Thus, the atomic weight of hydrogen being taken as 1, it is found that an atom of chlorine is 35.5 times as heavy as that, so that the atomic weight of chlorine is said to be 35.5. Now, in spite of the enormous difference between the weight of the two elements just mentioned, they combine in the same proportions by volume; and the union is known as hydrochloric acid, or HCl.
But in certain cases elements do not combine in equal proportions; for instance, an atom of oxygen will not combine with less than two of hydrogen. Further, with this we find that the three volumes are condensed into the space of two volumes—a very common phenomenon in the chemical combination of gases. The union of hydrogen and oxygen alluded to forms water, the chemical symbol of which is, consequently, H2O.
Chemical affinity, or chemical attraction, is the force which is exerted between molecules not of the same kind. Thus, in water, which, as we have seen, is composed of hydrogen and oxygen, it is affinity which unites these elements, but it is cohesion which binds together two molecules of water. In compound bodies, cohesion and affinity operate simultaneously; whilst in simple bodies, or elements, cohesion alone has to be considered. To affinity are due all the phenomena of combustion and of chemical combination and decomposition.
Certain gases, such as chlorine and nitrogen, and such substances as sulphur, carbon, and silicon, with many others, form acids in conjunction with hydrogen, or hydrogen and oxygen. These combine with greater or less facility with other elements which do not form acids, and are termed bases. A combination of an acid and a base is known as a salt. Salts the names of which end in -ide, such as chloride, sulphide, etc., are combinations of a metal with a non-metal. Monoxide means an oxide containing one atom of oxygen; dioxide one containing two atoms; protoxide means the first oxide, because it is the first or lowest of the oxides of the given metal in amount of oxygen present; the highest oxide is often known as peroxide. The terminations -ous and -ic are frequently used for the lower and higher oxides respectively. Examples:—
FeO, iron protoxide, or ferrous oxide.
Fe_{2}O_{3}, iron sesquioxide, or ferric oxide.
FeS_{2}, iron disulphide.
Sb2S3, antimony trisulphide.
The following symbols may be indicated as referring to compounds especially met with in brick-earths:—
CaO, lime, instead of calcium oxide.
Al_{2}O_{3}, alumina, instead of aluminium trioxide.
SiO_{2}, silica, instead of silicon dioxide.
Na_{2}O, soda, instead of sodium oxide.
K_{2}O, potash, instead of potassium oxide.
MgO, magnesia, instead of magnesium oxide.
In analysing a body, the first step consists in determining the nature of the elementary substances contained therein. That may be accomplished in the dry way by means of the blowpipe and accessories, as explained in the last chapter. Such an examination, as previously remarked, is known as a qualitative analysis. Or, it may be accomplished in the wet way by ordinary chemical examination. The next step is to determine the amount of the constituents present, and that is known as a quantitative analysis. In making a qualitative analysis, the chemist is assisted by the knowledge that certain basic substances and certain acids produce peculiar phenomena in the presence of known substances or preparations termed reagents.
There is a great difference between a chemical compound and a simple mixture of elements; and it is not always easy (e.g., some alloys) to say whether a substance is in the one state or the other. This distinction is well exemplified by the air we breathe. The chemist finds by analysis that the air is nearly constant in composition, containing essentially in 100 parts 76.8 by weight of nitrogen (including about 1 per cent. of the recently-discovered element, argon), and 23.2 of oxygen. Small proportions of water vapour, carbon dioxide, etc., may be ignored for our present purposes. In view of this comparatively uniform composition, the question at first arises as to whether the air is, or is not, a chemical compound? The answer is in the negative, for, amongst other things, it can be shown that the ratio of 76.8 to 23.2 is not that of the atomic weights of the two elements present, viz., 14 : 16, nor of any simple multiples of these.
We will now quote a few analyses of well-known earths, and explain each in turn: