50 HOW CROPS GilOW. 



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 2 O. In like manner the symbol 

 of carbonic acid is C O 2 . 



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 2 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 2 O 18 



Sulphydric acid H 2 S 34 



Sulphide of iron Fe S 88 



Oxide of Mercury Hg O 216 



Carbonic acid (anhydrous) C O 2 44 



Chloride of calcium Ca C1 2 111 



Sulphurous acid (anhydrous) S O 2 64 



Sulphuric acid S O 3 80 



Phosphoric acid P 2 O 5 142 



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 C 3 H 6 . 



