_Aluminum Compounds._—Sulphate of alumina is most commonly employed. It can be obtained in a state of considerable purity at a very moderate price, and important improvements in the methods used for its manufacture have been recently introduced. Potash and soda alums have no advantage over sulphate of alumina, and, in fact, are less efficient per pound, while their costs are greater. Chloride of alumina is practically equivalent to the sulphate in purifying power, but is more expensive. _Sodium Aluminate_ has been examined by Mr. Fuller, who states that experience has shown that its use is impracticable in the case of the Ohio River water. _Compounds of Iron._—Iron forms two classes of compounds, namely, ferrous and ferric salts. When the ferrous salts are applied to water, under certain conditions, ferrous hydrate is precipitated, but this substance is not entirely insoluble in water containing carbonic acid. Under some conditions the precipitated ferrous hydrate is oxidized by oxygen present in the water to ferric hydrate, and so far as this is the case, good results can be obtained. Ferrous sulphate is not as readily oxidized when applied to water as is the ferric carbonate present in many natural waters, and for this reason ferrous sulphate has not been successfully used in water purification. In the treatment of sewage, where the requirements are somewhat different, it has been one of the most satisfactory coagulants. Ferric sulphate acts in much the same way as sulphate of alumina, and is entirely suitable for use where sulphate of alumina could be employed, but it has not been used in practice, due probably to its increased cost as compared with its effect, and to the practical difficulties of applying it in the desired quantities due to its physical condition. _Metallic Iron: The Anderson Process._—The use of metallic iron for water purification in connection with a moderately slow filtration through filters of the usual form is known as Anderson’s process (patented), and has been used at Antwerp and elsewhere on a large scale, and has been experimentally examined at a number of other places. The process consists in agitating the water in contact with metallic iron, a portion of which is taken into solution as ferrous carbonate. Upon subsequent aeration this is supposed to become oxidized and precipitate out as ferric hydrate, with all the good and none of the bad effects which follow the use of alum. The precipitate is partially removed by sedimentation, while filtration completes the process. The process is admirable theoretically, and in an experimental way upon a very small scale often gives most satisfactory results, muddy waters very difficult of filtration, and colored peaty waters yielding promptly clear and colorless effluents.