SOME CHEMICAL CONCEPTS 123 



little figures placed below and immediately after the 

 chemical symbol state the number of atoms con- 

 tained in the molecule. Where no figure is given it 

 is understood that only a single atom is present. 

 When an expression has a figure on the line before 

 it, such as 2H 2 SO 4 , or 3H 2 SO 4 , it means that two or 

 three molecules of Sulphuric acid are concerned. It 

 is also possible to bracket subgroups in a compound, 

 and place a small figure below the line following 

 the bracket. In such a case there is that number 

 of the group in the compound. Thus, when the 

 chemist writes 



6NaCN+Fe(OH) 2 =Na 4 Fe(CN) 6 +2NaOH, 



he means, if six molecules of Sodium Cyanide (com- 

 posed each of one atom of Sodium, one of Carbon, 

 and one of Nitrogen) are mixed with one molecule 

 of Ferrous Hydroxide (composed of one atom of 

 Iron combined with two groups, each consisting of 

 one atom of Oxygen and one atom of Hydrogen, 

 reaction occurs, and we get [that is the chemical 

 meaning of the symbol = ] one molecule of Sodium 

 Ferrocyanide (consisting of four atoms of Sodium 

 combined with one atom of Iron, and six groups, con- 

 sisting each of an atom of Carbon and an atom of 

 Nitrogen), and two molecules of Caustic Soda (con- 

 sisting of one atom of Sodium combined with one 

 atom of Oxygen and one atom of Hydrogen) . 



The question naturally suggests itself how these 

 various compounds are held together. Various 

 workers have suggested explanations of the problem, 

 but we are concerned here with what happens rather 



