From the difference which exists between the constitution of gases and solids, their combination appears to be opposed by the elasticity of the former and the cohesion of the latter; but, nevertheless, under proper circumstances, both the simple and the compound gases combine with solids.

The simple gases are oxygen, chlorine, hydrogen, nitrogen. Of these, oxygen combines with all the known simple solids; which are carbon, boron, silicon, phosphorus, sulphur, selenium, iodine, bromine, and the metals; hydrogen requires that the solids be brought into a fluid state before it can combine with them; and nitrogen combines with one solid only, which is carbon.

a. Oxygen gas unites with carbon in two proportions, and forms carbonic acid, and carbonic oxide, which are gaseous fluids. Experiment has demonstrated that carbonic acid is composed of 27.27 parts of carbon and 72.73 of oxygen: thence, an atom of it must consist of two atoms of oxygen (8x2)=l6, and one of carbon - 6' 12, making the equivalent = 22.12. Carbonic oxide is composed of 41 parts by weight of oxygen and 28 of carbon; or of one atom of oxygen = 8, and one of carbon = 6.12, making the equivalent = 14.12. Oxygen combines with boron in one proportion only, forming boracic acid, which is a compound of one atom of boron = 10.9, and three atoms of oxygen (8 x 3) = 24, the equivalent being = 34.9; or 100 parts of boron and 228.57 of oxygen.

With phosphorus, oxygen unites in three proportions, and forms hypophosphorous acid, phosphorous acid, and phosphoric acid, which are all solid substances. The proportions of the constituents of the two last compounds are the following: phosphorous acid consists of two atoms of phosphorus = 31.4 and three of oxygen (8 x 3) = 24, making the equivalent

= 55.4: and phosphoric acid of two atoms of phosphorus =31.4, and five of oxygen (8 x 5) -40; thus affording an equivalent of = 71.4.

Oxygen combines with sulphur in the same manner, and forms hyposulphurous acid, sulphurous acid, and sulphuric acid; the first of which is supposed to be a solid, the second and third are gases. The constituents of hyposulphurous acid are two atoms of sulphur = 32.2, and two of oxygen = 16, making its equivalent number = 48.2; those of sulphurous acid are, one of sulphur = 16.1, and two of oxygen - 16, making its equivalent = 32.1, while sulphuric acid is formed by the union of one atom of sulphur = l6.l, to three atoms of oxygen = 24, making its equivalent - 40.1. The oxygen in these compounds is much more loosely combined than in the preceding; and thence the greater facility with which they are decomposed by combustibles.

Oxygen unites readily with the metals, forming solid compounds. In them the oxygen is condensed, while the cohesion of the metallic particles is merely weakened, but not overcome. It has been supposed that the metals combine with determinate proportions only of oxygen, that there are no intermediate combinations, and that, in general, there are only one and two degrees of metallic oxidizement; there are, however, some which unite with three proportions. The compounds formed by the combination of oxygen with the metals have a powerful action on the animal economy, and are consequently very important objects of pharmacy. In this state metals become, also, capable of combining with acids, and acquire still greater activity; and, as the degree of oxidizement varies, so the combination of the oxide in these different states with the same acid forms compounds differing from one another, and exerting various degrees of medicinal power.

b. Chlorine unites with all the metals, forming solid compounds, some of which are active remedial agents.

c. Hydrogen has a considerable affinity for the simple combustibles; but they do not combine unless the cohesive force, which keeps together the particles of the solid, be overcome, or the hydrogen be exhibited in a nascent state; and, therefore, it is chiefly by the decomposition of water that these combinations are effected. Owing to the great elasticity of hydrogen gas, all the known combinations of it with the simple combustibles, except one, are gases.

Hydrogen unites with carbon in four proportions, constituting, 1. olefiant gas, which is composed of one atom of carbon and one of hydrogen; 2. bihydruret, composed of two atoms of hydrogen and one atom of carbon; 3. bicarbureted hydrogen, composed of one atom of hydrogen and two of carbon; and 4. ether, composed of two atoms of olefiant gas and one of water.

"Sulphureted hydrogen is the most intimate of the combinations of sulphur and hydrogen. A red heat does not decompose it." It is commonly formed by the "decomposition of water by the compound agency of an acid, and a metal united to sulphur." In this case no obstable is raised to the combination of the hydrogen, which is nascent, by the attraction of cohesion, the sulphur being just separated from the metal.

The combination of hydrogen with phosphorus is also obtained by the decomposition of water, by boiling the phosphorus in a liquid alkali, which retains the phosphorus in a temperature sufficient for enabling it to effect the decomposition. The oxygen of the water unites with one portion of the phosphorus, and forms phosphoric acid; while, at the same time, the hydrogen unites with the other portion, and forms phosphureted hydrogen.

d. The combinations of nitrogen gas with the simple solid combustibles are not yet sufficiently understood, except its combination with carbon to form cyanogen,1 which is a compound of one atom of nitrogen and two atoms of carbon. This compound unites with combustibles, and forms new compounds; for example, iodide of cyanogen and bromide of cyanogen.

The compound gases do not enter into many combinations with solids, if the salts, which the acid gases form with alkalies, earths, and metallic oxides, be excepted, and those formed by ammonia with the solid acids. In general, they are decomposed. Thus, carbon, phosphorus, sulphur, and many metals decompose nitrous gas, nitric acid, and chlorine; and sulphurous acid is decomposed by the metals.

Such are the effects of the combinations of solids, liquids, and gases. The knowledge of the laws which regulate them, and the results of the combinations, are of the Utmost importance; the greater number of the operations of pharmacy consisting of the combination of substances, with a view either of obtaining compounds by their direct chymical union, or the products of chymical action resulting from their mutual decomposition.

1 This name was imposed upon this gaseous compound by Gay Lussac, who discovered it in 1815; and who found that, in combination with hydrogen, it forms prussic acid, which he therefore named hydrocyanic acid. It may be obtained from exposing dry bicyanide of mercury in a small retort to a moderate heat. It is a colourless, transparent, elastic fluid, with a strong disagreeable odour, soluble in water and alcohol, and highly inflammable, burning with a beautiful violet-coloured flame.