MINERAL WATERS, WELL-WATERS, ETC. 221 



V. The silicic acid is left undissolved on treating 

 the residue obtained by evaporation, with dilute hy- 

 drochloric acid. Should the water contain carbonate 

 of soda, it must be acidulated with hydrochloric acid 

 previously to evaporation. If gypsum be present, a 

 large quantity of water must be employed to redis- 

 solve it. 



VI. Boracic acid may be detected by mixing the 

 water with carbonate of soda, concentrating by eva- 

 poration to a small bulk, and acidifying with hydro- 

 chloric acid; if turmeric-paper be dipped in this 

 solution, and dried, it will become brown if boracic 

 acid be present. 



VII. The presence of nitric acid may be detected 

 by adding to the partially evaporated water or to the 

 residual salts, a few drops of water, colored with a 

 solution of sulphate of indigo, and mixed with some 

 hydrochloric acid, which has been boiled. On boiling, 

 the solution will be decolorized. 



Some other bodies, especially free chlorine, have 

 the same bleaching effect. 



Or if the concentrated solution is mixed with 

 several times its volume of pure strong sulphuric acid, 

 the mixture allowed to cool, and then a few drops of a 

 concentrated solution of sulphate of protoxide of iron 

 cautiously added so that the fluids do not mix, a red- 

 dish purple or dark-brown stratum is produced ac- 

 cording to the quantity of the acid present. Or the 

 very concentrated solution may be heated with metallic 

 copper and concentrated sulphuric acid, when yellowish 

 red vapors of nitrous acid make their appearance. 

 Or the dry residue may be mixed with anhydrous 

 sulphate of copper or oxide of lead, and heated in a 

 tube. If pieces of paper moistened with sulphate of 

 protoxide of iron are held in the tube, they will be 

 colored yellow or brown if nitric acid is-present. 



A very sensitive reaction consists in mixing the salt 

 19* 



