Similarphenomena to those that take place in liquefaction, occur in vapor izing any liquid. If a vessel of water be placed over the fire, a sound is pro duced by the successive vaporization and condensation of the particles in contact with the bottom of the vessel. As the liquid increases in heat, the sound becomes louder till it terminates in ebullition. At this point the tem perature ceases to rise, and remains stationary till the whole of the liquid is evaporated. To ascertain the quantity of heat consumed in vapourizing a given quantity of water, Dr. Black set a tin cup full of water at 50° on a red hot iron plate ; in 4 minutes it reached the boiling point, and in 20 minutes more it was all boiled off. From 500 to 212° the rise is 1620, which was gained in 4 minutes; but it took five times as long to be converted into vapour; hence 162 x 5=810° is the quantity of heat that disappears, or is rendered latent in the conversion of water into steam. By subsequent experiments, the latent heat of steam is found to be 967°, or 1,000°. The point at which liquids emit vapour of equal tension with the atmosphere, which is their true boiling point, differs in different liquids, as may be seen in the following table:— The boiling point of the same liquid varies with the atmospheric pressure, and also with the vessel the liquid is boiled in. Thus, in silver, the boiling point was found to be 211.775°, in common earthenware, 213.8°, at the mean pressure of the atmosphere. If the whole of the pressure be removed, liquors will boil and assume the vaporous state at 124° below their ordinary boiling points. Thus water will boil in vacuo at 88°, instead of 212° ; and alcohol, at 49°. Ou this principle Dr. Wollaston constructed his thermometric barometer, for mea suring heights. He found that a difference of 1° in the boiling point of water is occasioned by a difference of pressure equal to 0.589 of an inch on the barometer. If water be heated in a close vessel, or under extraordinary pres sure, its temperature may considerably exceed and as the steam will be always of the same temperature as the liquid, and will have its elasticity increased by heat, the vapour produced will considerably exceed the atmo sphere in elasticity, giving rise to what is called high pressure steam. At the freezing point of water, the vapour that rises will have sufficient elasticity to balance two-tenths of an inch of mercury in the barometer; at 212° it equals the atmospheric pressure (about 30 inches). Its elasticity at some other tem peratures is stated in the following table :— A pint of water at 40., on being converted into steam, forms 1,694 pints; or in round numbers, 1 cubic inch of water will form 1,728 inches, or 1 cubic foot of steam. We have already observed that the latent heat of steam is about 1,000°. This may be ascertained by evaporating a given weight of water, and condensing it into a known weight of cold water. This may be illus trated by an apparatus similar to the annexed cut. A given weight of water may be evaporated from the vessel a, the vapour of which will pass along the pipe c, and be condensed in the water in b. It will be found that the steam will raise the temperature of the water in b six or seven times more than an equal weight of boiling water would do. By ex perimenting in this manner, Dr. Ure has ascertained the latent heat of several vapours, as in the annexed table :— From the above table it will be seen that different bodies require different quantities of heat to enable them to assume the vaporous state. An analogous fact is, that different bodies require very different quantities of heat to elevate their temperatures a given number of degrees. if a pound of water at 60. be mixed with a pound of oil at 90% the resulting temperature will be 70* instead of the mean 75.. And conversely, if a pound of water at 90° be mixed with a pound of oil at 60°, the temperature of the mixture will be 80.. In the first experiment we see that the oil lost 20°, while the water only acquired 10° ; and in the second the oil gained 20°, while the water lost only 10°. Hence the specific heat of water is double that of oil; or the same quantity of heat that will raise the temperature of oil 20°, will only raise that of water 10°. The same fact may be shown by placing mercury, oil, and water, in an oven ; the mercury will be first heated, next the oil, and lastly, the water. An important practical illustration of the doctrine of specific heat is afforded by atmospheric air. The specific heat of air diminishes more slo4ly than its specific gravity. When air is expanded to a quadruple volume, its specific heat is 0.540 ; and when expanded to eight times the volume, its specific heat is 0.368. The den sities 1, 4, 4, 1, correspond nearly to the specific heats 5, 4, 3, 2. Hence may bee xplained the intense cold that prevails at the tops of high mountains, and also the great heat developed in the compression of gases. A compression equal to four-fifths is sufficient to ignite tinder ; and if a syringe of glass be used, a vivid flash of light is seen to accompany the compression.
We have now alluded to moat of the phenomena of heat that may be useftil in chemical investigations, except those which relate to the conduction and radiation, which we shall briefly illustrate.
It is well known that if a bar of iron, as a poker, be placed in the fire, the heat will in time be communicated to its remote end. It is also a matter of common observation, that a hot mass of iron, or a vessel of hot liquid sus pended in a room, will gradually become cooler until it attains the temperature of the surrounding medium. If we suppose a mass of iron, heated red hot, to be placed on a metallic pillar or support, in a still room, we shall find that it will lose its heat in three distinct ways. 1. If the metallic support be felt, it will be found to be hot, and we may consequently infer that a portion of the heat has been conducted away by its means. 2. If the hand be held over the hot body, considerably above it, a current of hot air will be perceived, which must convey another portion of caloric from the hot body. 3. If the hand be held at some distance from the side of the body, a distinct sensation of heat will be experienced ; and as this occurs when the hot body is inclosed in a vessel exhausted of its air, it is manifestly a different mode of cooling from the other two : in fact, a variety of experiments render it evident that the heat is pro jected from the hot body in right lines on every side. In the first of these modes of cooling, the heat is conducted slowly along the iron bar, which is denominated a conductor ; and the process is called the conduction of heat. In the second, the heat unites with the particles of air, and renders them speci fically lighter, in consequence of which they ascend, and another stratum of cooler particles descend and occupy their place; these in their turn become expanded and rise, and thus a constant ascending current is maintained. Caloric, therefore, is conducted from bodies in two ways ; it either imparts heat to the adjacent particles, which impart it to the next, and so on, without change of place, or it unites with the adjacent particles of the surrounding medium, and is conveyed upwards by the increased levity which it occasions. The third method of cooling in which the caloric is projected from the body in right lines, is called the radiation of caloric. The communication of heat by con tact is manifest in solids and liquids, although in the latter it is chiefly propa gated by the ascent of heated particles. If different solids be taken, and have one end exposed to a high temperature, one of them will become heated in a shorter time than another. Thus, if a piece of copper or iron wire, 3 or 4 inches long, be held in the hand by one end, while a spirit lamp is applied to the other, it will soon become so hot as to be intolerable ; while a glass tube, in similar circumstances, may be held within an inch of the flame with little inconvenience. The difference in the facility with which heat is transmitted
through bodies, will appear from the' following table:— From this table it appears that the metals are the best conductors of heat, though even among them there are striking differences. The different kinds of wood have very little conducting power, and hence are well adapted for handles to vessels that are exposed to heat. Bodies of a porous or spongy nature, especially fibrous substances, as wool, silk, feathers, fir, &c. are the worst conductors of heat; and from this circumstance derive their value as articles of clothing. It is, however, probable that the warmth of these substances is attributable rather to the impediments they offer to the motion of the air than from any inherent heat-retaining power. Confined air is a bad conductor of heat ; and if a quantity of it be enclosed among the interstices of the fur, wool, &c. it will furnish an effectual barrier to the egrets of caloric. On this account double windows and doors are found effectual in maintaining an equable temperature in our apartments. The conducting power of liquids by contact is so exceedingly small, that for a long time it was doubted whether they conducted at all. Accurate experiments made in vessels of ice, have, however, established the fact that liquids do conduct heat downwards, or by contact of their particles. If it be desirable to heat a liquid, it is well known that the heat should be applied at the bottom of the vessel, by which means the stratum of particles nearest the fire becomes lighter, and ascends, being forced up by the descent of the colder, and therefore heavier parts. This process continues until the whole has attained that degree of heat at which the liquid boils; the same occurs in heating a confined portion of air. Any circumstance that tends to impede the motion of the particles of liquids will diminish the facility with which they are heated or cooled. Water-gruel, soups, and other thick drinks, retain their heat for a considerable time; while more dilute liquids become cooled at the surface, the cooler parts subside, and the hot ones rise and come into contact with the atmosphere; these become cooled and sink, and thus the ptocess goes on till the whole attains the same temperature as the surrounding medium. It has been long known that the sun's rays proceed in right lines, and that they are capable of being reflected and refracted by mirrors and lenses so as to produce an intense heat. In like manner, if an iron ball be heated a little below redness, it will be found to emit rays of heat that are capable of being reflected and re fracted in a similar way. If two concave and polished metallic mirrors be placed opposite to each other, and at about eight or ten feet distant, as in the annexed engraving,) and the hot iron ball be placed in the focus of one, as at a, while in that of the other we place a piece of phosphorus b, resting on a lump of charcoal, or any bad conductor, in a few seconds the phosphorus will inflame. Now to produce this effect, it is manifest that rays of heat must have emanated from the iron ball, and falling on the nearer mirror, must have been reflected to the second mirror, by which they have been concentrated on the phosphorus. In this experiment we observe two important facts, the radiation and reflection of heat. Radiation may be considerably modified so as to be nearly destroyed by an alteration of the surface of the radiating body. Instead of the hot ball, Sir John Leslie used a tin cubic cannister filled with hot water ; and as a large body would stop the return of the rays, he used only one mirror, in the focus of which he placed one of the balls of his differential as here represented. Previous to placing the cubic canister a before the mirror b, its four vertical sides were coated with different substances—one with lamp-black, another with China ink, a third with isinglass, while the fourth was left naked, presenting a surface of polished tin. When this vessel, filled with hot water, was presented to the mirror, the ther mometer c immediately indicated an increase of temperature, varying according to the surface presented ; the lamp-black surface depressed the liquid of the thermometer 100°, the China ink 88°, the isinglass 80°, and the tin 12°. By a variety of similar experiments. Professor Leslie obtained the results in the fol lowing table:— The nature of the substance is not the only circumstance that influences radiation. In general, the more smooth and polished the surface, the more feeble in its radiating power. If the surface be roughened with a file, or other wise, its radiation is increased. It also appears that the radiation occurs not only from the superficial particles, but also from those immediately beneath them. With one coating of jelly it was found that the radiation was 38°; while a film of the same substance, four times thicker, produced a depression of 54°. When the thickness of the coating amounted to one-thousandth part of an inch, the radiation became diminished. If the same radiating surface be presented to different mirrors, we shall discover the differences in the reflective powers. By various experiments of this kind, the reflective powers of several substances were found to be as follows :— If we compare these tables, we shall find generally that the best radiators are the worst reflectors, and eke vend. It may easily be inferred, that those bodies that radiate most caloric, when heated above the temperature of the surround ing medium, will also absorb most rapidly when exposed to a temperature supe rior to their own. In the experiment with the lamp-black surface exposed to the mirror, the thermometer indicated a temperature of 100° ; if, however, the glass ball of the thermometer be covered with tin foil, the indication will be re duced to 20°. In the same manner, if the bright side of the canister be pre sented, the temperature will be 12°, but with the bulb covered, only 240. From these experiments, as well as from reasoning, it is evident that the power is equal to the radiating. Connected with this part of the subject is the effect of screens. When a thin deal board was placed between the canister and the focal ball, the thermometric effect was diminished, and this diminution was proportional to the thickness of the screen. A pane of glass interposed reduced the effect of radiation from 100° to 20°. The reduction was greatest when the screen was most distant from the canister : the thinnest gold leaf stopped the whole of the heat ; in general, those bodies intercept heat most effectually which are the worst radiators. From some more recent experiments of M. de la Roche, it is found that caloric acquires a more penetrating power as it proceeds from a source of higher temperature. A curious experiment was made by the Florentine Academicians, in which, instead of the hot canister, a large mass of snow wu placed before the mirror : in this case the ther mometer indicated a rapid depression of temperature, and it was at first inferred that rays of cold emanated from the snow and acted on the then mometer ; this supposition is, however, unnecessary, for it may easily be shown that all bodies radiate heat constantly. Even a mass of ice or snow may have its temperature higher than the surrounding air, and will, therefore, produce signs of heat in the thermometer. In the experiment just cited, the snow radiates caloric towards the mirror, and the thermometer, at the same time, radiates towards it, which is reflected towards the snow. If the snow were not placed in front of the mirror, the thermometer would receive as much caloric as it emits, and hence its temperature would remain constant ; but as the temper ature of the snow is lower than that of the thermometer, the latter receives less than it imparts, and its temperature falls.