We are taught by chemistry that elements are capable of combining only in definite proportions, and that each substance possesses a definite proportion peculiar to itself. That proportion is called the _atomic weight_ of the element; or, it is the relative weight of the atom of each substance compared with that of the lightest substance known, hydrogen. 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. Fe2O3, iron sesquioxide, or ferric oxide. FeS2, iron disulphide. Sb2S3, antimony trisulphide. The following symbols may be indicated as referring to compounds especially met with in brick-earths:--