_Compounds of Manganese._—Manganese forms compounds similar to those of iron, that is to say manganous and manganic salts, but their use in connection with water filtration has not been found possible. In addition, manganese forms a series of compounds, known as manganates and permanganates, quite different in their structure and action from the others. These compounds contain an excess of oxygen which they give up very readily to organic matters capable of absorbing oxygen, and because of this power, they have been extensively used in the treatment of sewage. Applied to the treatment of waters their action is very slight, and the compounds are so expensive that they have not been employed for this purpose. Theoretically the action is very attractive, as the oxygen liberated by their decomposition oxidizes some of the organic matter of the water, thereby purifying it in part, while the manganese is precipitated as a flocculent precipitate having all of the advantages pertaining to a precipitate of hydrate of alumina, and without the disadvantage of adding acid to the water, as is the case with the compounds of alumina and iron. These chemicals, when used in comparatively concentrated condition, have powerful germicidal actions, but in water purification the amounts which can be used are so small that no action of this kind results. The amount which can be applied to a water is limited to the amount which can be decomposed by the organic matters present in the water, and is not large. _The Use of Metallic Iron and Aluminum, with the Aid of Electricity._—Elaborate experiments were made at Louisville with metallic iron and aluminum oxidized and made available by the aid of electric currents. The use of iron with electric currents was tried in sewage purification some years ago, under the name of the Webster process, but was never put to practical use. The theory is to oxidize the iron or aluminum in contact with the water, with the formation of flocculent hydrates, by the aid of an electric current, thereby securing the advantages of the application of salts of these metals to the water without the disadvantage of the addition of acid. _Other Chemicals Employed._—A solution containing chlorine produced by electrical action has been suggested. Chlorine is a powerful disinfectant, and when used in large quantities kills bacteria. It is not possible to use enough chlorine to kill the bacteria in the water without rendering it unfit for human use. The nature of this treatment has been concisely described by Dr. Drown,[36] who shows that the electrically prepared fluids do not differ in their action in any way from well-known chemicals, the use of which would be hardly considered. The use of ozone and peroxide of hydrogen have also been suggested, but I do not know that they have been successfully used on a large scale. The same is true of many other chemicals, the consideration of which is hardly necessary in this connection.