« PreviousContinue »
(Comptes rendus, 23, 524), and also of Andrews (Qu. J. of Chem. Soc. 1, 27). 1.
b. According to the state in which one and the same vapour exists.
A given quantity of saturated vapour of any substance always contains the same total quantity of heat, whatever may be the pressure to which it is subjected, and whatever may be its elasticity and temperature: but of the heat thus contained in it, the quantity existing in the combined state increases as the pressure on the vapour diminishes, and consequently as its temperature falls-while, on the contrary, the quantity of free or uncombined heat increases as the pressure is increased, and consequently as the vapour becomes hotter. (Sharpe, Manch. Mem. 1813; also Ann, Phil. 19, 302. Clement & Desormes, Thénard, Traité de Chimie, ed. 4, 1, 81. Pambour, Institut, Nr. 256.)
One pound of saturated aqueous vapour, at any temperature whatever, forms, with 5} lb. of water at 0°, 63 lb. of water at 100°, i. e. the vapour parts with 550° of heat, in order to become liquid water. Such is the case with aqueous vapour of 100° C. and a tension = 1 atmosphere, aqueous vapour of 152° C. and a tension = 4 atmospheres, and aqueous vapour of various other temperatures and tensions. (Clement & Desormes.) The vapour of water contained in the atmosphere at 0° contains 650° of combined heat. The temperature and elasticity of vapour of water increase as its volume diminishes. In a vessel, whose sides would permit neither ingress nor egress of heat, saturated vapour of water at 100° might, by enlargement of the space, be converted into cold vapour of low tension, and, by continual narrowing of the space, into very hot vapour of high tension, without any liquefaction taking place in the latter case (unless the space were to become too small to allow of the water existing in the gaseous state).
Estimating the latent heat of vapour of water at 100° C. as equal to 550° C. (990° F.), and the free heat = 100° C. (180° F.), we have
At 650° (1202° F.), no more of the heat could exist in the latent state. This temperature probably corresponds to a degree of external pressure at which the expansion of water into vapour becomes impossible, and beyond which all the heat remains accumulated in the water in the free state. Aqueous vapour below 0° would contain so many more degrees of latent heat as its temperature was under 0°; e. g. vapour of water at - 20° C. would contain 670° of latent heat.
1. The law just stated is commonly known as Watt's law. According to Southern, on the contrary, the heat obtained by subtracting from the total heat the sensible heat indicated by the thermometer,—or that which is ordinarily called the latent heat of evaporation,-remains constant. The late elaborate researches of Regnault have shown that both these laws are incorrect.-Supposing the relation between the total quantity of heat and the temperature to be developed in a series of ascending powers of the temperatures, that is to say, of the form:
a= A + Bt + C + Dt + ... A, B, C.... being constants, the law of Watt would be expressed by a=A. Regnault, on the contrary, finds that the law may be represented, within the limits of error of experiment, by a = A + Bt,—and he obtains for A and B the values, A = 606-5, B = 0.305; so that the formula for calculating the total quantity of heat in steam at different temperatures becomes
a= 606:5 + 0.305 t. Calculated by this formula, the preceding table becomes: In Vapour at
682-7 [Vid. Works of the Cavendish Society, Vol. I. p. 294.] T.
D. Liquefaction and Solidification of Gases.
a. Liquefaction by external pressure and cooling. Since, according to the preceding, every gas takes up a greater space than the liquid or solid matter out of which it is formed, it must, when the space which it occupies is continually diminished, ultimately lose its gaseous form, and be thereby deprived of its heat of Auidity.-If the sides of the vessel could be heated from without at exactly the same rate as the gas
becomes hotter by compression; or, what comes to the same thing, if the compression could be produced in a vessel impervious to heat, liquefaction would not take place till a very considerable external pres
sure was exerted, sufficient to diminish the space within limits no longer compatible with the existence of the gaseous form. Under ordinary circumstances, however, the sides of the vessel retain their former temperature, and consequently deprive the gas, whose temperature has been raised by compression, of its excess of heat; hence the liquefaction is effected by a much smaller pressure. The gaseous particles, cooled down by the sides of the vessel, and thus rendered less elastic, are pressed together by the rest; and, according to the contraction of the space, there remains either no gas uncondensed, or a quantity whose density and elasticity are in accordance with the existing temperature.
Bodies, when not prevented by external pressure, or perhaps in some cases by cohesion, appear to be capable of combining with heat, and forming gas, even at the lowest known temperatures: hence it is probably impossible to produce liquefaction by cooling alone. Accordingly, most liquefactions of gases are effected by the joint action of cooling and pressure.—The smaller the affinity of the ponderable substance for heat, or, in other words, the less the elasticity of its vapour at the same temperature, the smaller is the amount of external pressure and cooling required for destroying its gaseous condition. Thus vapour of water at 100° C., at which temperature its elasticity = 0.76 met. of mercury, is liquefied by a pressure somewhat greater than 0.76 metre; and at 0, at which its tension = 0.00476 M. by a pressure somewhat greater than that.
Almost all the more permanent gases may be liquefied by pressure and cooling. Monge & Clouet liquefied sulphurous acid gas by cooling; Guyton Morveau liquefied ammonia; and Stromeyer arseniuretted hydrogen gas by similar means; but these experiments were disregarded, and their results partly attributed to the presence of small quantities of water, till Faraday (Phil. Transact. 1823, 160 and 189; also Kastn. Archiv, 1, 97), partly in conjunction with Sir Humphry Davy, made known the mode of liquefying a considerable number of gases.
Faraday's usual mode of proceeding is as follows. He introduces the liquid required for generating the gas into the shorter and closed arm of a strong glass tube bent at an angle, places over it some folded platinum foil, fills the longer arm with the
solid substance from which the gas is to be developed, seals the extrenity, raises the shorter arm so that the liquid may run down amongst the solid matter and act upon it, and then, after leaving the tube to itself for a day or two, dips the shorter arm into a freezing mixture, and the longer arm into warm water: the liquefied gas then collects in the shorter arm. For carbonic acid, the materials are; oil of vitriol and carbonate of ammonia; for sulphuretted hydrogen: concentrated hydrochloric acid and protosulphuret of iron; for oxide of chlorine: oil of vitriol and chlorate of potash; for hydrochloric acid: oil of vitriol and sal-ammoniac. In cases in which the development of gas does not take place without the application of heat, Faraday puts the whole of the ingredients into the longer arm:—for sulphurous acid: mercury and oil of vitriol; for cyanogen: cyanide of mercury; for ammonia: ammonio-chloride of silver; for chlorine: hydrate of chlorine. Oxygen, hydrogen, nitrogen, nitric oxide, phosphuretted hydrogen, and fluoride of silicon, Faraday did not succeed in liquefying.–Niemann (Br. Archiv, 36, 175) places the ingredients in a glass tube sealed at one end, bends the tube at an acute angle about 8 or 10 inches from the sealed end, and after sealing the other end, cools the shorter arm, the length of which is only 2 or 3 inches, by means of snow or cold water, while the
longer arm is heated by the sun, or by water at temperatures between 30° and 38°. The tube must be very strong, and should not be struck hard or touched with sand; for as long as the development of gas is going on, the danger of bursting is continually on the increase. The bursting is attended with violent detonation, the tube being often split into innumerable fragments (Niemann). Hence it is necessary to wear gloves and a mask, with thick glasses before the eyes.
Bussy effected the liquefaction of chlorine, cyanogen, and ammouiacal gas at a pressure but little superior to the ordinary pressure of the atmosphere, by transmitting these gases through a tube closed with mercury, widened, and covered with cotton at one part,—while sulphurous acid was dropped upon the cotton, and a stream of air directed on it (p. 274).— These gases may likewise be condensed in a freezing mixture made with chloride of calcium.-A tube, in which solid carbonic acid has been sealed up, is found to contain nothing but gas when heated, while liquid carbonic acid makes its appearance in the cold. (Mitchell.)
Faraday has lately succeeded, by the use of more powerful means, in liquefying all the known gases, with the exception of oxygen, hydrogen, nitrogen, nitric oxide, carbonic oxide, and coal-gas,--and solidifying a great number of them. (Phil. I'rans. 1845, I. 170; abstr. Phil. Mag. J. 26, 253.) The method adopted was to subject the gases to the joint action of powerful mechanical pressure and extreme cold. The first object was attained by the successive action of two air-pumps, the first having a piston one inch in diameter, the second only half an inch. The first produced a pressure of about 20 atmospheres, the second increased it to upwards of 50. The tubes into which the gas thus condensed was made to pass were of green bottle glass, from 1 to 1 of an inch in external diameter, and had a curvature in one
portion of their length adapted for immersion in a freezing mixture. The mixture employed consisted of solid carbonic acid and ether. The cold produced by it amounted to - 106° Fah. in the open air, and - 166° Fah. under the exhausted receiver of the air-pump.
Many gases, when subjected to this extreme degree of cold, were liquefied without the use of the condensing apparatus: this was the case with chlorine, cyanogen, ammonia, sulphuretted hydrogen, arseniuretted hydrogen, hydriodic acid, hydrobromic acid, carbonic acid, olefiant gas, nitrous oxide, and oxide of chlorine. Fluoride of silicon liquefied at a pressure of 9 atmospheres. The following were solidified when subjected to the action of the carbonic acid bath in vacuo: Hydriodic acid, hydrobromic acid, sulphurous acid, sulphuretted hydrogen, carbonic acid, oxide of chlorine, cyanogen, ammonia, and nitrous oxide. Faraday suggests the employment of solid nitrous oxide mixed with ether as a means of producing a lower temperature than any yet observed. The following gases did not solidify, even at the lowest temperature that could be obtained: Olefiant gas, finoride of silicon, fluoride of boron, phosphuretted hydrogen, hydrochloric acid, and arseniuretted hydrogen. --The six gases mentioned at the commencement of the preceding paragraph showed no signs of liquefaction when cooled by the carbonic acid bath in vacuo: hydrogen and oxygen at 27 atmospheres, nitrogen and nitric oxide at 50, carbonic oxide at 40, and coal-gas at 32 atmospheres. 1.
Every liquid may be regarded as a condensed gas, prevented by pressure from expanding itself in the gaseous form, but still retaining a portion of its latent heat, which probably bears a simple relation to the whole quantity of latent heat existing in the gas. In an empty space of infinite extent, all solid bodies would, at all temperatures, be converted into gas, without previously melting: but when the empty space is of limited extent, it becomes filled with the gas that is formed; and this gas exerts a pressure on the remaining portion of the solid body, and renders it capable of fusion.
The decomposition of a gas by cooling or pressure is always attended with the formation of a Cloud. This cloud is a mixtare of the uncon. densed gas with the very finely divided liquid or solid particles which separate from the gas, and produce cloudiness by irregular refraction of light or by their opacity. When these particles are solid, the cloud is sometimes designated by the term Fume.
Daniell's Sulphuric Ether Hygrometer (Qu. J. of Sc. 1820; also Gilb. 65, 169 and 403) simplified by Döbereiner and Körner (Gilb. 70, 135 and 139) and improved by Adie (N. Ed. J. of Sc. 1, 60; abstr. Schw. 56, 459) depends for its action on the condensation of the aqueous vapour contained in the air by cooling.
Sublimation and Distillation depend on the conversion of a substance into vapour and the condensation of the vapour in another part of the apparatus by cooling. The object of both these operations is, for the most part, to separate an easily vaporized body from one which is less volatile. The former is converted into vapour, while the latter remains as the residue of the sublimation or distillation,—the so-called Caput mortuum, or in the case of its being liquid, the Phlegma, &c. of the older chemists. The operation is called Sublimation, when the vapour conducted into the colder part of the apparatus condenses into a solid substance, and Distillation, when the vapour is converted into a liquid. In both operations the body to be vaporized is enclosed in an Alembic, Flask, or Retort. On the top of the alembic or flask is fitted a Head; and this, during the distillation is often connected with a Cooling or Condensing Tube which passes into the Receiver: when a retort is used, the vapour passes through the neck into the receiver, which is kept cool, and there condenses. À tube widened in the middle is often placed between the receiver and the neck of the retort.—The vapours may also be conducted from the retort, the flask, or the head of the alembic through a glass or metal tube, and there they may be condensed either in the manner recommended by Liebig, viz., by Weigel’s Condensing Apparatus (Taschenb. 1794, 129), in which the tube is surrounded with a metal cylinder through which cold water is continually flowing in a direction contrary to that of the vapour-or else by surrounding the tube with bibulous paper or linen kept moist by water constantly dropping on it.
Vapour, condensed into a solid body forms a Sublimate; that which condenses to a liquid, a Distillate. When the latter is subjected to a second distillation, to free it still further from less volatile bodies, the operation is called Rectification: by Cohobation is meant the distillation of the distilled product after it has been poured back either on what remains behind or on fresh material.
Since vaporization proceeds very rapidly in vacuo even at low temperatures, so likewise does distillation go on very quickly at low temperatures when the apparatus is exhausted of air, provided the receiver be kept colder than the retort. If, on the other hand, the apparatus be full of air
, the substance must be heated to the boiling point; otherwise the distillation will be very slow. It wil, howerer, be understood from what