SULPHUR 181 



the acid may be well diluted. If you should apply water 

 at first, the heat produced by its union with so much acid 

 would cause an immediate corrosive action upon the skin. 

 That the acid itself acts rather slowly, and its action may 

 be greatly hastened by putting some water with it, may be 

 seen in the following experiment. Drop a little of the acid 

 upon paper, and you will see that the decomposition takes 

 place slowly ; but add a few drops of water, and the decom- 

 position or corrosive action will be instantaneous from the 

 influence of the heat produced. 



247. Uses of Sulphuric Acid. The sulphuric acid that we 

 commonly use is, when it is the strongest, nearly one fifth 

 water by weight, and its tendency to absorb water makes 

 it very difficult to keep it of this strength. Exposed to air 

 it will continually absorb its moisture, and of course increase 

 in bulk. In air that seems to us perfectly dry this acid 

 will find some moisture to drink. The chemist sometimes 

 wishes to obtain some gas in an entirely dry state, and for 

 this purpose lets it pass through sulphuric acid. The work 

 is thoroughly done. As the gas bubbles up through the 

 acid it loses every particle of water, and comes out perfectly 

 dry. 



As sulphuric acid has such strong and varied chemical 

 powers, it is largely used in the arts. It is used, for exam- 

 ple, in bleaching, in dissolving indigo for use in dyeing and 

 calico printing, in manufacturing sodium carbonate and ni- 

 tric and hydrochloric acids, in the refining of gold and sil- 

 ver, in the purification of oils, in the manufacture of super- 

 phosphate of lime, so much used now in agriculture, etc. 



Sulphuric acid having two atoms of hydrogen which can be replaced by a 

 metal, forms two classes of salts, neutral and acid. Thus we have sodium sul- 

 phate, Na 2 S0 4 , which reacts neutral, and hydro-sodium sulphate, NaHSO 4 , 

 which reacts acid. The second forms when excess of acid is present ; on 

 heating the acid salt to redness, sulphuric acid is expelled and the neutral 

 salt remains. 



