are liquefied, as wax, pitch, glue, and glass; but the greater number become liquid at once, as ice in becoming water; and some pass at once into the state of vapour or gas, without having assumed at all the intermediate state of liquid. These last are sublimed, as it is called, and on cooling again may be caught in a powdery state, as seen in that form of sulphur, or of benzoin, termed the flowers of these substances. Of this class, also, are arsenic and the substance called iodine, which last, from the state of metallic-looking crystals, becomes at once, on being heated, a dense transparent gas of a rich purple hue, and in cooling re-assumes its solid crystalline form. Many solids, chiefly of the organic kingdom, such as wood, starch, gum, and ivory, are not melted by a strong heat, but undergo chemical changes; in other words, they are decomposed, and new compounds result. 586. Some metals, almost infusible alone, readily melt when heated with other metals. Thus, on heating together platinum and antimony in a spirit flame, the platinum, which is almost infusible, is readily melted, and forms an alloy with the antimony. There is a combination of three metals, which is remarkable in this respect, that it melts at the temperature of boiling water (212°). It is called fusible metal, and is composed of two parts of bismuth, one of lead, and one of tin. The lowest melting point of these three metals is that of tin, 442°. The melting points of some substances serve to measure temperature. All the under-mentioned bodies melt at what is called a black heat, i.e., a heat not visible in the dark. These are called their points of fusion :— Solidification of Gases. 401 There are slight variations in these melting points, in the tables given by different writers. It has been pointed out as a curious fact that no fusible solid can be heated above its true melting point without being liquefied; but some liquids can be cooled below the solidifying point mentioned and still remain liquid. This is the case with water. The melting point cf ice is always 32°, but the freezing point of water contained in glass tubes may be carried to 24° below this, and even if the tube be capillary, nearly to zero F. Saline matter dissolved in water lowers the freezing point. One part of common salt in four parts of water freezes at 4°. The freezing point of sea water, which contains between three and four per cent of salt, is 28°. There is a remarkable difference among solids in this respect. One is known to exist only in the solid state, namely carbon in the native form of diamond. The most intense heat of the voltaic arc does not cause its liquefaction or volatilization. It is merely converted into black amorphous carbon. Among liquids, alcohol has never been solidified; and among gases, oxygen has never been liquefied or solidified. 587. Solidification of Gases.—It has been shown by an illustration of the properties of camphor how a vapour may be solidified (Art. 582). It is the mere result of the withdrawal of heat or of cooling, under peculiar conditions. Ammonia, chlorine, and carbonic acid gases have been converted from liquids into solids. The lastmentioned gas will serve to illustrate the principle. Carbonic acid liquefied in a wrought-iron vessel is allowed to escape in a gaseous state through a perforated brass cap. So great a cold is produced by its sudden expansion, that a quantity of the gas is solidified in a snow-like form in the brass capsule, and it retains its solid state for some time in a cold vessel. The liquefaction of gases has been brought into commercial use. Thus, liquefied ammonia or sulphurous acid, by the intense cooling which each produces in assuming the gaseous state, have been used for converting water into thick slabs of ice in a few minutes. "Heat diffuses itself among neighbouring bodies until all have acquired the same temperature; that is to say, until all will similarly affect a thermometer." 588. An iron bolt thrust in among burning coals soon becomes red-hot like them. If it be the heater of a tea-urn, it will, when afterwards placed in its receptacle amidst the water, give part of 402 Heat and Cold Relative Terms. its lately-acquired heat to the water, until the water boils. Boiling water, again, soon imparts heat to an egg placed in it. A hundred objects enclosed in the same apartment, if tested after a time by a thermometer, will all indicate the same temperature. ་ 589. Heat and Cold.-When the hand touches a body of higher temperature than itself, it receives heat according to the law just explained, and it experiences a peculiar sensation called that of warmth when it touches a body of lower temperature than itself, it gives out heat for a like reason, and experiences another and very distinct sensation called that of coldness. Now warmth and coldness, considered as existing in the bodies themselves, although thus appearing opposites, mark only different degrees of the same object, temperature, contrasted by name for convenience-sake, in reference to the ordinary temperature of the persons speaking of them—just as any two nearest mile-stones on a road, although merely marking degrees of the same object, distance from a place, might receive from persons living between them the opposite names of east and west, or of north and south. It is to be remarked, moreover, that the sensation of heat is also producible by a substance colder than the hand in its ordinary state, provided it be less cold than some other substance touched immediately before, or than the usual temperature of the place; and the sensation of cold is producible under the opposite circumstances of the hand touching a comparatively warm body, but which is less warm than something touched just before. This explains the fact that the same body may at the same time, and to the same person, appear both hot and cold. Let two basins be filled with equal quantities of water-one at about 40°, and another at 90°—and a hand placed in each. In the former there will be a strong sensation of cold, and in the latter of warmth. The water in the two basins may now be mixed in a larger vessel, and both hands plunged into the mixture. Although the temperature, as indicated by a thermometer, is 65°, the water will feel warm to one hand and cold to the other, but there is the same amount of heat in both. A cellar of which the temperature does not vary, feels warm in winter and cold in summer. For a like reason, a person from India, arriving in England in the spring time, deems the air cool, while the inhabitants of the country may be diminishing their clothing because the heat to them might appear oppressive. Such facts as these show that heat and cold are relative terms depending on pre-existing sensations, and that that which is cold to one person may be warm to another. It is necessary, therefore, to look for more correct indications of temperature than our bodily sensations. Conduction of Heat. 403 Water as ice feels cold, but a thermometer may be cooled to ncar the freezing point of mercury (-40° F.). By plunging a thermometer so cooled into ice, the mercury in the tube will rise, showing that it is warmed by the ice, and demonstrating that the ice contains heat. "Heat spreads through solid substances by conduction, as it is called, with a progress proper to each substance.” 590. If one end of an iron rod be held in the fire, a hand grasping the other end soon feels the heat coming through it. The shorter and thinner the rod the more rapidly is this sensation perceived, and if a copper rod is substituted for the iron rod the heat is sensibly felt much sooner. Through a similar rod of glass, the transmission is much slower, and through one of wood or charcoal it is slower still. The hand would suffer pain from holding the iron before it felt any warmth in the wood or charcoal, although the inner end of the wood were blazing and the charcoal were red-hot. These facts show that different substances conduct heat with different degrees of rapidity, and on this property many interesting phenomena in nature and in the arts depend. Hence it is important to ascertain the rate of transmission, and to classify the substances accordingly. Various methods for this purpose have been adopted. For solids-similar rods of different substances, after being thinly coated with wax, have been placed with their ends in hot oil, and then the comparative distances to which in a given time the wax was melted, furnished one set of indications of the comparative conducting powers. Another method consisted in heating to the same degree equal masses of different substances, with a central cavity in each containing a thermometer. The substances were then plunged into the same bath to cool, until the thermometer fell in all to a given point; the differences of time which elapsed gave the relative rates of cooling. 591. The following is a simple method of showing the relative conductivity of solids. Place bars of copper and iron about nine inches long and a quarter of an inch in thickness on a block of wood, and by the side of them a similar bar cut out of charcoal. The ends of the three bars should be brought in contact, so that they may be at once easily moved into a wide spirit-lamp flame. They should so diverge as to be two or three inches apart at the opposite ends. Place three small fragments of phosphorus at equal distances on each of the bars, the first being placed at about three inches from the 404 Conductivity of Metals. flame, and the others at three inches apart. Gently bring the ends of the three bars into the flame, so that they may be equally heated, and place a screen so as to cut off draughts of air. It will be observed that the first phosphorus on the copper will soon melt and take fire. This indicates a temperature of 113°. At an interval after this, the first on the iron. The three portions on the copper will generally be melted and ignited by the time the second on the iron is reached. Charcoal, being a non-conductor, becomes red-hot at the heated end and burns before the first phosphorus on it is melted, provided we guard against the radiation of heat. A silver and an electro-plated tea-spoon may be distinguished by a difference in conductivity. If a small fragment of phosphorus is laid on the extremity of the handle of each teaspoon, and the bowls of the spoons are then dipped into boiling-hot water, the silver will be indicated by the earlier ignition of the phosphorus. The handles of silver teapots and kettles are frequently divided, and thin layers of ivory introduced for the purpose of preventing the rapid conduction of heat by this metal, which, in conducting power, takes precedence of all other metals. These and other experiments have shown, as a general rule, that density favours the passage of heat through a solid. Thus, the best conductors are the metals, and then follow in succession, diamond, glass, stones, earths, woods, &c, as here noted. The numbers standing after the names, mark the approximate conducting powers of metals in relation to silver, taken as a standard and called 100. These results have been obtained by Messrs. Wiedemann and Franz by the use of a thermo-electric pile (to be afterwards explained), applied to the extremities of bars of the different metals of similar size and treated under similar circumstances. These figures differ greatly from those which had been previously obtained by Despretz and hitherto relied on by physicists. 592. The presence of impurities, such as arsenic or carbon, in metals lowers their conducting power for the electric current, and |