strong sulphuric acid, a substance which has a great affinity for water, and above it a thin, shallow, porous capsule (fig. 187) containing a small quantity of water. By exhausting the receiver the water begins to boil, and since the vapours are absorbed by the sulphuric acid as fast as they are formed, a rapid evaporisation is produced, which quickly effects the freezing of the water.

By using liquids more volatile than water, more particularly liquid sulphurous acid, which boils at -10°, a degree of cold is obtained sufficiently intense to freeze mercury. The experiment

may be made by covering the bulb of a thermometer with cotton wool, and after having moistened it with liquid sulphurous acid, placing it under the receiver of the air-pump. When a vacuum is produced the mercury is quickly frozen.

Thilorier, by directing a jet of liquid carbonic acid on the bulb of an alcohol thermometer, obtained a cold of -100° without freezing the alcohol. With a mixture of solid carbonic acid, liquid protoxide of nitrogen and ether, M. Despretz obtained a sufficient degree of cold to reduce alcohol to the viscous state.

By means of the evaporation of bisulphide of carbon the formation of ice may be illustrated without the aid of an air-pump. A little water is dropped on a small piece of wood, and a capsule of thin copper foil, containing bisulphide of carbon, is placed on the water. The evaporation of the bisulphide is accelerated by means of a pair of bellows, and after a few minutes the water freezes round the capsule, so that the latter adheres to the wood.

CHAPTER VIII.

LIQUEFACTION OF VAPOURS AND GASES.

240. Liquefaction of vapours.-The liquefaction or condensation of vapours is their passage from the aëriform to the liquid state. Condensation may be due to three causes-cooling, compression, or chemical affinity.

When vapours are condensed, their latent heat becomes free, that is, it affects the thermometer. This is readily seen when a current of steam at 100° is passed into a vessel of water at the ordinary temperature. The liquid becomes rapidly heated, and soon reaches 100°. The quantity of heat given up in liquefaction is equal to the quantity absorbed in producing the vapour.

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Liquefaction of Gases.

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Liquefaction by chemical affinity.-The affinity of certain substances for water is so great as to condense the vapours in the atmosphere, even when they are far from their point of saturation. Thus, when highly hygroscopic substances, such as quicklime, potass, sulphuric acid, are exposed in the air, they always absorb aqueous vapour. Certain varieties of common salt exposed to the air absorb and condense so much aqueous vapour as to become liquid. Many other salts have the same property, and are hence called deliquescent salts.

Liquefaction by pressure.-Let us suppose a vessel containing aqueous vapour, a cylinder for instance, and in this cylinder a piston which can be depressed at will, like that represented in fig. 4 page 9. As the vapour is not at first in a state of saturation when the piston is depressed, it behaves like a true gas, the pressure increasing its elastic force and density without liquefying it. But the more the piston is depressed the smaller does the volume of the vapour become, and a point is ultimately reached at which the vapour present is just sufficient to saturate the space. From this point the slightest increase of pressure causes a portion of vapour to pass into the liquid state, and the liquefaction continues as long as the excess of pressure lasts; so that if the piston descends to the bottom of the cylinder all the vapour is condensed. In this experiment it is to be observed, that when once saturation is attained, provided there is no air in the cylinder, the resistance to the depression of the piston does not increase in proportion as it descends, which arises from the condensation of the vapour, and confirms what was previously said as to the maximum tension of vapour in a state of saturation.

But

Liquefaction by cooling.-Cooling, as well as pressure, only causes vapours to liquefy when they are in a state of saturation. when once a given space is saturated, the slightest lowering of temperature takes from the vapours the heat which gives them their fluidity, the attraction between the molecules preponderates, they agglomerate, forming extremely small droplets, which float in the air and are deposited on the surrounding bodies.

Vapours are ordinarily condensed by cooling. Thus, the vapours exhaled from the nose and mouth of animals first saturate the colder air in which they are disengaged, and then condense with a cloud-like appearance. It is owing to the same phenomenon that the vapours become visible which are disengaged from boiling water, those which rise from chimneys, the fogs formed above

rivers, and so forth. All these vapours are more apparent in winter than in summer, for then the air is colder, and the condensation more complete.

In cold weather, the windows in heated rooms are seen to become covered with dew on the inside. The air of these rooms is in general far from being saturated with vapour, but the layers of air in immediate contact with the windows become colder; and as the quantity of vapour necessary to saturate a given space is less, the colder this space, a moment is reached at which the air in contact with the windows is saturated, and then the vapours they contain are quickly deposited. In a time of thaw, when the air is hotter on the outside than on the inside, the deposit is formed on the outside. To the same cause is due the deposit of moisture formed on walls, which is expressed by saying that they sweat; an unsuitable expression, for the moisture does not come from the walls but from the atmosphere. The walls are colder than the air, and they lower the temperature of the layers in contact with them, and condense the vapours. A similar effect is produced when in summer a bottle of wine is brought from the cellar, or when a glass is filled with cold water; a deposit of dew is formed on the surface of these vessels. The same phenomenon does not occur in winter, for then the temperature of the atmosphere being the same as that of the bottle, or even lower, the layers of air in immediate contact with it are not cooled.

241. Heat disengaged during condensation.-It has been seen that any liquid in vaporising absorbs a quantity of heat. This heat is not destroyed, for in the converse change it reappears in the sensible state; that is to say, capable of acting on our organs and on the thermometer. For instance, we know that a pound of water absorbs in vaporising 540 units of heat (237); that is to say, a quantity of heat necessary to raise 540 pounds of water from o° to 1°: conversely, a pound of water at 100°, which is liquefied and gives a pound of water at 100°, causes 540 units to pass from the latent to the sensible state, an amount of heat which is utilised in heating by steam.

242. Application to heating by steam.-The quantity of heat which becomes free when aqueous vapour is condensed is utilised in the arts for heating private houses, hot-houses, and public buildings. Steam is produced in boilers like those used in steam engines, and passes from thence into metallic tubes concealed behind the wainscot, or into columns which serve at the same time as ornaments

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Stills.

237

for rooms. The steam condensing in these tubes gives up a considerable quantity of heat, which they impart to the surrounding air. 243. Distillation. Stills.-Distillation is an operation by which volatile liquid may be separated from substances which it holds in solution, or by which two liquids of different volatilities may be separated. The operation depends on the transformation of liquids into vapours by the action of heat, and on the condensation of these vapours by cooling.

The apparatus used in distillation is called a still. Its form may vary greatly, but consists essentially of three parts; Ist, the body,

A (fig. 188), a copper vessel containing the liquid, the lower part of which fits in the furnace; 2nd, the head, B, which fits on the body, and from which a lateral tube, C, leads to, 3rd, the worm, S, a long spiral tin or copper tube, placed in a cistern kept constantly full of cold water. The object of the worm is to condense the vapour, by exposing a greater extent of cold surface.

To free ordinary well water from the many impuritics which it contains, it is placed in a still and heated. The vapours disengaged are condensed in the worm, and the distilled water arising from the condensation is collected in the receiver, D. The vapours in condensing rapidly heat the water in the cistern, which must,

therefore, be constantly renewed. For this purpose a continual supply of cold water passes into the bottom of the cistern, while the lighter heated water rises to the surface, and escapes by a tube in the top of the cistern.

Brandy is obtained from wine by means of distillation. Wine, consisting essentially of water, alcohol, and colouring matter, when heated in a still to a temperature between 78° and 100°, the alcohol, which boils at 78°, vaporises, while water, which only boils at 100°, remains behind, or at all events only passes over in small quantity. The liquid resulting from this distillation, is brandy, which is essentially a mixture of alcohol and water.

244. Liquefaction of gases.-We have already seen that a saturated vapour, the temperature of which is constant, is liquefied by increasing the pressure, and that, the pressure remaining constant, it is brought into the liquid state by diminishing the temperature.

Unsaturated vapours behave in all respects like gases. And it is natural to suppose, that what are ordinarily called permanent gases are really unsaturated vapours. For the gaseous form is accidental, and is not inherent in the nature of the substance. At ordinary temperatures sulphurous acid is a gas, while in countries near the poles it is a liquid; in temperate climates ether is a liquid, at a tropical heat it is a gas. And just as unsaturated vapours may be brought to the state of saturation and then liquefied by suitably diminishing the temperature or increasing the pressure, so by the same means gases may be liquefied. But as they are mostly very far removed from this state of saturation great cold and pressure are required. Some of them may indeed be liquefied either by cold or by pressure; for the majority, however, both processes must be simultaneously employed. Few gases can resist these combined actions, and probably those which have not yet been liquefied, hydrogen, oxygen, nitrogen, binoxide of nitrogen, and carbonic oxide, would become so if submitted to a sufficient degree of cold and pressure.

One of the most remarkable experiments on the liquefaction of gases is that made by Thilorier to liquefy and solidify carbonic acid. The principle of the method was first devised and applied by Faraday. The apparatus consists of two cast iron cylinders with very thick sides, of 5 to 6 quarts capacity. They are hermetically closed, and are connected by means of a leaden tube. In one of these cylinders, called the generator, are placed the substances by whose chemical action carbonic acid is evolved