50 HOW CEOPS GEO"W. 



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 j)refixing 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 comjjound is usually termed 2, formula. 

 Subjoined is a table of the formulas of some of the com- 

 pounds that have been already described or employed. 



FOEIIULAS OF COIIPOUXDS. 



Name. Formula. Molecular weight 



Water 



Sulphydrio acid 

 Sulphide of iron 

 Oxide of Mercury 

 Carbonic acid (anhydrous) 

 Chloride of calcium 

 Sulphurous acid (anhydrous) 

 Sulphuric acid 

 Phosphoric acid 



Empirical and Rational Formulas,- 

 many different formulas can be made for a body of com- 

 plex character. Thus, the carbonate of ammonia, whose 

 composition has ah-eady been stated, (p. 49,) and which 

 contains 



1 atom of Nitrogen, 



1 " " Cnrbon, 



3 atoms " Oxygen, and 



5 " " Hydrogen, 

 may be most compactly expressed by the symbol 

 N C O. H,. 



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