174.]

LATENT HEAT OF LIQUIDS.

367

the south be instantly converted into water, and sweep before it, not merely the habitations of man and their tenants, but trees, rocks, and hills. Such fearful catastrophes do now and then occur, when a volcano like Etna pours forth a stream of lava over its snow-clad sides: the flood that then ensues is even more destructive than the fiery torrent itself. The latent heat of water is greater than that of any other body, but in all cases of liquefaction there is a similar disappearance of heat; the quantity which becomes latent varying with the nature of the substance.

Person (Ann. Chim. Phys. 1847 [3], xxi. 295, and 1848, xxiv. 129 and 265) has determined the quantity of heat absorbed during the fusion of a considerable number of bodies, and he concludes that the latent heat of fusion is obtained by multiplying the difference between the specific heat of the substance in its liquid and its solid form by a number obtained by adding the number 160 (an experimental constant furnished by researches upon the latent heat of water) to the number of degrees Centigrade indicating the melting point of the substance in question.*

If = the latent heat, d the difference of the specific heat in the liquid and in the solid state, t the melting point °C., the latent heat may be calculated by the formula

(160+ t) d = l.

The results obtained with the metals do not accord with Person's theory, as the difference of their specific heats in the solid and liquid states is very trifling; but for other bodies the result calculated corresponds pretty closely with that furnished by experiment. If Person's view be correct, a consequence which he ingeniously draws from it is, that the absolute zero of temperature would fall at -160° C. (-256° F.). On the thermo-dynamical theory, the absolute zero is at 273° C. (-459°4 F.).

368

FREEZING MIXTURES.

[174.

The numbers in the second column of the table represent the number of degrees of temperature that an equal quantity or mass of water would be raised by the passage of each of the bodies enumerated, from the liquid to the solid state, or they may be taken as the number of pounds of water that would be raised 1° C. by the heat emitted during the congelation of one pound of each of the substances included in the table-i.e., the number of units of heat those in the third column indicate the degrees on Fahrenheit's scale to which the same weight of water would be raised by a similar experiment.

(175) Freezing Mixtures.-The chemist avails himself of the fact that heat disappears during liquefaction, for the purpose of procuring artificial cold: the action of freezing mixtures depends upon this principle. Many salts, while undergoing solution, produce a very considerable reduction of temperature.

For example: 100 grms. (about 4 ounces) of nitre and 100 of sal-ammoniac, each in fine powder, when mixed with 200 grms. of water, reduce the thermometer from 50° to 10° (10° to -12° C.). Equal parts of ammonic nitrate and water reduce the temperature from 50° to 4° (10° to 16° C.). So, likewise, equal parts of water, of powdered crystallized ammonic nitrate, and of sodic carbonate, also crystallized and in powder, effect a reduction from 50° to -7° (10° to -22° C.). In like manner, the solution of crystallized sodic sulphate in commercial hydrochloric acid is attended with a rapid reduction of temperature: this mixture is employed in the common refrigerators, 5 parts of the acid being poured upon 8 parts of the salt reduced to powder: the temperature may thus be reduced from 50° to 0° (10° to 18° C.).

The most convenient mixture, however, when procurable, consists of 2 parts of pounded ice (or, better still, of fresh snow) and I part of common salt. A steady temperature of 4° (-20°C.) can by its means be maintained for many hours Again, a mixture of 3 parts of crystallized calcic chloride and 2 of snow will produce a cold sufficient to freeze mercury; if, before making the mixture, both the vessels in which the experiment is to be performed and the chloride be cooled to 32°, such a mixture will cause a thermometer when plunged into it to fall to -50° (− 45° C.).

Even during the liquefaction of a metallic alloy by mercury, the same fact is observed; thus an alloy may be formed by melting together 207 parts of lead, 118 parts of tin, and 208 parts of bismuth; if this be granulated, by pouring it, when melted, into water, it may be dissolved in 1600 parts of mercury, and will cause a thermometer, if immersed in it, to sink from 63° to 14° (17° to - 10° C.).

It is owing to this absorption of heat during the liquefaction of solids, that not only in the melting of ice, but in the much higher temperatures required for the fusion of many of the metals, the temperature remains stationary so long as any portion of the mass remains unmelted; the excess of heat is transferred to the unmelted solid by conduction, and is rapidly absorbed by it during its liquefaction.

175-1

FUSING-POINTS.

369

The following table contains the temperatures at which several substances, metallic and non-metallic, enter into fusion:

The fusing-point of a mixture of analogous bodies is generally considerably below that of either of its separate components. Alloys, for example, often have a melting-point much below that of any of the metals which enter into their formation, as is seen in the case of fusible metal, and of the alloy of potassium and sodium, which is liquid at the ordinary temperature. It has long been practically known to the glass-maker and the metallurgist that mixtures of various silicates fuse at a temperature far below that required to melt any of them alone. A similar increase of fusibility is observed when many of the chlorides are mixed together before exposing them to heat. A mixture of equivalent quantities of sodic and potassic carbonate melts below the fusing-point of either salt separately, and is often used to effect the fusion of siliceous minerals in analysis. Schaffgotsch found that potassic acetate melts at 558° (292° C.), sodic acetate at 606° (319° C.), but a mixture of the two salts in equivalent proportions fuses at 435° (224° C.). In like manner potassic nitrate melts at 642° (339° C.), sodic nitrate at 592° (311° C.), but a mixture of the two salts in equivalent proportions liquefies

370

INFLUENCE OF PRESSURE ON FUSING-POINT.

[175·

as low as 430 (221° C.), or 162° (90° C.) below the meltingpoint of the most fusible of the two salts.

A mixture of crystallizable fatty acids also commonly melts at a temperature below that of either when separated.

The melting-point of ice is perfectly stationary* at o° C.; but

* Sir W. Thomson (Phil. Mag. 1850 [3], xxxvii. 123), in confirmation of the results anticipated from a mathematical investigation made by his brother, and communicated to the Royal Society of Edinburgh, January, 1849 (Trans. Roy. Soc. Edin. 1849, xvi. 575), found experimentally that the freezing-point of water, a liquid which expands at the moment of congelation, is lowered to a minute but measurable extent by exposing the water to pressure. Some preliminary experiments showed, that for a pressure of 8.1 atmospheres the point of congelation was lowered o°0588 C.: by a pressure of 16.8 atmospheres it was reduced 8°1288: the calculated numbers being o°0605 and o°·1261 respec

FIG. 134.

E

D

e

tively. At the end of the last century Dr. Hutton, in some remarks on Major Williams's experiments on the expansive force of freezing water, pointed out that preventing the expansion will prevent the freezing, and the water will remain fluid, whatever the degree of cold may be (Trans. Roy. Soc. Edin. 1786, ii. part 2, 27). Bunsen, on the other hand, found the melting-point of paraffin and of spermaceti to be raised by increasing the pressure. Spermaceti, for instance, under the atmo spheric pressure, solidified at 117°9 (47°7 C.), but under a pressure of 150 atmospheres it solidified at 123°8 (51° C.); both these bodies contract at the moment of solidification, and as had been anticipated by Thomson, the melting-point was raised. Hopkins found this to hold good for still higher pressures; his experiments comprised not only spermaceti, but also was and stearin. The experiments of Mousson (Ann. Chim. Phys. 1859 [3], lvi. 252) upon this point are very remarkable. He contrived an apparatus, in which he was able to subject ice to a pressure which he estimated at 13,000 atmospheres, and by which its volume was reduced by 13 hundredths of that which it occupied at -0°C. He found that under this enormous pressure ice frozen at o° C. remelted, and continued liquid belew 0°4 (− 18° C.). This apparatus consisted of a steel bar, A, fig. 134, in the axis of which a cylindrical cavity, B, was drilled. This cavity was closed below by a conical copper plug, ƒ, which was kept in its place by the screw, G. Above, the cavity was made slightly conical, and to it was fitted the copper core, e, upon which the steel piston, D, rested, and could be pressed down with enormous force by means of the screw, E, worked by a lever attached to the screw, F. In making the experiment, the apparatus was closed at top; it was then inverted, and a loose copper rod, d, was introduced, after which it was filled up to c with water, and subjected to a low temperature. As soon as the water was completely frozen, the plugƒ was

B

f

175.]

INFLUENCE OF SALTS ON FREEZING-POINT.

371

water which contains salts in solution has a lower point of congelation. Sea-water, for example, freezes at 27°4 (−2°5 C.), much of the salt separating, and purer water floating in the form of ice; whilst water which is saturated with sea-salt sinks as low as -4° (-20° C.) before freezing (71).

Rüdorff (Pogg. Annal. 1861, exiv. 63) finds that in saline solutions generally, the freezing-point is below that of pure water, but the degree to which it is lowered varies with the nature of the salt employed. Almost the only salts which are well adapted to this inquiry are the chlorides and nitrates of the metals of the alkalies and alkaline earths, as few other salts possess the requisite solu

introduced, and the apparatus securely closed. It was then restored to its usual position, and immersed in a freezing-mixture at -20°C. After allowing it to acquire this low temperature, the greatest degree of compression which could be applied was brought to bear upon the ice within. The ice was thus liquefied, and the copper rod, d, was found to have fallen to the bottom of the water, which immediately solidified again on relaxing the pressure. Boussingault has repeated this experiment and carried the temperature as low as 24° without causing solidification. (Comp. Rend. 1871, lxxiii. 77.)

H. C. Sorby (Proceed. Roy. Soc. 1863, xii. 538) has made some interesting observations upon the influence of pressure upon the solubility of salts, in which he has obtained results analogous to these upon the freezing-points of liquids. He finds in cases where, as is usual, the volume of the water and of a salt after solution is less than the volume of the water and the salt separately, that the solubility is increased by pressure; but that in cases where, as when sal-ammoniac is dissolved in water, the volume of the solution is greater than that of the water and the salt taken separately, the solubility is lessened by a small but measurable amount. For sal-ammoniac this diminution for a pressure of 100 atmospheres is equal to 0637 per cent. of the quantity of the salt in solution. Sorby calculates that the energy with which this salt tends to dissolve in a solution containing I per cent. less than would be dissolved without pressure is such, that any unit of salt would in dissolving give rise to a mechanical energy equal to that required to raise 171 times its own weight to the height of one metre.

On the contrary, salts which expand in crystallizing from solution must, under pressure, overcome mechanical resistance in that change, and as this resistance is opposed to the energy of crystallization, the salt is rendered more soluble. The extent of the influence of pressure, and the mechanical value of the energy of crystalline polarity, vary in different salts. For instance, a pressure of 100 atmospheres would increase the solubility of crystallized cupric sulphate as much as 3183 per cent., whereas it would increase the solubility of sodic chloride to the extent of only o'419 per cent. The energy with which this latter salt tends to crystallize from a solution containing I per cent. more than would be dissolved without pressure is such, that any unit of salt in dissolving would give rise to a mechanical energy sufficient to raise 157 times its own weight to the height of one metre; whereas, in the case of cupric sulphate, this energy is only sufficient to raise 7 times its own weight to the same height. Of course, if the solution were still more supersaturated, the energy of crystallization would be greater, and vice versâ.