70 CHEMISTRY FOR AGRICULTURAL STUDENTS 



finely divided platinum, an instance of chemical action being 

 brought about by contact with a third substance ("contact 

 action"). On the other hand, sulphurous acid is directly 

 oxidised at ordinary temperatures. The two anhydrides and 

 acids may be distinguished as sulphurous and sulphuric. 

 Analysis shows that equal weights of sulphur and oxygen are 

 combined in sulphurous anhydride, and that sulphuric 

 anhyd.ide contains of oxygen half as much again. What 

 formulae should be ascribed to the two compounds ? 



The use of the fumes of burning sulphur, in presence of water vapour, as 

 a bleaching agent, is partly due to the oxidation of sulphurous to sulphuric 

 acid at the expense of the combined oxygen of the colouring matter, 

 colourless substances being formed. In presence of water vapour the 

 fumes are also a useful disinfectant, probably for the same reason. 



XVII. SULPHURIC ACID 



It is found by experiment that a single combining weight of 

 water combines with a single combining weight of sulphuric 

 anhydride to form sulphuric acid. Its formula is therefore 

 SO3.H2O or H2SO4. The "oil of vitriol" of commerce is 

 sulphuric acid, with 2 to 6 per cent, of additional water. Its 

 specific gravity is 1.84. 



Pour some oil of vitriol into a small beaker containing water. 

 Is there any evidence of chemical combination and the 

 formation of a hydrate of sulphuric acid } Expose some strong 

 sulphuric acid in a beaker to the air for a few days. Does the 

 liquid increase in volume, indicating absorption of water 

 vapour ? The chemical attraction between sulphuric acid and 

 water may be made use of for drying gases. 



In what proportion does sulphuric acid unite with bases to 

 form salts. Pour some pure sulphuric acid of known strength into 

 a small flask, cover with a watch glass, and weigh. Transfer 



