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. Fcrro 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.

11. Evaporation and Ignition. If the water be evaporated to dryness, and

ignited in a glass tube, the presence of organic matter may be inferred by the odor

and smoke evolved, as well as by the charring. Another mode of detecting organic matter is by adding nitrate (or acetate) of lead to the inspected water, and collecting and igniting the precipitate ; when globules of metallic lead are obtained if organic matter be present. The putrefaction of water is another proof of the presence of this matter. Nitrate of silver is the best test for the presence of chloride of soda or

common salt. By adding a small quantity of this to the common well water of New-York, a copious, white, flocculent precipitate is immediately formed, which is the chloride of soda. The same test, however, applied to the Croton water, produces no discoloration whatever. Purification of Common water. By filtration, water may be deprived of living beings and of all suspended impurities ; but substances held in solution, cannot