104 CHEMICAL NOMENCLATURE AND NOTATION, 



lent of sulphur, and three of oxygen, that is one equivalent 

 of sulphuric acid ; and sulphate of iron itself consisting of 

 one equivalent of each of the preceding compounds, may be 

 represented as follows : 



Fe O S O 3 , or 

 FeO-f-S O 3 , or 

 Fe O, S O 3 , 



The sign plus ( + ) or the comma, being introduced in the 

 second and third formulae, to indicate a distribution of the 

 elements of the salt into its two proximate constituents, oxide 

 of iron, and sulphuric acid, which is not so distinctly indicated 

 in the first formula. It may often be advantageous to make 

 use of both the comma and the plus sign in the same formula, 

 and then it would be a beneficial practice to use them as in the 

 following formula for the double sulphate of iron and potash : 



FeO, SO 3 + KO, SO 3 , 



in which the comma is employed to indicate combination more 

 intimate in degree, or of a higher order than the plus sign, 

 namely, of the oxide with the acid in each salt, while the com- 

 bination of the two salts themselves is expressed by the 

 sign +. 



The small figures in the preceding formulae affect only the 

 symbol or letter to which they are immediately attached. Larger 

 figures placed before and in the same line with the symbols apply 

 to the compound expressed by the symbols. Thus 3 S O 3 , 

 means three equivalents of sulphuric acid ; 2 Pb O, two equiva- 

 lents of oxide of lead. But the interposition of a comma or plus 

 sign prevents the influence of the figure extending farther, thus 



2 Pb O, Cr O 3 , or 



2PbO + Cr O 3 , 



is two proportions of oxide of lead, and one of chromic acid, 

 or the sub-chromate of lead. To make the figure apply to 

 symbols separated by the comma or plus sign, it is necessary 

 to enclose all that is to be affected within brackets, and place 

 the figure before them. Thus, 



2 (Pb O, Cr O 3 ) 



means two proportions of the chromate of lead. The following 

 formulae of two double salts with their water of crystallization, 

 exhibit the application of these rules : 



Iron-alum, or the sulphate of peroxide of iron and potash, 

 KO, SO 3 + Fe 2 O y 3SO 3 + 24 HO 



