The name given to all elastic aeriform fluids (except the atmospheric air) which retain that state at all ordinary temperatures and pressures. For a long time the gases were supposed to be permanently elastic; but about the year 1823 Sir Humphrey Davy,assisted by Mr. Faraday, succeeded in reducing several of them to a liquid state by subjecting them to great pressure, and an extreme degree of cold; upon removing the pressure, and restoring the natural temperature, the liquids became again converted into gases. This discovery induced these gentlemen to institute a series of experiments with the view of ascertaining whether the vapours arising from the gases thus liquefied might not be rendered available as mechanical agents in lieu of steam, and be applicable to the same purposes. These experiments were detailed in a paper read at the Royal Institution, and from this paper we select the following results: -

Sulphuretted Hydrogen, which condenses readily at 3° Fahr, under a pressure equal to the elastic force of an atmosphere compressed to one-fourteenth, had its elastic force increased to that of an atmosphere compressed to one-seventeenth by an addition of 47° of temperature. Liquid muriatic acid at 3° Fahr, exerted an elastic force of 20 atmospheres; by an increase of 22° of temperature, its force was increased to 25 atmospheres; and by a farther addition of 26°, its force became equal to 40 atmospheres. Carbonic acid at 12° Fahr, exerted a force of 20 atmospheres; and at 32° Fahr, its pressure was equal to 36 atmospheres, making an increase of pressure equal to 13 atmospheres by an increase of 20° of temperature; and this immense force of 36 atmospheres being exerted at the freezing point of water. From the above experiments, it will be seen that great accessions of force are obtained by very slight additions of heat; and Sir Humphrey observes, in the Memoir, that - "if future experiments should realize the views here developed, the mere difference of temperature betwixt sunshine and shade, and air and water, will be sufficient to produce results that have hitherto been obtained only by a great expenditure of fuel." Upon this subject Mr. Tredgold, in his excellent Treatise on the Steam Engine, observes, - "l think it will be found that two other circumstances should be considered in estimating the fitness of compressed gases as mechanical agents. First, the distance through which the force will act; for if this distance of its action be less in the same proportion as its force is increased by compression, no advantage will be gained; the power of a mechanical agent being jointly as its force, and the space through which that force acts. Secondly, the quantity of heat required to produce the change of temperature, is also to be considered; for if the mechanical power requires as great an expenditure of heat as common steam, no advantage will be gained; in fact, the only prospect they afford of being useful, is through lessening the extent of the surface to be heated." Mr. Tredgold then gives the following Table of the gases liquefied by Mr. Faraday, with their densities as far as can be ascertained, and with a column to show their mechanical power as compared with steam, according to the spaces through which they act. from which it will be seen that in effect they are all inferior to the latter.

The quantity of fuel requisite for their vaporization is not known.

Spec. Grav. Air - 1.

Spec. Grav. of Liquid, Water - 1.

Temperature.

Pressure in Atmosphere

Carbonic acid gas . .

1.527

32°

36

Sulphuric acid gas . .

2.777

1.42

45

2

426

Sulphur. hydrogen gas .

1.192

.9

50

17

530

Euchlorine gas . . .

2.365

Nitrous oxide ....

1.527

45

50

Cyanogen

1,818

.9

45

3,6

395

Ammonia.....

.596

.76

50

6.5

1057

Muriatic acid gas . . .

1.285

50

40

Chlorine

2.496

1.33

50

4

440

Steam of water . . .

.48

1.000

212

1.

1711