1. Gases may be mixed together in the same manner as liquids, and with nearly similar results. Some never intimately combine, or are merely mechanically mingled, while others unite closely, and form new chymical compounds, possessing properties very different from those of their components.

Table of pharmaceutical gases which may he mixed together without any apparent change in their state.

i. Gases that may be mixed, but which do not combine :

Oxygen with carbonic acid gas. . Hydrogen with muriatic acid, olefiant gas, sulphureted hydrogen, and ammoniacal gas. Nitrogen with almost all the other gases.

ii. Pharmaceutical gases which mix without any change, but may be made to combine:

Oxygen with chlorine, hydrogen, nitrogen, carbonic oxide, sulphurous acid, and protoxide of nitrogen. Hydrogen with iodine and nitrogen.

iii. Pharmaceutical gases which mix without change, but may be made to decompose each other:

Oxygen with carbureted hydrogen, sulphureted hydrogen, cyanogen, and ammonia.

Hydrogen with carbonic acid, nitrous gas, nitrous oxide, and sulphurous acid.

Although these gases, when simply mixed, do not chymi-cally combine, or act on each other, yet the mixtures, even independent of agitation, are homogeneous compounds, or the gases do not arrange themselves according to their gravities, but are all equally diffused in the mixture, every portion of it containing exactly the same proportion of each of the mixed gases, and when once mixed they never afterwards separate. The bulk, also, after mixture, is exactly equal to the sum of the bulks of the gases which have been mixed; or each gas occupies the same space as when separate; and the specific gravity of the mixture is exactly the mean of that of the gases mixed. Hence the mixture of these gases appears to be a species of combination, similar to that which takes place in mixing together vinegar and water, or similar liquids; and that this is actually the case appears from the experiments of Mr. Dalton', who found that two gases of different specific gravity, when merely brought into contact, the lightest being placed uppermost and the heaviest undermost, will mix together spontaneously, if left at rest; and the same effect will take place if the gases be put in separate vessels, communicating with each other by a tube only, as in the experiments of M. Berthollet 2, who supposes that the gases dissolve one another, while Mr. Dalton conceives that they are merely mechanically combined.

Vapours and gases unite in nearly the same manner as gases and gases; and this combination enables the vapour to sustain the pressure of the incumbent atmosphere, which it could not otherwise support without being condensed. They are also retained together by a species of affinity, sufficient to cause their intimate and uniform mixture, but not strong enough to produce chymical combination.

2. Of the gaseous bodies which chymically unite when they are mixed, "some combine in all circumstances, by mere mixture; others unite only in particular states."

1 Phil. Mag. xxiv. 8. 2 Statique Chymique, i. 274. 487.

Table Of Some Of The Gases Which Unite By Simple Mixture, And Form Pharmaceutical Products

The two first of these products are vapours, the third is a liquid, and the rest are solid bodies.

a. Oxygen and binoxide of nitrogen unite in two different proportions; or 100 volumes of oxygen gas are capable of uniting with 133, and also with 200 volumes of binoxide of nitrogen. The first proportions produce nitric acid, which, as binoxide is a compound of 66 2/3 volumes of nitrogen, and 66 2/3 of oxygen, therefore appears to be a compound of 166§ volumes of oxygen combined with 66f of nitrogen; the second produce nitrous acid, which appears on the same principles to be a compound of 100 volumes of oxygen united to 200 of nitrogen. It is, however, probable that these gases will combine in different proportions from the above, and produce nitric acid, containing various proportions of nitrous gas in solution. The immediate effect of their combination is the formation of a yellow-coloured vapour.

b. Ammoniacal gas and aqueous vapour combine the moment they are brought into contact; and are condensed to a liquid; but the exact proportions are unknown.

c. Ammoniacal gas and muriatic acid gas unite when equal volumes of each are brought into contact; and the result of the mixture is a mutual condensation into a white powder, or muriate of ammonia. If 100 cubic inches of muriatic acid gas, therefore, weigh 59.80, and the same bulk of ammoniacal gas 18.67 grains troy, muriate of ammonia must be a compound of three parts of muriatic acid by weight, united to 1 part of ammonia, or of 1 proportion of each.

d, Ammoniacal gas and carbonic acid unite in equal volumes, and condense into a solid salt, which is carbonate of ammonia. e. The combination of ammoniacal gas with sulphureted hydrogen in equal volumes forms also a solid compound.

Table Of Some Of The Gases Which Mix Without Chymically Combining, But May Be Made To Combine; And Of The Pharmaceutical Products Formed By The Combinations

a. The two first combinations may be effected by combustion, and the third by electricity. It has been supposed that the heat in these cases acts indirectly only, and produces the combination by forcibly expanding one portion of the gas, and thence producing a sudden compression in the neighbouring portions, so that some of the atoms of the two gases, being brought within the sphere of action of the attraction of affinity, combine; while the caloric evolved by this union occasioning the same expansion to be constantly renewed, the whole gaseous mixture is by degrees combined. This theory is confirmed by the experiments of Biet, which proved that oxygen and hydrogen gases can be made to combine by simple pressure.

b. Oxygen gas and sulphurous acid gas probably combine when simply mixed together, but the fact has not been ascertained in a decisive manner. They undoubtedly combine in a red heat; it is, however, probable that the combination is not direct, but that a portion of the sulphur is first separated, and then enters into combustion.

All these gases suffer condensation when they combine, as displayed in the following table1:

1 This Table is copied from that drawn up by Dr. Thomson (Syst. of Chymistry, 5th edit. iii. 47.)

Table Of The Principal Pharmaceutical Gases Which Mutually Decompose Each Other When Mixed Together

The first decomposition is attended with combustion.

a. Chlorine and ammonia, when brought into contact, excite spontaneous combustion; one fourth of the ammonia is decomposed; the hydrogen of that portion uniting with the chlorine, and forming muriatic acid, which enters into combination with the remaining ammonia, forming the muriate, whilst the nitrogen is dissipated.

b. Chlorine with sulphureted hydrogen is also formed into muriatic acid, whilst the sulphur is precipitated.

c. Chlorine with nitrous gas has no reciprocal action : it is brought to the state of common muriatic acid; while the nitrous gas is converted into nitric acid. The requisite proportions, according to Humboldt1, are equal bulks of each gas.

d. Sulphureted hydrogen gas and nitrous gas mixed together in a dry state suffer spontaneous decomposition; sulphur is deposited, and protoxide of nitrogen, ammonia, and water, are produced.

Table Of Some Gases Which Mix Without Spontaneous Decomposition, Hut Which May Be Made To Decompose Each Other By Peculiar Treatment

Some of these decompositions are rapidly produced by combustion; others take place without combustion, and are, consequently, very slow.

Oxygen and sulphureted hydrogen gases, when mixed together, do not suffer any change; " but, if the mixture be made to approach an ignited body, combustion immediately takes place, and the products vary according to the proportion of the gases mixed." In all cases a great proportion of the sulphur is deposited, and some sulphurous acid is formed, owing to the caloric evolved by the combustion of the hydrogen setting fire to a portion of the sulphur.

The vapours of ether and of alcohol detonate when certain proportions are mixed with common air, or rather with the oxygen it contains; and the products are carbonic acid' and water; the quantity of the former being very considerable,

1 Ann. de Chymie, torn, xxviii. p. 142. F 4 when the vapour fired is that of alcohol. If the proportion of the ethereal vapour be one cubic inch, which should weigh 0.7 grains, and that of the oxygen 6.8 inches, weighing 2.3 grains, the products will be 4.6 inches of carbonic acid, and a nortion of water.

The following Table shows the quantity of oxygen necessary for decomposing 100 inches of each of the above gases.

When 100 measures of nitrous oxide are mixed with 100 of hydrogen, and fired by the electric spark, a complete combustion of the hydrogen and decomposition of the nitrous oxide take place, and water and nitrogen are produced. The superior affinity of the hydrogen over nitrogen is, in this case, aided by the caloric which is evolved during the combustion of the former gas.

Nitrous gas does not detonate with, nor decompose, any of the combustible gases which have been just considered; but when moist iron is placed in contact with nitrous gas, the hydrogen evolved by the decomposition of the water for the oxidizement of the iron decomposes the nitrous gas, converting it into nitrous oxide, and forming ammonia.

The other cases of decomposition enumerated in the table are slowly produced by the continued action of electricity, without any combustion taking place.

The combinations of gases with gases are not immediately effected for the purposes of pharmacy, but several of them occur during many of the operations for the preparation of the saline and metallic compounds; and, therefore, require to be known and understood for comprehending the theory of these operations.