50 HOW CROPS GROW. 
When a compound contains more than one atom of an 
element, this is shown by appending a small figure to the 
symbol of the latter. For example: water consists of 
two atoms of hydrogen united to one of oxygen, the 
symbol of water is then H,O. In like manner the symbol 
of carbonic acid is C O,,. 
When it is wished to indicate that more than one mole- 
cule of a compound exists in combination or is concerned 
in a chemical change, this is done by prefixing a large 
figure to the symbol of the compound. For instance, 
two molecules of water are expressed by 2 H, O. 
The symbol of a compound is usually termed a formula. 
Subjoined is a table of the formulas of some of the com- 
pounds that have been already described or employed. 
FORMULAS OF COMPOUNDS. 
Name. Formula, Molecular weight. 
‘ Water H, O 18 
Sulphydric acid HS 34 
Sulphide of iron : FeS 88 
Oxide of Mercury Hg O 216 
Carbonic acid (anhydrous) CO, 44 
Chloride of calcium Ca Cl, 111 
Sulphurous acid (anhydrous) SO, 64 
Sulphuric acid 8 O, 80 
Phosphoric acid P, O, 142 
Empirical and Rational Formulas,—It is obvious that 
many different formulas can be made for a body of com- 
plex character. Thus, the carbonate of ammonia, whose 
composition has already been stated, (p. 49,) and which 
contains 
1 atom of Nitrogen, 
1 “ Carbon, 
3 atoms “ Oxygen, and 
5 “ “ Hydrogen, 
may be most compactly expressed by the symbol 
N CO, H,. 
