When there are two oxides of the same base, both of which are _salifiable_,--in naming the salts, the term _proto_ is prefixed to the acid of the salt formed by the lowest, and per to that of the higher oxide; as, the _Proto_sulphate of Iron, or Sulphate of the Protoxide; the _Per_sulphate of Iron, or Sulphate of the Peroxide. Many salts contain more than one atom of acid to each atom of base. In that case, the usual prefixes expressive of quantity are adopted: thus, the _Bi_sulphate of Potash contains twice as much Sulphuric Acid as the neutral Sulphate, etc. On the other hand, there are salts in which the base is in excess with regard to the acid, and which are usually known as "basic salts;" thus, the red powder which deposits from solution of Sulphate of Iron, is a _basic_ Persulphate of Iron, or a Sulphate of the Peroxide of Iron with more than the normal proportion of oxide. _Nomenclature of the Hydracid Salts._--The composition of these salts being different from those formed by Oxygen Acids, the nomenclature varies also. Thus, in neutralizing Hydrochloric Acid with Soda, the product formed is not known as Hydrochlorate of Soda, but as _Chloride of Sodium_; this salt, and others of a similar constitution, being _binary_, and not _ternary_, compounds. The salt produced by Hydrochloric Acid and _Ammonia_ however is often called "Muriate or Hydrochlorate of Ammonia," although more strictly it should be the _Chloride of Ammonium_. ON SYMBOLIC NOTATION. 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 NO{5}, not NO{5}AgO. 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, Fe{2}O{3} represents two equivalents of Iron united with three of Oxygen, or the "Peroxide of Iron;" SO{3}, one equivalent of Sulphur with three of Oxygen, or Sulphuric Acid.