liquids, e. g. water, when kept perfectly at rest, may be cooled several degrees below the melting point, and in many cases will solidify instantaneously, particularly when disturbed, the temperature at the same time rising to the melting point. (Vid. pp. 9, 10.)-Respecting the difficult freezing of water in vessels exhausted of air, vid. August and Kries. (Pogg. 52, 184, and 636.)

Pure water when agitated can be cooled but very little below the melting point; but with aqueous solutions of salts, even when they are strongly agitated, the cooling may be carried a degree or even several degrees below the melting point, the material (whether glass, lead or copper, of which the vessel consists, appearing to have no influence on the result. (Despretz.)

a. Quantity of salt dissolved in 1000 parts of water.

b. Maximum of cooling, before solidification, of a saline solution contained in a copper vessel surrounded with a freezing mixture and stirred. c. Melting point, or temperature at the commencement of solidification.

When more than 24.693 parts of carbonate of soda are dissolved in 1000 parts of water, the solution when agitated deposits crystals of the salt instead of ice. It is remarkable that the efflorescent, less soluble carbonate of soda lowers the freezing point of water more than carbonate of potash; on the other hand, the more soluble sulphate of soda lowers it further than sulphate of potash. (Despretz.)

When ice at 0° is mixed with an equal quantity of water at 75°, all the ice melts and the water produced has the temperature of 0°. Consequently, 75 degrees of heat (= 135° Fah.) are absorbed from the water and enter into a state of chemical combination, in order to convert an equal quantity of ice at 0° into water at 0°. If water at 0° be mixed with ice below 0°, the water freezes,-and the temperature of the ice, when the right proportions have been observed, rises to 0°. 1lb. of ice at 75° mixed with 1lb. of water at 0° would give 2lb. of ice at 0°. In this case, latent heat is set free and raises the temperature of the ice.

Whilst, according to this result, the latent heat of water appears to be 75°, that of tin is only 13.314°, and of lead 5-358°. (Rudberg, Pogg. 19, 125; comp. G. A. Erman, A. & F. Swanberg, Despretz, Pogg. 20, 282; 26, 291; 52, 177). According to the last mentioned observers, latent heat, like specific heat, appears to vary nearly in the inverse ratio of the atomic weights of the bodies.

De la Provostaye & Desains make the latent heat of water 79.25 C.; Regnault found it to be 79.24 by one experiment, and 79.06 by another. The mean of these results is 79.2° 142·6° (Fah.)

Person (Pogg. 70, 300; N. Ann. Chim. Phys. 21, 295; abstr. Ann. Chem. Pharm. 74, 179) has determined the latent heat of fusion of 13 substances, and likewise their specific heat in the liquid state. lowing are the results:

The fol

The latent heat 7 of these bodies may be expressed by the formula

(160+) = 1

where t denotes the melting point in centigrade degrees, = Cc the

difference of specific heat in the solid (= c) and the liquid state (= C ). For example, we have

The above equation shows that-To obtain the latent heat of a body, the difference of its two specific heats must be repeated as many times as there are degrees between 160° and its melting point.

From this it is probable that -160° C. or -256° F. is the absolute zero of heat; since otherwise we shall be obliged to admit that, at temperatures below -160°, the difference between the specific heats in the solid and liquid states must for all bodies be equal to nothing.

The preceding formula is not applicable to the metals, inasmuch as for these bodies the value of is almost nothing. Person is however of opinion that if the metals could be retained in the liquid state below their ordinary melting points, their specific heat (= Ċ) in this state would be found to be much greater than it is at temperatures above the melting point, and consequently = C — c would have a more considerable value.

According to Person, the latent heat of fluidity is not a constant quantity, but, like the latent heat of vaporization, variable with the temperature. Thus, 1 kilogramme of ice at 20° C. requires 10 units of heat to bring it to 0° and 79-2 units to melt it; in all therefore 89.2 units. Now if water at 0° be cooled to -20°, it loses 20 units of heat,—since, according to Person's experiments, the specific heat of water does not alter sensibly at temperatures below 0°. Consequently, when it freezes, it can only give up 8912 20 69.2 units of heat. T.

Most bodies contract in passing from the liquid to the solid state; the following only are known to expand in undergoing this change: water, bismuth, and cast-iron, according to Reaumur; copper, according to Karsten; silver according to Persoz (Chim. molecul. 240); certain alloys of bismuth, and likewise oxide of lead, according to Manx. The expansion of antimony, observed by Reaumur, is not confirmed by the experiments of Manx. Water, according to Leroyer & Dumas, expands in freezing by of its volume; bismuth, according to Manx, at least. Solid pieces of these substances float on the melted portion. The outer portion of these liquids being generally the first to solidify, the enclosed portion which still remains liquid splits the solid crust or the vessel in cooling, and forms an opening by which a portion flows out. Melted bismuth into a glass tube bursts it in solidifying. (Marx, Schw. 58, 454; J. pr. Chem. 22, 135.) This expansion seems to be connected with the fact that such bodies, water, e. g., attain their greatest density several degrees above their freezing points, and when cooled below the particular temperature which gives the maximum density, expand more and more as they approach the freezing point. (p. 225.)

Metallic alloys often exhibit two solidifying points, that is to say,

when the melted mass is cooling, the temperature remains constant for some time at two different points. This takes place when the metals are not combined in the stoichiometrical proportion in which they unite into a solid compound: in such a case the metal which is in excess solidifies first, and the more intimate compound at a lower temperature. (Pogg. 18, 240.)

On the compressibility of liquids-which is of very small amount, and at least in the case of water follows Mariotte's law-vid. Oersted (Schw. 52, 9); Colladon & Sturm. (Ann. Chim. Phys. 35, 113; also Pogg. 12, 39, and 161.)

2. Vaporization.

All ponderable substances have an affinity for heat, in consequence of which they endeavour to combine with it and form Elastic Fluids or Gases. Every gas consists, therefore, of heat and a ponderable substance, the Ponderable Basis or Material.

This affinity differs greatly in amount. Substances which have great tendency to assume the gaseous form are called Volatile; those in which this tendency is small are called Fixed; Corpora volatilia and fixa. On the whole, the volatility of bodies follows the same order as their fusibility; nevertheless, water is more volatile than mercury or oil of vitriol. -The more intimate the union between the ponderable body and heat, the greater is the difficulty of depriving the body of its gaseous form by external pressure and cooling,-the more permanent, therefore, is the gas. A gradual transition nevertheless exists from those gases which are either not condensable at all, like oxygen, or only by very strong pressure and cooling, like carbonic acid,-to those which, like the vapour of iron, exist only at the highest known temperatures. Gases may therefore be divided into Permanent Gases, which retain their gaseous form under the ordinary atmospheric pressure and at 0°, and Vapours which, under these circumstances, return to the liquid or the solid state.

Heat is contained in gases in larger quantity than in liquids; it predominates in them, overcomes the cohesion of their ponderable matter, and imparts to them, on the contrary, a tendency to expand without limit when not prevented by external obstacles. This expansive tendency of gases is their Elasticity or Tension. In the same gas, and at the same temperature, the tension varies, according to Mariotte's law, in the direct ratio of the density. It is only when gases are brought by pressure and cooling near the point at which they assume the liquid form, that the density increases somewhat more rapidly than the elasticity.-This peculiarity is exhibited not only by vapours, but also by those more perma nent gases which may be liquefied by strong pressure, e. g., sulphuretted hydrogen, ammonia, cyanogen, and (according to Oersted) sulphurous acid gas. If equal measures of air and ammoniacal gas are exposed to the same pressure, the latter is condensed in a higher degree, as if it were exposed to a stronger pressure, and this difference increases considerably as the pressure becomes greater:

A condensation of air corresponding to a pressure of 3.863 met. of the mercurial column, produces, therefore, the same condensation of ammoniacal gas as if the pressure were that of a column of 4.132 met. (Despretz, Ann. Chim. Phys. 34, 335, and 443; also Pogg. 9, 605; also Schw. 51,

VOL. I.

8

108.)-Even carbonic acid gas, which is not so easily liquefied as the gases just mentioned, has a density somewhat too great, even under the ordinary pressure; it does not exactly follow Mariotte's law, till the pressure is reduced to less than of an atmosphere. (Wrede, Ann. Pharm. 38, 140.)*

All ponderable substances, when in the gaseous form, are reduced to their utmost state of expansion and rarity, and pass through openings which are impervious to liquids and solids. All gases are transparent; most of them colourless.-The gases of chlorine, oxide of chlorine, selenium, and sulphur, are yellow, of bromine and hyponitric acid red,-of iodine violet, and of indigo reddish purple.

A. Conditions of Vaporization.

a. A certain space must be allowed.

Every ponderable substance takes up a greater space in the gaseous than in the liquid or solid state. If, therefore, a solid or liquid body be closely confined by an unyielding envelope, no formation of gas can take place at any temperature, unless the envelope be burst open. If, on the contrary, the body be situated in a vacuum, a portion of it-corresponding to the magnitude of the empty space, the temperature, and the nature of the substance itself-will be converted into gas.

When a certain quantity of gas has been generated in an empty space, the expansive tendency of the gas already produced hinders the further vaporization of the remaining substance: for this would produce compression of the gas previously formed. Equilibrium is therefore established between the elasticity of the generated gas and the tendency of the remaining matter to combine with heat and form more gas,—and thus the further production of gas is prevented. The empty space is now saturated, as it were, with gas at the given temperature, or a saturated gas has been produced. But if the temperature be raised, the affinity of the heat for the remaining matter will overcome the elasticity of the gas previously generated, and a further production of gas will take place: a larger quantity of gas is thus accumulated in the same space,-in consequence of which the elasticity increases, and gradually attains such a degree of force that equilibrium is again established, and the further accumulation of gas is stopped. The higher therefore the temperature, the greater is the quantity of matter vaporized in vacuo, and the greater are the density and elasticity of the gas or vapour produced.

The gas contained in any space is unsaturated, when an additional quantity of the same vaporizable substance introduced into it is likewise converted into gas. Thus carbonic acid gas, at ordinary pressures and temperatures, is an unsaturated gas; for liquid carbonic acid introduced into a space filled with it vaporizes in large quantity,-and it is not till the vaporization is ended, and a portion of carbonic acid still remains liquid, that the gas can be regarded as saturated. The same is true of all gases which are permanent at ordinary pressures and temperatures.

Alcohol, ether, or rock-oil enclosed in a tube of strong glass or iron is completely converted into vapour on the application of heat, only when the space not occupied by the liquid is somewhat greater than the volume of the liquid itself. With rock-oil the empty space may be somewhat smaller than with alcohol, and with ether still less. Alcohol when thus

*See also Regnault. (Pogg. 77, 531.)