Larger figures placed before and in the same line with the symbols, affect the _whole compound_ which the symbols express: thus, 2 SO{3} means two equivalents of Sulphuric Acid; 3 NO{5}, 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 S{2}O{2}, AgO S{2}O{2}, 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 S{2}O{2}) AuO S{2}O{2} + 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:-- FeO SO{3} + 7 HO. These atoms of water are driven off by the application of heat, leaving a white substance, which is the Anhydrous salt, and would be written simply as FeO SO{3}. The _plus_ sign however is often employed in token of simple _addition_, no combination of any kind being intended. Thus the decomposition which follows on mixing Chloride of Sodium with Nitrate of Silver may be written as follows:-- NaCl + AgO NO{5} = AgCl + NaO NO{5}; that is,-- Chloride of Sodium _added to_ Nitrate of Silver. = Chloride of Silver _and_ Nitrate of Soda. ON EQUIVALENT PROPORTIONS. When elementary or compound bodies enter into chemical union with each other, they do not combine in indefinite proportions, as in the case of a mixture of two liquids, or the solution of a saline body in water. On the other hand, a certain definite weight of the one unites with an equally definite weight of the other; and if an excess of either be present, it remains free and uncombined. Thus, if we take a _single grain_ of the element Hydrogen--to convert that grain into Water there will be required exactly 8 grains of Oxygen; and if a larger quantity than this were added, as for instance _ten grains_, then two grains would be over and above. So, to form Hydrochloric Acid, 1 grain of Hydrogen takes 36 grains of Chlorine:--for the _Hydriodic Acid_, 1 grain of Hydrogen unites with 126 grains of Iodine. Again, if separate portions of metallic Silver, of 108 grains each, are weighed out,--in order to convert them into Oxide, Chloride, and Iodide of Silver respectively, there would be required Oxygen 8 grains. Chlorine 36 " Iodine 126 "