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to the former, according as the temperature rises or falls; then from the liquid to the aëriform state, or conversely. These various changes of state we shall now investigate under the name of fusion, solidification, vaporisation, and liquefaction.
Fusion is the passage of a solid body to the liquid state by the action of heat. This phenomenon is produced when the force of cohesion which unites the molecules is balanced by the force of repulsion (4); but as the cohesive force varies in different substances, the temperature at which bodies melt does so likewise. For some substances this temperature is very low, and for others very high, as the following table shows :
Fusing points of certain substances.
264 Butter .
. · · 335 Phosphorus
Zinc . . Potassium
450 Stearine . . . . 60 Silver
Silver . . . 1000 White wax . . . 65 Gold . . . . 1250 Sodium . . . . 90 Iron . . . . 1500
Some substances, however, such as paper, wood, wool, and certain salts, do not fuse at a high temperature, but are decomposed. Many bodies have long been considered refractory; that is, incapable of fusion ; but, in proportion as it has been possible to produce higher temperatures, their number has diminished. Gaudin has succeeded in fusing rock crystal by means of a lamp fed by a jet of oxygen ; and more recently Despretz, by combining the effects of the sun, the voltaic battery, and the oxy-hydrogen blowpipe, has melted alumina and magnesia, and softened carbon, so as to be flexible, which is a condition near that of fusion.
Some substances pass from the solid to the liquid state without showing any definite melting point; for example, glass and iron become gradually softer and softer when heated, and pass by imperceptible stages from the solid to the liquid condition. This intermediate condition is spoken of as the state of vitreous fusion. Such substances may be said to melt at the lowest temperature at which perceptible softening occurs, and to be fully melted when the further elevation of temperature does not make them more fluid; but no precise temperatures can be given as their melting points.
220. Laws of fusion.- It has been experimentally found, that the fusion of bodies is governed by the two following laws :
I. Every substance begins to fuse at a certain temperature, which is invariable for each substance if the pressure be constant.
II. Whatever be the intensity of the source of heat, from the moment fusion commences, the temperature of the body ceases to rise, and remains constant until the fusion is complete.
For instance, the melting point of ice is zero, and a piece of this substance exposed to the sun's rays, placed in front of a fire or over a lamp, could never be heated beyond this temperature. Exposure to a more intense heat would only accelerate the fusion, the temperature would remain at zero until the whole of the ice was melted.
221. Latent heat.-Since, during the passage of a body from the solid to the liquid state, the temperature remains constant until the fusion is complete, whatever be the intensity of the source of heat, it must be concluded that, in changing their condition, bodies absorb a considerable amount of heat, the only effect of which is to maintain them in the liquid state. This heat, which is not indicated by the thermometer, is called latent heat, or latent heat by fusion, an expression which, though not in strict accordance with modern ideas, is convenient from the fact of its universal recognition and employment.
An idea of what is meant by latent heat may be obtained from the following experiment. If a pound of water at 80° is mixed with a pound of water at zero, the temperature of the mixture is 40°. But if a pound of pounded ice at zero is mixed with a pound of water at 80°, the ice melts, and two pounds of water at zero are obtained. The pound of ice at zero is changed into a pound of water also at zero, but as the hot water is also lowered to this temperature, what has become of the 80° of heat it possessed ? They exist in the water which results from the ice; their effect has neither been to heat it nor to expand, but simply to impart fluidity to it. Consequently, the mere change of a pound of ice to a pound of water at the same temperature requires as much heat as will raise a pound of water through 80°. This quantity of heat represents the latent heat of the fusion of ice, or the latent heat of water.
Every substance in melting absorbs a certain amount of heat, which, however, varies materially.
The enormous quantity of heat absorbed by ice in melting explains how it is that so long a time is required for thaw. And conversely it is owing to the latent heat of water, that even when its temperature has been reduced to zero so long a time is required before it is entirely frozen. Before it can be so it must give out the heat which had been consumed in its liquefaction : it thus becomes a source of heat which retards the solidification. Faraday has calculated that the heat given out by a cubic yard of water in freezing is equal to that which would be produced by the combustion of a bushel of coals.
222. Solidification, Those substances which are liquefied by heat revert to the solid state on cooling, and this passage from the liquid to the solid state is called solidification. If this solidification takes place at a low temperature it is frequently spoken of as congelation.
In all cases the phenomenon is subject to the following laws :
1. Every body, under the same pressure, solidifies at a fixed temperature, which is the same as that of fusion.
II. From the commencement to the end of the solidification, the temperature of a liquid remains constant.
Thus if lead begins to melt at 335°, melted lead in like manner when cooled down begins to solidify at 335o. Moreover, until it is completely solidified, the temperature remains constant at 335o. This arises from the fact, that the liquid in proportion as it solidifies restores the heat it had absorbed in being melted. The same phenomenon is observed whenever a liquid solidifies.
Many liquids, such as alcohol, ether, and bisulphide of carbon, do not solidify even at the lowest known temperature. Pure water solidifies at zero; salt water at – 2.5o, olive and rape oils at -6°; linseed and nut oils at -27°.
Water presents the remarkable phenomenon, that when it solidifies and forms ice its volume undergoes a material increase. In speaking of the maximum density of water we have already seen that, on cooling, it expands from 4 degrees to zero; it further expands on the moment of solidifying, or contracts on melting, by about 10 per cent. One volume of ice at oo gives O'908 of water at o', or I volume of water at oo gives 1.102 of ice at the same temperature.
The increase of volume in the formation of ice is accompanied by an expansive force which sometimes produces powerful mechanical effects, of which the bursting of water pipes and the breaking of jugs containing water are familiar examples. The splitting of stones, rocks, and the swelling up of moist ground during frost, are caused by the fact that water penetrates into the pores and there becomes frozen.
The expansive force of ice was strikingly shown by some experi
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 cylindri. cal 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 sodium and 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.