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Crystallisation.

217

ments of Major Williams in Canada. Having quite filled a 13inch iron bomb-shell with water, he firmly closed the touch-hole with an iron plug weighing 3 pounds, and exposed it in this state to the frost. After some time the iron plug was forced out with a loud explosion, and thrown to a distance of 415 feet, and a cylinder of ice 8 inches long issued from the opening.

From the expansion which water undergoes in freezing, it is clear that ice must be less dense than water; and this in fact is the case, for ice floats on the surface of water. In the polar seas, where the temperature is always very low, masses of floating ice are met with which are called ice-fields. They rise out of the sea to a height of 4 or 5 yards, and are immersed to a depth of 7 or 8 yards, and they frequently extend over 40 miles. True mountains of ice, or icebergs, are found floating on those seas; they have not the same extent, but attain very great heights.

Cast-iron, bismuth, and antimony expand on solidifying like water, and can thus be used for casting; but gold, silver, and copper contract, and hence coins of these metals cannot be cast, but must be stamped with a die.

223. Crystallisation.-When bodies pass slowly from the liquid to the solid state their molecules, instead of becoming grouped in a confused manner, generally acquire a regular order and arrangement, in virtue of which these bodies assume the geometrical shapes of cubes, pyramids, and prisms, etc., which are perfectly definite, and are known as crystals. Flakes of snow, when looked at under the microscope, ice in the process of formation, sugar candy, rock crystal, alum, common salt, and many other substances afford well-known instances of crystallisation.

Two methods are in use for crystallising substances; the dry way and the moist way. By the first method bodies are melted by heat, and then allowed to cool slowly. The vessel in which the operation is performed becomes lined with crystals, which are made apparent by inverting the vessel and pouring out the excess of liquid before the whole of it is melted. Sulphur, bismuth, and many other metals are thus easily crystallised. The second method consists in dissolving in hot water the substance to be crystallised, and then allowing it to cool slowly. The body is then deposited on the sides of vessels in crystals which are larger and better shaped the more slowly the crystallisation is effected. In this manner sugar candy and salts are crystallised.

224. Solution.-A body is said to dissolve when it becomes liquid in consequence of an affinity between its molecules and

those of a liquid. Gum arabic, sugar, and most salts dissolve in

water.

During solution, as well as during fusion, a certain quantity of heat always becomes latent, and hence it is that the solution of a substance usually produces a diminution of temperature. In certain cases, however, instead of the temperature being lowered, it actually rises, as when caustic potass is dissolved in water. This depends upon the fact that two simultaneous and contrary phenomena are produced. The first is the passage from the solid to the liquid condition, which always lowers the temperature. The second is the chemical combination of the body dissolved with the liquid, and which, as in the case of all chemical combinations, produces an increase of temperature. Consequently, as the one or the other of these effects predominates, or as they are equal, the temperature either rises, or sinks, or remains constant.

225. Freezing mixtures.-The absorption of heat in the passage of bodies from the solid to the liquid state has been used to produce artificial cold. This is effected by mixing together bodies which have an affinity for each other, and of which one at least is solid, such as water and a salt, ice and a salt, or an acid and a salt. Chemical affinity accelerates the fusion, the portion which melts robs the rest of the mixture of a large quantity of sensible heat, which thus becomes latent. In many cases a very considerable diminution of temperature is produced.

If the substances taken be themselves first previously cooled down, a still more considerable diminution of temperature is occasioned.

Freezing mixtures are frequently used in chemistry, in physics, and in domestic economy. The portable ice-making machines which have come into use during the last few years, consist of a cylindrical metallic vessel divided into four concentric compartments. In the central one is placed the water to be frozen; in the next there is the freezing mixture, which usually consists of sulphate of sodiumand hydrochloric acid; 6 pounds of the former and 5 of the latter will make 5 to 6 pounds of ice in an hour. The third compartment also contains water, and the outside one contains some badly conducting substance, such as cotton, to prevent the influence of the external temperature. The best effect is obtained when pretty large quantities, 2 or 3 pounds, of the mixture are used, and when they are intimately mixed. It is also advantageous to use the machines for a series of successive operations.

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226. Vapours.-We have already seen (108) that vapours are the aëriform fluids into which substances, such as ether, alcohol, water, and mercury, are changed by the absorption of heat.

In respect to the property of disengaging vapours, liquids are divided into two classes, volatile liquids, and fixed liquids. The first are those which have a tendency to pass into the state of vapour, at the ordinary or even at lower temperatures; such, for instance, are water, ether, chloroform, alcohol, which rapidly disappear when exposed to the air in open vessels. To this class belongs a numerous family of liquids met with in nature, such as essence of turpentine, oil of lemons, of lavender, of thyme, of roses, etc.

Fixed liquids, on the contrary, are those which emit no vapour at any temperature; such, for instance, are the fat oils, as olive, rape, etc. When strongly heated these oils are decomposed, and give rise to gaseous products; but they do not emit vapours of the same nature as their own. There are some known as drying oils, which become thicker in the air; but this is in consequence of their having absorbed oxygen, and not in consequence of evaporation.

Some substances give vapours even in the solid state. Ice gives an instance of this, as is seen in dry cold winters, where the snow and ice quite disappear from the ground, without there having been any fusion. Camphor and odoriferous bodies, in general, present the same phenomenon.

227. Elastic force of vapours.- Vapours formed on the surface of a liquid are disengaged in virtue of their elasticity; but this force is generally far lower than the pressure of the atmosphere, and hence liquids exposed to the air only evaporate slowly.

The following experiment renders evident the elastic force of vapours. A bent glass tube has the shorter limb closed (fig. 179) ; this branch and part of the longer are filled with mercury. A drop of ether is then passed into the closed leg, which in virtue

of its lower density rises to the top of the tube at B. The tube thus arranged is immersed in a water bath at a temperature of

C

Α

Fig. 179.

about 45°. The mercury then sinks slowly in the short branch, and the space AB is filled with a gas which has all the appearance of air. This gas or aeriform fluid is nothing but the vapour of ether, whose elastic force CA counterbalances not only the pressure of the column of mercury, but also the atmospheric pressure exerted at C.

If the water in the vessel be cooled, or if the tube be withdrawn, the mercury gradually rises in the short leg, and the drop of liquid which seemed almost to have disappeared is re-formed. If, on the

contrary, the water in which the tube is immersed be still more heated, the drop diminishes and the mercury descends further in the short leg; thus showing that fresh vapours are formed, and that the elastic force increases. This increase of tension with the temperature continues as long as any liquid remains to be vaporised.

The crackling of wood in fires is due to the increased tension of the vapours and gases formed in the pores of the wood during combustion. In roasting chesnuts it is usual to slit the outer skin; the object of this is to allow the vapour formed to escape, for otherwise it would acquire such a tension as to burst the chesnut and scatter the particles far and wide.

228. Formation of vapours in a vacuum.—In the previous experiment the liquid changed very slowly into the vaporous condition; the same is the case when a liquid is freely exposed to the air. In both cases the atmosphere is an obstacle to the vaporisation. In a vacuum there is no resistance, and the formation of vapours is instantaneous, as is seen in the following experiment. Four barometer tubes, filled with mercury, are immersed side by

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Formation of Vapours in a Vacuum.

221

side in the same trough (fig. 180). One of them, A, serves as a barometer, that is, only contains dry mercury, and a few drops of water, alcohol, and ether are respectively introduced into the tubes, B, C, D. When the liquids reach the vacuum a depression of the mercury is at once produced. But this depression cannot be produced by the weight of the liquid, for it is but an infinitely small fraction of the weight of the displaced mercury. Hence, in the case of each liquid, some vapour must have been formed whose elastic force has depressed the mercurial column, and as the depression is greater in the tube D than in the tube C, and greater in this than the tube B, it is concluded that, for the same temperature, the elastic force of ether is greater than that of alcohol vapour, and that this in turn has a greater elastic force than that of water. If the depression

[graphic]

Fig. 180.

be measured by means of a graduated scale, it will be found that at a temperature of 20° the elastic force of ether is twenty-five times as great as that of water, and that of alcohol almost four times as great. From these experiments we obtain the two following laws for the formation of vapours:

I. In a vacuum all volatile liquids are instantaneously converted into vapour.

II. At the same temperature the vapours of different liquids have different elastic forces.

229. Limit to the formation and to the tension of vapours. Saturated space.-The quantity of vapour which can be formed in a given space, whether at the ordinary or at higher temperatures,

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