heated acquires increased mobility; expands to twice its original volume, and is then suddenly converted into vapour;-this change takes place at 207° C. (404-6° Fah.), when the alcohol occupies just half the volume of the tube. If the tube is more than half filled with alcohol, it bursts when heated. A glass tube one-third filled with water becomes opaque when heated, and bursts after a few seconds. If this chemical action of the water on the glass be diminished by the addition of a little carbonate of soda, the transparency of the glass will be much less impaired; and if the space occupied by the water be of the whole tube, the liquid will be converted into vapour at about the temperature of melting zinc. (Cagniard de la Tour.) If liquid carbonic acid sealed in a glass tube occupies of the volume of the tube at 0°, it neither increases nor diminishes in bulk when heated, because the increase of volume produced by heating is just compensated by the diminution caused by vaporization. If it occupies of the space at 0°, its volume diminishes when it is heated, and increases when it is cooled, because the loss by evaporation preponderates over the expansion. If it takes up of the space, its volume increases by heat and contracts by cold, as in a thermometer; but at + 30° (86° Fah.) the whole is converted into vapour. The gas, which at 0° occupies the space above the liquid carbonic acid would, if condensed, yield of its volume of liquid acid; and that which is produced at + 30° would give ( Gm.) of its volume of liquid carbonic acid at 0°. (Thilorier.)

When a volatile substance is heated, the tension of the gas increases in a much higher ratio than its density, because the heat not only increases the elasticity by producing more gas, but likewise by its expanding force imparts a higher degree of elasticity to the gas already formed. In the case of water the following table has been calculated by Despretz.

The higher the temperature the greater is the difference; hence the advantage of high-pressure steam-engines. (Despretz.)

Lastly, when the volume of the empty space and the temperature are given, the quantity of gas produced varies with the nature of the body: 1. According to the affinity between the ponderable matter and heat: for the weaker this affinity, the smaller will be the quantity of gas which by its tension will be able to stop the further production of gas; and the greater the affinity, the denser and more elastic must be the gas produced before the process of evaporation ceases;-2. According to the atomic weight of the substance: for the atomic weight is closely connected with the specific gravity of the gas (pp. 52 and 64).

The most volatile among terrestrial substances, so far as they have not been prevented by combination with others, have especially assumed the gaseous form; and the aeriform mixture thus produced constitutes the Atmosphere or Common Air surrounding the earth. The weight with which this mass of air presses on the earth's surface varies with atmo

spheric changes and with the height of the locality. At the level of the sea it is mainly equal to the pressure which would be produced by a column of mercury of the height of 28 Paris inches 0-9 line, or 29.92 English inches, or 0.76 mètre; and the pressure corresponding to this height is taken as the normal barometric pressure, and called Atmospheric Pressure, Pressure of the Atmospheric Column, Atmosphere. If a glass tube about 1 metre in length, and closed at one end, be carefully filled with mercury and inverted in that liquid, the pressure of the air acts in such a manner that the mercury within the tube stands about 0.76 met. above the level of that without. The space above the mercurial column is called the Torricellian vacuum. The pressure of the atmosphere amounts to about 15 lbs. on a square inch of surface, or 1033 grammes on a square centimetre.

A volatile body at the surface of the earth is placed, as it were, in a vessel with moveable sides, which press it in all directions with a force equivalent to the weight of a column of mercury 0.76 met. high. If the body is enclosed in a flexible bag, or shut off by mercury or in any other manner from the immediate contact of the air, its conversion into vapour will not take place unless the elasticity of its vapour at the given temperature be at least equal to the atmospheric pressure, or sufficient to support a column of mercury 0.76 met. high. Since, however, this elasticity increases with the temperature, there exists for each body a certain temperature at which the elasticity of its vapour is a balance for the pressure of the atmosphere, and consequently cannot be restrained by that pressure. This temperature is the Boiling Point of the body. At this point it · passes into vapour, in spite of the atmospheric pressure, as soon as the additional heat required to volatilize it is supplied.

If a substance, the tension of whose vapour at the given temperature is less than the atmospheric pressure, be introduced in excess into the Torricellian vacuum-that is to say-into a space in which the pressure of the atmosphere is balanced by a corresponding column of mercury, the substance fills the empty space with vapour having an elasticity corresponding to the temperature. By this the column of mercury in the barometer-tube is depressed through a height corresponding to the elasticity: for the tension of the vapour now supplies the place of part of the column of mercury. In this manner the elasticity of a vapour, when less than the atmospheric pressure, may be estimated in lines or millimetres of the mercurial column.

If heat be now applied to the unvaporized portion of the body situated in the vacuum, the density and elasticity of the vapour increase; the vapour depresses the mercury more and more, the higher the temperature is raised, and the more elastic the vapour in consequence becomes; till at length the mercury within the barometer-tube stands at the same level as that without. This temperature again is the boiling point: for the tension of the vapour is now of itself a balance for the pressure of the air.

As the atmospheric pressure increases or diminishes, the boiling point -or temperature at which the body assumes the gaseous form-rises or falls; it is usual, however, to state the boiling-point as corresponding to a pressure of 0.76 metre.

In the following data of the elasticities of different gases, it is to be observed that the gas or vapour is supposed to be in contact with unvaporized matter; and therefore that a saturated vapour or gas is produced. The more permanent gases are exposed to the different temperatures in contact with unvaporized matter in a sealed glass tube, a narrow gradu

ated glass tube filled with air and closed with a drop of mercury (Manometer) being placed in the same tube to indicate the pressure by the various degrees of compression of the enclosed air. The more volatile substances are introduced into the Torricellian vacuum, where they depress the mercurial column in proportion to the elasticity of their vapours.

The elasticity of the more permanent gases is expressed in whole atmospheres. In this table, D F denotes Davy & Faraday (Phil. Transact. 1823, 160 and 189); N, Niemann (Br. Archiv. 36, 175); B, Bunsen (Pogg. 46, 95); T, Thilorier (Ann. Chim. Phys. 60, 434); M, Mitchell (Sill. Amer. J. 1840, 177, also Ann. Pharm. 37, 354).

According to Oerstedt and Swendson, the tension of sulphurous acid at +21-25° amounts to 3.269 atmospheres.

According to Wach (Schw. 50, 33), the tension of sulphurous acid at 6.75 3.2 and at 19.5° 4.36 atmospheres.-The boiling-point of sulphurous acid (at a pressure of 0.76 metre) is about 10°, that of cyanogen -20°, of ammonia 33.7°, and of carbonic acid about 50°. Phosphuretted hydrogen, hydriodic acid, hydrobromic acid, and hydrochloric acid gases may, according to Bunsen, be liquefied by cooling to 90°.

The elasticity of vapours is expressed sometimes in atmospheres, sometimes in metres, sometimes in inches and lines. The tension of the vapour of water at low temperatures is the mean tension calculated by Kämtz (Schw. 42, 385) from the experiments of Rouppe, Schmidt, and Ure; that of the same vapour at high temperatures is given by Arago and Dulong (Ann. Chim. Phys. 43, 74; also Pogg. 18, 437; also Schw. 59, 167); that of vapour of mercury by Avogadro (Ann. Chim. Phys. 49, 369); that of the vapour of sulphuret of carbon by Marx (Schw. 62, 460), and those of the vapours of alcohol, ether, rock-oil, and oil of turpentine by Ure. (Phil. Trans. 1818, p. 338; also Schw. 28, 328.) Some of the following series are merely extracts. Degrees of Reaumur and Fahrenheit will also be found here and there.

• According to a former statement, 5.

The tension of the vapour of sulphuret of carbon at 12° C. amounts to 0-2 metre (Berzelius & Marcet); at 22.5° to 0.3184 M. (Cluzel); at 46.6° to 0.76 M. (Gay-Lussac); at 57.6° to 14, at 160° to 77, and at 171° to 8.95 atmospheres. (Davy & Faraday.)—On the tension of the vapours of water, alcohol, and ether, comp. Dalton (Ann. Phil. 15, 130) and August. (Pogg. 13, 122.)

[Calculation of the tension of aqueous vapour at different temperatures by F. Von Wrede. (Pogg. 53, 225.)]

Magnus (Pogg. 61, 225) and Regnault (N. Ann. Chim. Phys. 11, 334; 13, 196) have also determined the tension of the vapour of water at different temperatures. The results obtained by these philosophers, whose experiments were made independently of each other, present the most remarkable agreement, as will be seen from the following table.