CHEMICAL NOMENCLATURE 271 



that 1 volume of hydrogen combines with 1 volume of chlorine to 

 form '2 volumes of hydrochloric acid gas, that is, by Avogadro's 

 Law, a certain number of molecules of hydrogen combine with the 

 same number of molecules of chlorine to form twice that number 

 of molecules of hydrochloric acid gas ; so that 1 molecule of 

 hydrogen must combine with 1 molecule of chlorine to form 2 

 molecules of the compound. Moreover, as each molecule of 

 hydrochloric acid gas must contain at least 1 atom of hydrogen 

 and 1 atom of chlorine, then the 2 molecules must contain 

 at 1?a*t 2 atoms of each element, and therefore 1 molecule of 

 hydrogen (which gives rise to 2 molecules of hydrochloric acid 

 gas) must contain at least 2 atoms of hydrogen, and similarly, 

 each molecule of chlorine must contain at least 2 atoms of chlo- 

 rine. This is the number of the atoms in the molecule of these 

 elements if the compound we have considered contains 1 atom 

 of each element, and this, the simplest explanation, is that which 

 really holds. The formula for hydrochloric acid gas may there- 

 fore be written as HC1. This result which we have found for 

 hydrogen and chlorine, is also generally true for other gaseous 

 elements ; so that we may state, as a fairly general rule, that 

 The molecule of a gaseous element contains 2 atoms. 



Formula for Water. The formation of water gives a slightly 

 more complicated case, since it was found that 2 volumes of 

 hydrogen with 1 volume of oxygen form only 2 volumes of 

 water vapour. From this it follows, however, by Avogadro's 

 Law, that 2 molecules of hydrogen with 1 molecule of oxygen 

 form 2 molecules of water, and hence from what has been 

 previously stated, 4 atoms of hydrogen combine with 2 atoms of 

 oxygen to form 2 molecules of water. Each molecule of water 

 therefore consists of 2 atoms of hydrogen with 1 atom of 

 oxygen. We can consequently represent the molecule of water 

 by the formula H 2 O. 



Chemical Equations. We may now learn how to represent 

 the formation of compounds by what are termed chemical 

 eifiKitiuns. The formulae of the reacting bodies are placed on 

 the left hand side of the equality sign, and those of the 

 products on the right. 



(1) 2H. + : , = 2H 2 0;i 



that is, 



2 mols. hydrogen with 1 mol. oxygen form 2 mols. of water. 



1 Tn this formula for water the small 2 only refers to the H, the large 2 to the 

 whole group, and similarly iu the other formulae employed. 



