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MEASUREMENT OF THE LATENT HEAT OF VAPOURS.

387

glass tube bent twice at right angles, and terminating in a bulb at each extremity; as shown in fig. 139. In making the instrument, one of these bulbs is partially filled with water, which is then made to boil briskly; the steam thus generated expels the atmospheric air through a capillary opening left in the other bulb, and when the instrument is thus freed from air, and filled only with water and vapour of water, the aperture is sealed. To make use of it, the water is all collected into one bulb, and the empty bulb is plunged into a freezing-mixture; the aqueous vapour which this bulb contains is thereby condensed, and evaporation occurs rapidly from the surface of the liquid in the other bulb; its sensible heat is thus diminished, and the water in a few minutes begins to freeze. The bulb containing the water should be protected from currents of air by enclosing it in a glass, as shown in the figure.

(186) Measurement of the Latent Heat of Vapours.-Equal masses of different liquids require very different quantities of heat to convert them into vapour. The quantity of heat which is thus rendered latent may be determined by distilling over a given mass of the liquid, and condensing it in a large volume of water, the temperature of which is noticed before and after the experiment. Suppose the latent heat of steam to be 536° C.; a litre of water converted into steam would on recondensation raise the

improvements in the mounting. A simpler form of apparatus, in which the evaporation of ammonia is made use of for the production of a low temperature, was exhibited in London at the International Exhibition of 1862 by MM. Carré and Co.

FIG. 140,

Fig. 140 represents the apparatus: A is a strong boiler of wrought iron, filled for three quarters of its capacity with a concentrated solution of ammonia; B, a wrought iron annular condenser, shown in section, connected with the boiler by pipes specially arranged with a view to prevent the liquid from boiling over into the receiver. In order to use the instrument, the boiler is laid upon its side, with the condenser upwards, for about ten minutes, so as to allow all the liquid to drain back into the boiler, the expulsion of the liquid being facilitated by heat- C ing the condenser with a lamp. The boiler is then heated very gradually by means of a stove or large gas-burner, and the condenser plunged into a vessel of cold water, through which a stream of cold water is kept running. A little water is placed in the cup, D, in the bottom of which is a screw-cock communicating with the interior. Distillation is next proceeded with, until the temperature of the boiler has risen to about 270° (132° C.), when the ammonia will have been in a great measure expelled from the liquid, and condensed in the receiver under the pressure of its own particles. The boiler is then withdrawn from the fire, the water in the cavity, B, is allowed completely to drain away, a cork is put into the hole at the bottom of the space, B, and a little alcohol is poured into the cavity, after which the vessel containing the water to be frozen is introduced. The boiler is plunged into a vessel of cold water, and kept cool by a rapid current of cold

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MEASUREMENT OF THE LATENT HEAT OF VAPOURS. [186.

temperature of 10 litres 53°6 C. It is found that a litre of water, if converted into steam of 100°, and condensed again into the liquid form, would raise about 5 litres of water from o° to 100° C.

FIG. 141,

We owe to Andrews (Q. J. Chem. Soc. 1848, i. 27) a careful determination of the latent heat of a number of vapours: fig. 141 shows the mode of procedure which he adopted. The liquid to be tried is placed in the flask A, the neck of which has a very short bend, and is connected with a glass receiver, B, furnished with a spiral condensing-tube, terminating at d; this receiver is placed in a vessel c, with a considerable quantity of water, which has been accurately weighed. The liquid is distilled over into B: the quantity that condenses is carefully weighed, and the rise of temperature experienced by the water used for condensation is estimated by a very sensitive thermometer, t. The whole is enclosed in an outer tin-plate vessel furnished with a lid, which acts as a screen, and it is further protected from the radiation of the lamp by the tinplate screen R; s is a light glass tube for agitating the water. The result obtained has, however, to be corrected by other experiments for the heat absorbed by the metallic parts of the apparatus, and for that which is lost by radiation during the time that the experiment lasts; allowance has also to be

water, whilst the condenser, c c, is wrapped in flannel, and the apparatus is left to itself. As the water in A becomes cool, it re-dissolves the condensed ammoniacal gas which evaporates rapidly from the vessel, c, producing a great and sudden depression of temperature.

If air finds admission to the interior, the rapidity of congelation may be greatly reduced. In such a case it becomes necessary to get rid of the air, and it may be expelled in the following manner:-The temperature of the boiler having been raised to about 140° (60° C.), water having been placed in the cup, D, the screw at the bottom of the cup which communicates with the receiver is slightly relaxed; if air be present, the disengaged gas will carry a portion of the air with it, which will rise to the surface of the water, whilst the ammonia is dissolved: as soon as the escaping gas is wholly dissolved by the water, the screw must be closed firmly, and the distillation proceeded with as above directed.

M. Raoul Pictet, of Geneva, has employed liquid sulphurous anhydride, condensed by a pressure of two or three atmospheres, and allowed to volatilize in a vessel from which the gas is removed by a pump and returned to the condenser. As a circulating fluid, dilute glycerin is used instead of salt water.

Mr. King has recently contrived a machine in which, by means of the expansion of compressed air, he is enabled to produce an amount of cold equal to that generated by the ether machine for an equal consumption of coal. (Jas. Young)

186.]

LATENT HEAT OF VAPOURS.

389

made for the heat which the condensed liquid has given out after its liquefaction, in cooling down from its boiling-point to the temperature of the water used in the condenser.

The results obtained in this delicate branch of inquiry by Despretz, and by Brix, which, however, embraced a much smaller number of liquids, agree pretty closely with each other and with the experiments of Andrews. These results, with some of those obtained by Favre and Silbermann, who also have made numerous experiments upon this subject (Ann. Chim. Phys. 1853 [3], xxxvii. 461), are given in the following table:

The numbers which represent the latent heat of equal volumes of each vapour are obtained by multiplying the numbers in the fourth column by the molecular weight of each compound, divided

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LATENT HEAT OF VAPOURS.

[186. by 18, the number for the molecule of aqueous vapour, H2O. The numbers contained in the second column indicate the quantities of water in grammes, the temperature of which would be raised 1o C. by condensation into the liquid form of a gramme of the vapours of each of the various liquids mentioned; the liquid condensed being supposed in each case to be at the temperature of its own boiling-point. For instance, the conversion of one gramme of steam at 100° C. into water at 100° would raise 536.67 grammes of water from o° to 1° C. So the condensation of one gramme of the vapour of alcohol at 78°-4 into liquid alcohol at 78°4 would heat 208-92 grammes of water from o° to 1° C.

(187) The density of steam increases directly as its pressure. Watt concluded from his experiments that the same mass or quantity of steam, whatever its density may be, contains the same quantity of heat, its latent heat being increased in proportion as its sensible heat is diminished or absorbed.

For in

But only

Regnault has, however, shown by a series of laborious experiments, that although the assumption of this rule may not lead to serious errors in practice, and although, consequently, there is but little saving of fuel in performing evaporations at a low temperature, yet that it is not strictly correct. It is true that the latent heat decreases as the sensible heat rises, but this diminution is less rapid than the rise in sensible heat; for, in reality, the sum of the latent and sensible heat increases as the temperature rises, by a constant quantity, equal for each degree to °305 c: this may be seen in the subjoined table, in which it is assumed that the sensible heat of steam may be neglected for all degrees below zero Fahrenheit::

391

188.]

DISTILLATION.

Latent and Sensible Heat of Steam at different Temperatures.

It must be borne in mind that equal volumes of different liquids produce very different volumes of vapour. Water furnishes a much larger volume than any other liquid, a cubic inch of water at 100° C. expanding to nearly a cubic foot of steam at 100°, or more accurately to 1694 times its volume. The following table shows the volume of vapour which is furnished by a cubic inch of four different liquids, at their respective boiling-points. Equal volumes of different vapours, taken at the boiling-points of their respective liquids, consequently possess very different densities, as is shown by the last column of the table:

(188) Distillation.-The rapid formation of vapour during ebullition is often made use of by the chemist for the purpose of separating liquids from solids,-as in the ordinary case of distilling water to free it from the impurities dissolved in it, or for the separation of two liquids which differ in volatility, as in procuring spirit of wine from a fermented liquor. In such operations the arrangements for condensation acquire considerable importance; they are of various kinds, but the worm-tub is the apparatus most usually employed it consists of a spiral pipe called a worm, which is