Standard of Volume for Gases.-The next point that we have to determine is, how best to represent the constitution of the numerous gases which are known to us; or in other words, how to represent the kind and the quantity of the elements which are present in equal volumes of them. It is obviously necessary, if we would compare them, to deal with equal volumes. It would appear at first sight that the best and simplest plan would be to state in all cases the composition of one volume of each gas. It is quite possible to do so, and indeed it does not very much matter what volume we take as the standard. But if we select one volume we are compelled to make constant use of fractions in representing the composition of gases, and it is therefore usual to take two volumes as the standard of comparison. A single example will make this plain.

We have already seen that 1 volume of chlorine combines with 1 volume of hydrogen to form 2 volumes of hydrochloric acid. If we wish to state the composition of 1 volume of hydrochloric acid, we must do so by saying that it contains a volume of chlorine and a volume of hydrogen. But on the 2 volume system the fractions are avoided. 2 volumes of chlorine are said to combine with 2 of hydrogen to form 4 of hydrochloric acid; and 2 volumes of hydrochloric acid contain 1 of chlorine and 1 of hydrogen.

Let us therefore agree to state in every case the composition of 2 volumes of gas. If we do so, we must never represent less than 2 volumes as taking part in any chemical process. We must in fact assign arbitrarily to the smallest quantity of any gas that is ever concerned in any chemical change a volume of 2. It is almost needless to add that our facts will then be equally true if we apply them to 2 millionths of a cubic inch or to 2 gallons.

Chemical Symbols and Formula.-Each one of the elements is denoted in the system of modern chemistry by a symbol, which is either the first letter, or the first and some other characteristic letter, of its Latin name. Thus, oxygen is denoted by O; hydrogen by H; carbon by C; chlorine, which begins with the same letter, by Cl; calcium by Ca, and so on. In most cases the Latin name is the same as the English, but in a few instances it is different and the symbol must be

remembered carefully. The following are the exceptions : Antimony is Sb., from stibium; copper, Cu., from cuprum; gold, Au., from aurum; iron, Fe., from ferrum; lead Pb., from plumbum; mercury, Hg., from hydrargyrum; potassium, K., from kalium; silver Ag., from argentum; sodium Na., from natrium; tin, Sn., from stannum ; and tungsten, W., from wolfram. Compounds are described by formula, which consist of the symbols of the elements composing them. Thus the formula for hydrochloric acid is HCl., for mercuric oxide, HgO., and so on.

Symbols then are employed to denote the elements, but they are also employed, by a very useful extension, to denote definite weights of them. What these weights are we have now to consider, confining ourselves for the present exclusively to elements in the state of gas.

To begin with, let us represent the standard 2 volumes of hydrogen by the symbol H, and 2 volumes of chlorine by the symbol Cl. How are we in this case to represent hydrochloric acid, which, as we have already seen, contains in 2 volumes, 1 of hydrogen and 1 of chlorine? 2 volumes of this compound must evidently be represented by such a formula as this: H Cl. This is inconvenient, and yet such cases will be of incessant occurrence, unless we alter our representation of the elements. Accordingly we describe the 2 volumes of hydrogen as H, and the 2 volumes of chlorine as Cl2, and the formula for hydrochloric acid, which contains of each, then becomes HCİ.

To take another instance. The symbol N might be applied to 2 volumes of nitrogen, if we did not know that 2 volumes of ammonia contain only 1 volume of nitrogen united to 3 volumes of hydrogen. If we use the symbol N for 2 volumes of nitrogen and H for 2 volumes of hydrogen, ammonia must be described by the inconvenient formula N H1. But calling nitrogen N2, and hydrogen H2, both of these are doubled, and the formula for ammonia becomes NH..

Again, with phosphorus: we might apply the symbol P to 2 volumes of phosphorus vapour if it were not for such compounds as phosphine gas, 2 volumes of which contain only one quarter as much phosphorus as is present in 2 volumes of phosphorus vapour. But by describing phosphorus as P and phosphine as PH,, we get over the difficulty; for the

formulæ show plainly, and in whole numbers, that one contains four times as much phosphorus in 2 volumes as the other. We thus gain more definite ideas of the signification which may be applied to the symbols and formula of gases. For the sake of distinctness they may as well be put in the form of definitions. :—

1. The symbol of an elementary gas is a letter or letters, used to denote the smallest fraction of the weight of the normal two volumes of it that is ever found in two volumes of any compound

gus.

2. The formula, whether of an elementary or compound gas, must always exhibit the composition of two volumes of it. It consists of symbols.

Thus, the symbol for hydrogen is H; the formula, H2; the symbol for phosphorus is P; the formula, P. Compounds must always be described by formulæ, the term "symbol" being reserved for elements.

Relative Weight of Gases.-It has already been shown (page 23) that the specific gravity of a body means its weight as compared with the weight of an equal volume of some other body which is taken as a standard. The standard for gases is hydrogen, one volume of which is said to weigh 1. We have therefore only to double the specific gravities of the gases (which have been carefully determined by experiment) to find the relative weights of the standard 2 volumes of each gas. If 2 volumes of hydrogen weigh 2 (2 grains, 2 pounds, or 2 hundredweight), 2 volumes of chlorine will weigh 71, 2 volumes of oxygen, 32, and so on with the rest. We must now study the relative weights of the different elements which go to make up the weight of 2 volumes of each of the more important gases, elementary and compound. It will be best to take the elementary gases first. Constitution of Elementary Gases. It has already been seen that the formula for 2 volumes of any elementary gas is determined by the compounds which contain that gas. formula for hydrogen is H2, because compounds are known which contain in 2 volumes only half as much hydrogen as is contained in 2 volumes of the pure element. Now what is true of a large volume must also be true of any volume, however minute, and we are therefore led to the conclusion that even the smallest conceivable volume of hydrogen

The

gas that exists in a separate state must be capable of division into two parts, and as the weight of 2 volumes of hydrogen has already been defined to be 2 (hydrogen being the standard), each of these constituent parts must have a weight of 1.

Atoms.* The name atom is, for reasons that will be explained hereafter, applied to the quantity of each elementary gas that is denoted by its symbol. 2 volumes of hydrogen denoted by the formula H,, weigh 2, and consist of 2 atoms, each denoted by the symbol H, and each weighing 1. In like manner, 2 volumes of phosphorus vapour are said to consist of 4 atoms; the formula for 2 volumes being P1 and the symbol for each atom P. The atom in this case weighs 31. In the case of mercury and a few other elements the 2 volumes of gas are said to contain but 1 atom (weighing, in the case of mercury, 200), because no gas is known which contains in 2 volumes less than 200 of mercury.

We can now add the definition of an atom to those given above:

3. The atom of an elementary gas is the quantity denoted by its symbol; that is, the smallest fraction of the weight of 2 volumes of it that is ever found in 2 volumes of any other gas.

These remarks will sufficiently explain the following table (p. 70) which exhibits the composition of all the more important elementary gases. It is only necessary to point out that two of them, oxygen and sulphur, occur in two places. Two different modifications of each of these gases are known, which differ from one another in weight. The atoms are, however, the same in each case.

Constitution of Compound Gases. It will now be obvious that two volumes of every compound gas must contain two or more atoms of two or more kinds. The formula for a compound gas must denote the number and kind of atoms that there are in two volumes of it. Hydrochloric acid is an example of the simplest kind of gaseous compound, two

* I use this word in its present connection with regret. It would certainly be better to exclude it altogether from an account of gases which is independent of the truth or untruth of the atomic theory. But there is no word in use which exactly answers to the conditions, and I hold the coinage of a word to be far too serious an experiment to be undertaken in an elementary treatise, or by any but a leader in science. Might not the word "prime," originally employed by Wollaston, be conveniently revived for this purpose?

volumes of it containing, as we have already seen, one atom of hydrogen and one of chlorine. Other gases contain atoms of three and even four different elements, and there are sometimes as many as twenty, thirty, or even more atoms in two volumes. In fact, we know no limit to the number of atoms which may enter into the composition of a compound gas.

The following table (p. 71) shows the constitution and formulæ of a few of the more important compound gases. The rules according to which they are named will be explained hereafter.

Compound Gases containing non-volatile Elements.-Many gases contain atoms of elements which have not as yet been converted into the condition of gas, or which, even if they do exist as gas, have never had their specific gravities accurately determined in that condition. In these cases the atomic weight (weight of the atom) of the non-volatile element can be found by exactly the same rule as that given before, namely, by observing the smallest weight of the element that ever enters into the composition of 2 volumes of a compound