These measurements, as far at least as temperatures within fifteen or sixteen degrees of 100° C. are concerned, have also been confirmed in a remarkable manner by determinations of the boiling-point of water at different heights on the Pyrenees, and also on Mont Blanc. (Vid. Pogg. 75, pp. 360, 365, 368; also Ann. Chim. Pharm. 56, 162.) ¶

The progression of the tension is not affected by the freezing-point, at least in the case of water and hydrocyanic acid; at 0° ice and water have the same tension; it is not, therefore, affected by the greater cohesion of ice. (Gay-Lussac, Ann. Chim. Phys. 70, 419.)

Volta's (Schw. 52, 98) and Dalton's law, according to which the vapours of different substances have the same tension at an equal number of thermometric degrees above or below their boiling-points, is regarded as incorrect by Schmidt, Mayer, Despretz, Ure, and others, according to the results of their own experiments: it is nevertheless remarkable that this law is pretty nearly true in the case of many substances. From the boiling-point of sulphurous acid, e. g. to the temperature at which its tension is equal to 2 atmospheres, there is, according to the preceding table, an interval of 17° C.; in the case of water, the corresponding interval is about 20°; of alcohol 18°, and of ether 20°: but in the case of rock-oil and oil of turpentine, which are liquids of less simple constitution, it is about 30°. Davy also (Ann. Chim. Phys. 20, 175) puts forward a view founded on his experiments with sulphuret of carbon, chloride of phosphorus, and alcohol, favourable to Dalton's law.-Dove's (Pogg. 23, 291) comparison of the tensions of permanent gases, according to Faraday, with that of vapour of water according to Arago and Dulong, likewise speaks in favour of this law.

Sulphurous acid is also nearly in accordance with the law, but sulphuretted hydrogen is not; respecting this last gas, however, it must be observed that the determinations of its tension by Davy & Faraday and by Niemann differ considerably. According to this law, the boiling-point of nitrous oxide should be 158°, of carbonic acid 146°, of hydrochloric acid 130°, and of ammonia · 53° (Dove); according to Bunsen, however, the boiling point of ammonia is 337°.

If a substance B be enclosed in a space already filled with the vapour of another substance A, which may be atmospheric air or any other gas,no portion of A being present in the non-gaseous form, B being in immediate contact with A, and the two substances not being capable, under the given circumstances, of entering into chemical combination with one another, then, whether the elasticity of the gas produced by B be greater or less than that of the gas A, B will produce exactly the same quantity of gas as if it were situated in a vacuum at the same temperature, with this difference, however, that in the first case the conversion of B into gas takes place almost as quickly as if it were in vacuo; in the second very slowly and only on the surface, where the gas A is in contact with the

substance B. The effect takes place more or less quickly in proportion to the rarity or density of the gas A. (Volta, Schw. 52, 98, Dalton.)

Ice evaporates in the open air at temperatures much below 0°: the chlorides of potassium, sodium and antimony do not evaporate at a red heat in a covered crucible; but when the crucible is opened, and a current of air thereby produced, vaporization takes place. Zinc volatilizes in carbonic oxide gas (when oxide of zine is ignited in contact with charcoal) at a lower temperature than when heated alone. Iodine, whose boiling point is 175°, passes over with vapour of water at 100°. (GayLussac. Similarly, water mixed with alcohol evaporates when heated, together with the alcohol vapour. When the aqueous solutions of different salts are evaporated, part of the salt is likewise given off; this may however be explained by supposing the salt to be mechanically carried over: at all events Faraday (J. of Roy. Inst. 1, 70; also Pogg. 19, 545) found that the water evaporated at ordinary temperatures from saline solutions is free from salt. The cases of reciprocal affinity from the mutual adhesion of gases likewise belong to this head. (pp. 125, 126.) -The more diffusible the gas, the more quickly do bodies evaporate in it; the effect takes place more quickly therefore in hydrogen gas.

It may be supposed that when the elasticity of the gas A is greater than that of the gas which would be produced from B at the given temperature, no part of B can be converted into gas, because the gas already present must by its elasticity compress the substance B sufficiently to prevent the vaporization. The formation of gas is likewise prevented when the substance B is separated from the gas A by a moveable sheet ef matter, a bladder for example. The gas A must always be in immediate contact with the substance B, and the formation of gas takes place only at the points of contact. This phenomenon is explained by Berthollet (Statique Chim. 1, 280) on the hypothesis of a chemical solution of the gas B in the gas A,-with respect to which it must be particularly observed that neither the density nor the chemical nature, but only the volume of the gas A has any influence on the quantity of the gas B produced. Dalton, on the other hand, explains it by supposing-either that one gas acts as a vacuum to the other, or that the unequal magnitudes of the globules of the two gases give rise to an internal motion and uniform distribution of their particles. The phenomenon is perhaps best explained by supposing that the adhesion of the existing gas A for the gas B, which is to be produced, is in all cases more powerful than the pressure which A, whatever may be its density, exerts upon the substance B, inasmuch as the adhesion increases with the density,-and consequently, that the body B diffuses itself with its proper elasticity through any other gas in the same quantity as in vacuo. (Compare pp.

22, 23.)

When a gas B is generated within an unyielding envelope the interior of which is already occupied by another gas A, both gases together press against the sides, each with its own proper tension. If for example the tension of the gas A and that of the gas By, the tension or elasticity of the gaseous mixture=x+y.

If, on the contrary, the sides of the vessel in which the gaseous mixture is generated yield to such an extent that the gas remains constantly under the pressure x, the mixed gases first expand (since according to Mariotte's law the elasticity of a gas varies inversely as its volume) in the ratio of a x+y. But by this expansion of the gases A and B, the elasticity of the gaseous mixture is diminished in such proportion that

x

it is reduced to a: the elasticity of the gas A therefore becomes (x + y

:x=x:

=

x y

x y x + y

and that of the B
gas

(a

x + y : x = y:

x y x +

With this the further expansion ceases, supposing that no x + y portion of the substance B remains unvaporized. If however this be the case, then-since the space has become larger-a fresh quantity of B is converted into gas, and the gas B regains the elasticity = y: and since by this action the elasticity of the gas and therefore also its expansion increases, the formation of gas and the expansion of the gaseous mixture go on till-as Dalton has shown-the volume of the gaseous mixture is to the original volume of the gas A as x: x-y. For since in this gaseous mixture, after its complete expansion under the pressure x, the gas B has an elasticity =y, that of the gas A must be only x y, in order that the elasticity of the gaseous mixture may exactly balance the external

pressure a.

Examples in which the preceding table of the elasticity of gases is made use of:-If at a temperature of 25° C. and under a pressure corresponding to 336 Par. lines, water be introduced in excess into a vessel containing one measure of dry atmospheric enclosed by mercury, thensupposing that the mercury stands at the same level within and without the vessel, and that the atmospheric pressure remains constant,-one measure of air will, by taking up vapour of water (since the tension of vapour of water at 25° C. or 20° R. is equal to 10.08 lines), be expanded in the ratio of 336 10'08 336 325.92: 336 When alcohol evaporates in dry air at 0° C. and under a pressure of 30 English inches, the volume is increased in the ratio of 30- 04 30 296: 30; and in the case of ether at 0° C. the expansion is as 30 — 6·2 : 30 = 23·8 : 30.

b. The affinity of Heat for the Ponderable Substance must be superior to all other forces acting upon the same.

a.

To the Cohesion of the Ponderable Substance.

Since a body introduced into a space devoid of air fills that space with a quantity of gas corresponding to the temperature-since this quantity, though it decreases with the temperature, cannot be reduced to nothing excepting at the absolute zero-since all substances, at least at high temperatures, are visibly converted into gas, and must therefore afford some, although perhaps but a small quantity at low temperatures-finally, since this formation of gas takes place in a space filled with air in the same quantity as in vacuo, only with less rapidity,-it may be concluded that of every substance situated on the surface of the earth, a certain, though perhaps extremely small portion must be converted into gas. That no diminution of weight can however be observed in pieces of metal, &c. even after many years, may result either from this circumstance, that at a certain temperature considerably below the boiling point, the cohesion and gravitation of the ponderable substance may overcome its affinity for heat-which indeed is Faraday's view (Ann. Phil. 28, 436; also Pogg. 9, 1); or possibly from this, that when once the several bodies on the earth's surface have introduced into the atmosphere certain portions of their gases or vapours corresponding to the existing temperature, these gases may exert a pressure on the remaining matter sufficient to

prevent its further vaporization-the fluctuations of temperature on the surface of our earth being perhaps too small, in comparison with the wide interval between the ordinary temperatures of solid bodies and their boiling points, to cause any sensible condensation of these gases when the atmosphere cools, or to give rise to fresh formations of the same gases when a rise of temperature takes place,-unless perhaps many meteoric phenomena may be due to some such cause.

Ice evaporates in vacuo at temperatures below -40°, sulphuric ether at-51°, at which temperature it is solid (Configliacchi), sulphuret of carbon at 62. (Marcet.) The Torricellian vacuum is in reality filled with vapour of mercury. In the open air ice evaporates far below 0°, mercury at +15.5°, but not at -6.7° (Faraday); oil of vitriol does not evaporate at ordinary temperatures. (Bellami, Pogg. 9, 7.)-By enclosing two substances in a flask filled with air, keeping them separate from one another, the one in the solid form, the other in the state of aqueous solution, and exposing the flask for four years to the ordinary temperature of the air, the following results are obtained: Crystallized oxalic acid and crystallized oxalate of ammonia evaporate over in small quantities towards aqueous solution of chloride of calcium; also protochloride of mercury towards aqueous solution of chloride of calcium; also protochloride of mercury towards aqueous solution of potash, and nitrate of ammonia in minute quantity to dilute sulphuric acid. The following on the contrary do not volatilize: sal-ammoniac or common salt towards dilute sulphuric acid; arsenious acid or calomel to caustic potash; aqueous solution of iodide of potassium to chloride of lead; common salt dissolved in water to crystallized nitrate of silver; sulphate of soda dissolved in water to crystallized hydrated chloride of barium; aqueous solution of chloride of calcium to carbonate of soda; and aqueous solution of sulphate of copper or persulphate of iron to hydrated ferrocyanide of potassium. In most of these cases, water evaporates over to the solid body and dissolves it; but the solution does not mix with that of the salt previously dissolved in water. (Faraday, J. of Roy. Inst. 1, 70; also Pogg. 19, 545.)

B. To the Affinity of the volatile Substance for any other less volatile body with which it may be combined.

Substances which from their own nature have a strong tendency to assume the gaseous form, so that when separated from other bodies, they exist for the most part only in the state of gas,-often, when combined with other bodies, either become wholly incapable of passing into the gaseous condition, or undergo that change only at high temperatures, because the affinity of the other body acts in opposition to that of heat.

Oxygen in red oxide of mercury, peroxide of manganese, and other metallic oxides requires a red heat to convert it into gas: so also does carbonic acid when combined with lime and other oxides of metals. Most metallic oxides do not part with their oxygen even at the highest temperatures, but are partly converted into vapour before reduction and condense again to the state of solid oxide in the cold.

Water, which when pure, boils at 100° exhibits a much higher boiling point when combined with salts, sulphuric acid, phosphoric acid and other less volatile substances.

The following table gives the boiling points of various aqueous solutions according to Faraday (Ann. Chim. Phys. 20, 324) and Griffiths (Qu. J. of Sc. 18, 90; also Pogg. 2. 227).-Column A contains the names of the