100 
ORIGINAL COMMUNICATIONS. 
comprehended by reference to those commencing with the 
letter C in the table. 
Several compounds, of frequent occurrence in combination, 
have received arbitrary symbols, not expressive of the ele- 
ments entering into their composition. Thus water has been 
represented by Aq, ammonia by Am, and cyanogen by Cy. 
The union of symbols to denote chemical combinations 
constitutes formulae. When the union of two or more sub- 
stances to form one compound is to be represented, the most 
obvious mode is to unite their symbols by the algebraic sign +, 
thus Hg-f-0 expresses protoxide of mercury, Hg-j-S proto- 
sulphuret of mercury, &c. 
These symbols denoting single equivalents, it is necessary 
when either or both elements enter into the combination in a 
different proportion, to express this by the use of numbers in 
connection with the symbols to be multiplied by them. These 
numbers are either prefixed to or placed after and a little above 
the symbol. When the number is placed in the latter position 
the sign -j- is omitted, as unnecessary. Thus, periodide of 
mercury, a combination of one equivalent of mercury with 
two of iodine, may be represented either by Hg-f2l, or 
Hgl 2 , and the sesquiodide, consisting of two equivalents of 
mercury and three of iodine, by 2Hg-f 3l, or Hg 2 I 3 . 
The extension of this mode of notation to very complicated 
compounds, would render the formulae inconveniently long. 
To avoid this, the sign -\- is frequently omitted, and the for- 
mula thereby rendered more simple. To introduce a still 
more concise expression, Berzelius has proposed, instead of 
using in all instances the symbols for oxygen, sulphur, sele- 
nium, and tellurium, to substitute for each a particular sign, 
to be placed over the symbol, representing the body with 
which they may be respectively combined, the number of 
these signs denoting the number of equivalents in the com- 
bination. Oxygen he represents by a dot (.), sulphur by a 
comma (,), selenium by a small dash (-), and tellurium by a 
cross (+). Thus Hg and Hg represent protoxide and deu- 
