382

MEASUREMENT OF HEIGHTS BY BOILING-POINT.

[182.

once made to enter into ebullition by placing it under the receiver of the air-pump, and exhausting the air; by this means water may be made to boil at a temperature of 70° (21° C.). Indeed liquids in general boil in vacuo at from 60° to 140° (33° to 77° C.) below their ordinary point of ebullition when under a barometric pressure of 760mm.

This result may be shown by boiling some water in a Florence flask, and corking up the flask whilst the steam is escaping rapidly. Upon pouring cold water over the upper part of the flask the steam is condensed, its pressure is removed, and the water begins to boil briskly; but in this case the bubbles nearly all rise from the surface, not from the bottom of the liquid. A simple proof that steam from boiling water possesses an elasticity equal to that of the atmosphere is obtained by repeating the last experiment with a tin canister instead of a globular flask. On corking up the canister and pouring cold water over it, the steam within is suddenly condensed, a vacuum is produced, and the canister is crushed in by the pressure of the external air.

The reduction of temperature at which boiling takes place is advantageously applied in the preparation of vegetable extracts the medicinal properties of which would be impaired by the ordinary temperature of 100° C., and by exposure to the air. The apparatus consists of a still and a receiver, which are connected by an air-tight joint, and are filled with steam to expel atmospheric air, and then hermetically sealed; on cooling the receiver, rapid evaporation and ebullition take place at a temperature much lower than that of the usual boiling-point of the liquid. A modification of this process is used in the manufacture of sugar, both in the concentration of the cane-juice and in the subsequent evaporation of the syrup.

(183) Measurement of Heights by the Boiling-Point.-As might be expected in consequence of the diminution of atmospheric pressure, it is found that on ascending from the earth's surface the temperature at which water boils becomes gradually lower. In descending a mine the effect is reversed, and the boiling-point becomes proportionately elevated. De Saussure observed that on the summit of Mont Blanc, which is 15,650 feet (nearly three miles) above the sea-level, water boils at 18507 (85°4 C.); and Wisse determined the boiling-point upon Mount Pichincha, at an altitude of 15,668 feet (4775'6 metres), to be 185°29 (85°16 C.) whilst the barometer stood at 437mm.6. The observation of the point at which water boils at any particular elevation furnishes an easy means of determining its altitude above the sea-level; a difference of about 327 metres in elevation produces a variation of 1o C., or 596 feet of ascent produces a variation of 1° F. in the boiling-point of water.

The following table shows the temperature at which water boils at the corresponding heights of the barometric column, calculated by Regnault, and confirmed by direct observation:

184.]

HIGH-PRESSURE STEAM.

Boiling-Points of Water at different Pressures.*

383

The necessity of attending to the height of the barometer at the time of making a careful observation upon the boiling-point of any liquid will now be obvious. It has been ascertained that at pressures near 760mm. of mercury a variation in the height of the barometric column of about 27mm. makes a difference of 。° C., or that of one-tenth of an inch in the barometric column makes a difference of more than a sixth of a degree F. in the boiling-point; so that within the range of the barometer in this climate the boiling-point of water may vary nearly 3° C., or 5° F.

(184) High-Pressure Steam.-As a reduction of the pressure lowers the boiling-point, so an augmentation of the pressure raises it. To demonstrate this fact an apparatus has been contrived, consisting of a small iron boiler (fig. 137), furnished with three apertures in the lid, through one of which a thermometer

* For an extended table of this kind, vide Regnault, Ann. Chim. Phys. 1845 [3], xiv. 206; or Dixon On Heat, p. 269.

384

FIG. 137.

stem is passed air-tight; through the second, a long glass tube open at both ends is inserted; the lower extremity of this tube plunges below the surface of mercury placed in the boiler, above which a quantity of water is introduced; the third aperture must be furnished with a stop-cock. It will be found, on applying heat, that so long as free communication with the atmosphere is permitted through the open stop-cock, the temperature of ebullition will remain steadily at 100° C.; but by closing the cock, the steam may be confined, and as fresh portions of steam continue to rise from the water, the pressure on the surface increases, as is shown by the rise of the mercury in the open tube; the boiling-point also becomes higher; until when the mercury stands at 760mm., or 30 inches, and the pressure on the surface is equal to that of an additional atmosphere, the thermometer marks a temperature of 120°.8 C. By continuing the heat without allowing the steam to escape, the boiling point rises still higher and the pressure of the steam increases with increasing rapidity as the temperature rises, as is shown by the following table, founded on the experiments of Regnault :

These results differ but little from those obtained under the direction of Dulong and Arago, by a commission appointed for the purpose many years ago by the French Government. They found the temperature of steam of 20 atmospheres to be 417°4 (214°2 C.), and calculated that if the pressure rose to 50 atmospheres the temperature would amount to 510°4 (265°.8 C.).

185.]

PRODUCTION OF COLD BY VAPORIZATION.

385

It will be observed that the increase of pressure, for equal increments of temperature, is more rapid at high than at low temperatures, and this circumstance (in addition to the greater simplicity of construction of the machinery in high-pressure engines) is one of the principal reasons for the increased economy of power obtained in employing high-pressure steam as a motive power, when compared with that furnished by the use of lowpressure engines. But it is only when in contact with a body of water from which fresh steam is constantly rising, that the pressure augments in this manner, and thus is sufficient to rend asunder the strongest boilers. If dry steam alone be heated, it follows the law which regulates the expansion and pressure of gaseous bodies in general (134, 197).

High-pressure steam whilst confined is always of the temperature of the water from which it is produced; it is, therefore, often used in the arts to supply a steady temperature above that of 100° C. It is found that the solvent powers of water are much increased by the elevation of temperature caused by preventing the free escape of the steam. Papin's digester is an apparatus designed to effect this object; it is simply a strong iron vessel, furnished with a safety-valve for regulating the pressure at which the steam is allowed to blow off. The water may thus be kept steadily at any required temperature above 100° as long as is requisite. The gelatin of bones may by this means be easily extracted from the earthy matter, although the bones may be boiled for hours in water at 100° C. without undergoing any such change.

(185) Production of Cold by Vaporization. In all cases, whether volatilization occur above the usual boiling-point or below it, heat is absorbed in large quantity. If a few drops of ether be allowed to fall on the hand, the liquid disappears rapidly in vapour, and produces the sensation of cold. Indeed, the boiling of one liquid may be attended with the freezing of another which is brought into its vicinity. Place, for example, a drop or two of water between two watch-glasses, pour a little ether into the upper glass, and, having introduced them into the receiver of the airpump, exhaust the air; the ether will speedily boil, and the water between the two glasses will be frozen, by the rapid abstraction of heat which it has experienced during the conversion of the ether into vapour. Water, as Leslie has shown, may even be

Mr. Harrison has contrived an ingenious freezing-apparatus upon th's principle: one form of the instrument is figured in the Pharmaceutical Journal

386

PRODUCTION OF COLD BY PROCESSES OF EVAPORATION.

FIG. 138.

[185.

frozen by the rapid absorption of heat occasioned by its own evaporation. This experiment may be performed by supporting a watch-glass (T, fig. 138) containing water, over a dish of oil of vitriol, s, under the receiver of the air-pump, P. A piece of cork hollowed at its upper surface, and charred by being placed in a flame, furnishes a good support for the water in consequence of its being a bad conductor of heat. On exhausting the air, the water evaporates quickly, the vapour being

removed with great avidity by the oil of vitriol as fast as it is formed; and in two or three minutes the water which remains in the watch-glass becomes converted into ice.

Water is also frozen by its own evaporation in the Cryophorus, which derives its name from κpúos, frost, popòs, bearing, in allusion to its mode of action;

FIG. 139.

condensation of the vapour being effected by the application of a freezing-mixture, at a distance from the evaporating surface. The apparatus consists of a long

(1857, xvi. 477). About ten gallons of ether are placed in a small metallic multitubular boiler, which is immersed in a strong solution of salt and water, contained in a wooden trough cased in a non-conducting material. The boiler is connected with an exhausting pump, by working which the ether is caused to evaporate rapidly, and the temperature of the boiler is proportionably reduced, at the expense of that of the salt water around it. This salt water is made to flow off gradually, through a channel containing a series of metallic vessels, each capable of containing 14 or 15 lb (5 or 6 kilogr.) of the water to be frozen. The salt water, which is reduced at first to a temperature of about - 4°5, is returned again by a small pump when its temperature has risen to about -2° C. and it is again made to flow around the boiler in which the ether is evaporating, so that a perpetual circulation of cold salt water is maintained: the ether, the volatilization of which has caused the reduction of temperature, is condensed in a worm by means of a current of cold water, and is returned, with scarcely any loss to the boiler. Mr James Young found in working the apparatus on a large scale that, for every ton of coal consumed in working the pumps, I ton of ice was produced, though this probably is not by any means the maximum quantity of ice which the apparatus could produce by suitable