The list of symbols employed to represent the various elementary bodies is given at [page 306].—Commonly the initial letter of the Latin name is used, a second or smaller letter being added when two elements correspond in their initials: thus C stands for Carbon, Cl for Chlorine, Cd for Cadmium, and Cu for Copper.
The chemical symbol however does not simply represent a particular element; it denotes also a definite weight, or equivalent proportion, of that element. This will be explained more fully in the succeeding pages, when speaking of the Laws of Combination.
Formulæ of Compounds.—In the nomenclature of compounds it is usual to place the Oxygen or analogous element first in the case of binary compounds, and the acid before the base in the ternary compounds, or salts; but in representing them symbolically this order is reversed: thus, Oxide of Silver is written AgO, and never as OAg; Nitrate of Silver as AgO NO5, not NO5AgO.
The juxtaposition of symbols expresses combination; thus, FeO is a compound of one proportion of Iron with one of Oxygen, or the "Protoxide of Iron," If more than one equivalent be present, small figures are placed below the symbols: thus, Fe2O3 represents two equivalents of Iron united with three of Oxygen, or the "Peroxide of Iron;" SO3, one equivalent of Sulphur with three of Oxygen, or Sulphuric Acid.
Larger figures placed before and in the same line with the symbols, affect the whole compound which the symbols express: thus, 2 SO3 means two equivalents of Sulphuric Acid; 3 NO5, three equivalents of Nitric Acid. The interposition of a comma prevents the influence of the large figure from extending further. Thus, the double Hyposulphite of Soda and Silver is represented as follows:—
2 NaO S2O2, AgO S2O2,
or two equivalents of Hyposulphite of Soda with one of Hyposulphite of Silver; the large figure referring only to the first half of the formula. Sometimes brackets, etc. are employed, in order to render a complicated formula more plain. For example, the formula for the double Hyposulphite of Gold and Soda, or "Sel d'or," may be written thus;—
3 (NaO S2O2) AuO S2O2 + 4 HO.
In this formula, the plus sign (+) denotes that the four atoms of water which follow, are less intimately united with the framework of the salt than the other constituents.
The use of a plus sign is commonly adopted in representing salts which contain water of crystallization. Thus, the formula for the crystallized Protosulphate of Iron is written as follows:—