Gases are aeriform fluids, possessed of very different properties, but all agreeing in that peculiar kind of elasticity which constitutes aeriform bodies.

1. Constitution Of Gases

The particles of gases, like those of liquids, are movable upon each other; but gases differ from liquids in possessing elasticity, or that power which allows them to be compressed into a smaller bulk; and by which, however large a portion of any gas contained in a vessel be taken away, the small portion which is left is enabled to expand so as to fill the vessel. The bulk of common air may be thus easily reduced or increased 3000 times; and, indeed, there does not appear to be any limit to expansion. These properties of airs depend on the repulsion which exists between their component particles: the force of which, according to Newton, is always inversely as the distance of their centres from each other. As gases contain a larger proportion of combined caloric than any other class of bodies, it is very probable that caloric is the cause of the repulsion which exists between their particles, or of their elasticity; and hence the addition of sensible heat to gases increases their elasticity, while the abstraction of it, or the application of cold, diminishes it.

No degree of compression yet attained, nor any abstraction of caloric, can alter the constitution of air; but by compression several of the gases may be reduced to liquids, and by the simple abstraction of caloric all the vapours can be reduced to the liquid, or even the solid state.

Arrangement of pharmaceutical gases according to their composition.1

I. Simple Gases

1. Oxygen.

2. Hydrogen

3. Nitrogen.

4. Chlorine.

5. Sulphur vapour.

6. Iodine vapour.

II. Compound Gases

a. Simple gases combined.

7. Hydriodic acid.

8. Binoxide of nitrogen

9. Muriatic acid gas.

10. Ammonia.

11. Steam.

b. Oxygen and a solid base.

12. Carbonic acid gas.

13. Sulphurous acid gas.

14. Sulphuric acid vapour.

c. Hydrogen and a solid base.

15. Cyanogen.

16. Olefiant gas.

17. Sulphureted hydrogen.

1 Thomson's Chymistry, 4th edit. iii. 437.

d. Fluorine, Chlorine, Cyanogen, with a base. 18. Hydrocyanic (Prussic) acid.

e. Triple or quadruple compound gases.

19. Sulphuric ether.

20. Nitrous ether.

21. Vapour of alcohol.

22.-------------oil of turpentine.

All these gases are invisible, except chlorine, which has a yellowish green colour; but when gases of very different specific gravity are mixed together, they become in a certain degree visible. With respect to the specific gravity of gases, there is a greater difference between them under the same pressure, and at an equal temperature than exists between liquid substances; a circumstance which must depend either on a difference of the repulsive force, or of the weight of the atoms in different gases.

A Table Of The Density And Weight Of 100 Cubic Inches Of The Pharmaceutical Gases At The Temperature Of 60&Deg;, And A Barometrical Pressure Of 30&Deg

Water is a constituent of almost every gaseous body; and the quantity of it contained in each depends upon the bulk, not the density, of the gas. It also appears probable, that the weight of it contained in 100 inches of all gases under the same pressure, and at the same temperature, is very nearly the same. It can be separated, in a great degree, by sulphuric acid, very dry alkalies, lime, and other matters which have a powerful attraction for water; but the whole of the moisture cannot be absorbed by these substances; and it is therefore undetermined whether gases can exist independent of the presence of water. The quantity present in any gas is regulated, in a great degree, by the temperature: for, if this be high, a much larger proportion of moisture can be retained in the elastic form; but in a low temperature it is deposited.

Vapours differ from gases in several particulars. Their "elasticity does not increase as the pressure, like that of gases:" they can be condensed by pressure, and by the abstraction of caloric, into liquids; and even some of those bodies which are regarded as real gases, such as ammonia and chlorine, are reduced by pressure at a low temperature to the liquid form. The elasticity of the majority of vapours is sensible at a high temperature only; but some become sensibly elastic at the common temperature.