=Mineral Impurities.=—Mineral impurities are dissolved by water in its course through the soil, and so will vary with the character of the latter. 1. The water obtained from _granitic_ formations contains very little mineral matter, often not more than two to six grains per gallon. _Clay slate water_ is also generally very pure, as is the water from hard _trap rocks_. 2. The water from _millstone grit_ and _hard oolite_ is very pure, often containing only four to eight grains per gallon, chiefly calcium and magnesium sulphate and carbonate. 3. _Soft sand-rock waters_ usually contain thirty to eighty grains per gallon of sodium salts, with a little lime and magnesia. 4. _Loose sand and gravel waters_ vary greatly. They may be almost free from mineral matter, or the solids may be more than seventy grains per gallon, including much organic matter. 5. Waters from the _lias clays_ vary somewhat, but commonly contain a large quantity of calcium and magnesium sulphates. 6. _Chalk waters_ generally contain from seven to twenty grains of calcium carbonate, with smaller quantities of other salts. 7. _Limestone and magnesian limestone waters_ differ from the last, in containing more calcium sulphate and less calcium carbonate, as well as much magnesium sulphate and carbonate in the dolomite districts. 8. _Selenitic waters_ contain calcium sulphate in considerable quantities. 9. _Clay waters_ usually possess the characters of water from surface wells, and are objectionable. 10. _Alluvial waters_ generally contain a large amount of various salts, including the various calcium, magnesium, and sodium salts. 11. _Artesian well water_ varies greatly in composition. It may contain a large amount of sodium and potassium salts, or a small quantity of iron, or calcium salts. The commonest and most important mineral constituent of water is calcium carbonate, next to this calcium sulphate. These two salts are the chief causes of =hardness of water=. For practical purposes as regards use in domestic matters and in manufactures, the most important classification of waters is into _hard_ and _soft_. The degree of hardness varies within wide limits—from rain-water, which has no hardness at all, to the water from new red sandstone rocks which sometimes possesses a hardness of 90 degrees; or wells in the gravel, in which it may be as much as 152 degrees. The following _classification of waters_, according to the degree of hardness, beginning with the least hard and gradually increasing in hardness, is from the sixth report of the Rivers Pollution Commissioners:—1. Rain-water. 2. Upland surface. 3. Surface from cultivated land. 4. River. 5. Spring. 6. Deep-well. 7. Shallow-well water.