_Tests of the usual impurities in Common Water._ The following are the tests by which the presence of the ordinary constituents or impurities of common waters may be ascertained. 1. EBULLITION.--By boiling, air and carbonic acid gas are expelled, while carbonate of lime, (which has been held in solution by the carbonic acid) is deposited. The latter constitutes the crust which lines tea-kettles and boilers. 2. PROTOSULPHATE OF IRON. If a crystal of this salt be introduced into a phial filled with the water to be examined, and the phial be well corked, a yellowish-brown precipitate (sesquioxide of iron) will be deposited in a few days, if oxygen gas be contained in the water. 3. LITMUS. Infusion of litmus or syrup of violets is reddened by a free acid. 4. LIME WATER. This is a test for carbonic acid, with which it causes a white precipitate (carbonate of lime) if employed before the water is boiled. 5. CHLORIDE OF BARIUM. A solution of this salt usually yields, with well water, a white precipitate insoluble in nitric acid. This indicates the presence of sulphuric acid (which, in common water, is combined with lime). 6. OXALATE OF AMMONIA. If this salt yield a white precipitate, it indicates the presence of lime, (carbonate and sulphate.) 7. NITRATE OF SILVER. If this occasion a precipitate insoluble in nitric acid, the presence of chlorine may be inferred. 8. PHOSPHATE OF SODA. If the lime contained in common water be removed by ebullition and oxalic acid, and to the strained and transparent water, ammonia and phosphate of soda be added, any magnesia present will, in the course of a few hours, be precipitated in the form of the white ammoniacal phosphate of magnesia. 9. TINCTURE OF GALLS. This is used as a test for Iron, with solutions of which it forms an inky liquor, (tannate and gallate of iron). If the test produce this effect on the water before, but not after boiling, the iron is in the state of carbonate; if after, as well as before, in that of sulphate. Tea may be substituted for galls, to which its effects and indications are similar. _Ferro cyanide of potassium_ yields, with solutions of the sesqui-salts of iron, a blue precipitate, and with the proto-salts a white precipitate, which becomes blue by exposure to the air. 10. HYDROSULPHURIC ACID. (_Sulphuretted Hydrogen._) This yields a dark (brown or black) precipitate, (a metallic sulphuret) with water containing iron or lead in solution.