THE VOLATILE PART OF PLANTS. 35 



symbol is H 2 0. In like manner the symbol of carbon 

 dioxide is C0 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 0. 



The symbol of a compound is usually termed a formula 

 and if correct is a molecular formula and shows the com- 

 position of one molecule of the substance. Subjoined is 

 a table of the molecular formulas of some of the com- 

 pounds that have been already described or employed. 



FORMULAS OP COMPOUNDS. 



Name. Formula. Molecular Weight. 



Water H 2 O 18 



Hydrogen Sulphide H 2 S 34 



Iron Moiiosulphide FeS 88 



Mercuric Oxide HgO 216 



Carbon Dioxide CO, 44 



Calcium Chloride CaCl 2 111 



Sulphur Dioxide SO 2 64 



Sulphur Trioxide SO, 80 



Phosphorus Pentoxide P 2 6 5 142 



Empirical and Rational Formulas. It is obvious 

 that many different formulas can be made for a body of 

 complex character. Thus, the carbonate of ammonium, 

 whose composition has already been stated (p. 33), and 

 which contains 



1 atom of Nitrogen, 

 1 atom of Carbon, 

 3 atoms of Oxygen, and 

 5 atoms of Hydrogen, 



may be most compactly expressed by the symbol 



NC0 3 H 5 . 



Such a formula merely informs us what elements and 

 how many atoms of each element enter into the compo- 

 sition of the substance. It is an empirical formula, 

 being the simplest expression of the facts obtained by 

 analysis of the substance. 



National formulas, on the other hand, are intended to 

 convey some notion as to the constitution, formation, or 



