AMOUNT OF IRON REQUIRED TO RENDER WATER OBJECTIONABLE. Three hundredths of a part in 100,000 of metallic iron very rarely precipitate or cause any trouble. Five hundredths occasionally precipitate, and this amount may be taken as about the allowable limit of iron in a satisfactory water. One tenth of a part is quite sure to precipitate and give rise to serious complaint. Two or three tenths make the water entirely unsuitable for laundry purposes, and are otherwise seriously objectionable, and will hardly be tolerated by a community. Under some conditions ground-waters carry as much as 1 part in 100,000 of iron, and such waters are hardly usable. In iron-removal plants an effluent containing less than 0.05 is regarded as satisfactory. One containing less than 0.02, as is the case with many plants, is all that can be desired. The percentage of removal is of no significance, but only the amount left in the effluent. CAUSE OF IRON IN GROUND-WATERS. Natural sands, gravels, and rocks almost always contain iron, often in considerable amount. The iron is usually combined with oxygen as ferric oxide, and in this condition it is insoluble in water. Water passing through iron containing materials will not ordinarily take up iron. When, however, the water contains a large amount of organic matter in solution, this organic matter takes part of the oxygen away from the iron, and reduces the ferric oxide to ferrous oxide. The ferrous oxide combines with carbonic acid, always present under these conditions, forming ferrous carbonate, which is soluble and which goes into solution. Surface-waters nearly always carry free oxygen, and when such waters enter the ground they carry oxygen with them, and the organic matters in the water use up the free oxygen before they commence to take oxygen away from the iron of the ground. It is thus only in the presence of organic matters, and in the absence of free oxygen, that the solution of iron is possible. It sometimes happens that the organic matters which reduce the iron are contained in the soil itself, in which case iron may be taken up even by water originally very pure, as for instance, by rain-water. Generally speaking, iron is everywhere present in sufficient quantity in the strata from which ground-waters are obtained, and wherever the conditions of the organic matters and oxygen necessary for solution occur, iron-containing waters are secured, and the iron is usually present in the earth in such quantity that the water can dissolve as much as it will take up for a long series of years, or for centuries, without exhausting the supply. There is thus little prospect of improvement of such waters from exhaustion of the supply of iron.