ON CHEMICAL SYMBOLS. 101 
toxide of mercury, Hg and Hg, protosulphuret and bisulphuret 
of mercury, Cu and Cu, protoseleniuret and biseleniuret of 
copper, and Bi and Bi, prototelluriet and bitelluriet of bis- 
muth. 
Two equivalents of the radical are frequently denoted by 
drawing a line through the lower part of the symbol, but for 
the convenience of printing, a dash placed beneath the symbol 
is usually substituted; thus As represents two equivalents of 
arsenic, P two of phosphorus, and so on in the case of other 
symbols. This expression for the doubling of the radical is 
not to be employed unless in connection with the dot, comma, 
&c, over the symbol denoting the union with oxygen, sulphur, 
&c. For example, arsenious acid, consisting of two equiva- 
lents of arsenic and three of oxygen, may be represented by 
2As + 30, or As 2 3 , or As, but not by As + 30, or AsO 3 ; 
and sesquisulphuret of antimony, by 2Sb + 3S, or Sb 2 S 3 , or 
Sb, but not by Sb + 3S, or SbS 3 . 
When these primary compounds are to be expressed in any 
multiple of a single equivalent, the manner will vary accord- 
ing to the formula used. Where the elements of the primary 
compound are united by the sign +, the whole compound is 
enclosed between parentheses, and the multiple placed outside. 
But when the more concise mode of notation, which dispenses 
with the sign -f , is adopted, the parentheses are unnecessary; 
thus the formula for two equivalents of deutoxide of mercury is 
2(Hg + 20) or 2Hg0 2 , or 2Hg, or Hg 2 . 
The union of these primary combinations gives rise to more 
complicated formulae. The principles on which these are 
founded are, however, the same as when elementary bodies 
are concerned, if we consider the elementary formulae as the 
expression of a single idea. The object here is not only to 
express the elements and the number of their equivalents in 
the compound, but likewise to point out the mode in which 
these elements and the compounds formed by them are united. 
To attain this object, the formula of each primary compound, 
