Alum is a double sulphate of potash and aluminium, and in this case breaks into potassium sulphate which remains in solution, and a basic aluminic sulphate. This basic sulphate of aluminium, the composition of which is undetermined, precipitates as a more or less gelatinous and flocculent mass, and carries down with it the foreign matters and humus bodies. The sulphuric acid set free in the formation of the basic aluminic sulphate attacks the earthy and alkaline carbonates, which are always present, and forms with them sulphates, setting carbonic acid free. Aluminic sulphate acts like alum. Aluminic acetate and ferric acetate do not give such good results. In later years an extensive use of alum has been made in the many processes of purifying water. It is not improbable that aside from its effect in precipitating matter mechanically by envelopment with the precipitating basic aluminic sulphate, the alum exerts a distinct coagulative action on the albuminous substances in the water, rendering them insoluble, and thus causing their precipitation; perhaps the same or similar effect that alum produces in the tanning of leather. By the addition of a minute amount of alum, water is rendered capable of a most perfect jnechanical filtration. The fact that alum is cheap, and can be obtained in quite a pure state at any drugstore, places it within the reach of every one. Its sharp taste precludes the possibility of its being swallowed by mistake. But even should it be swallowed by mistake, no great harm would be likely to ensue. If it can be proved that alum not only clarifies a water, but also removes from it disease germs and ptomaines, its use will prove of incalculable value to the human race. The investigation of the effects of alum on drinking water falls under several heads, viz.: (1.) Clarification of the water by settling; (2.) Clarification of the water by filtration; (3.) Use of water clarified by alum in manufacturing; (4.) Removal of disease germs; (5.) Removal of ptomaines; (6.) Removal of organic matter. The investigation must needs be both chemical and biological. Only the first and part of the second cases have so far been examined. It is evident that to obtain practical results in the clarification of water by alum, it must be added in such small amounts as to leave no unnecessary excess, and that neither taste nor physiological action should be imparted to the water. Prof. Peter T. Austen, Ph. D., and Francis A. Wilber, M. S., write about their practical experiments with alum as follows :

"At the time of our experiments (January, 1885) the New Brunswick (N. J.) city water was quite turbid from clayey and other matters, so that we were able to obtain some very reliable results. To determine the effect of alum as a precipitating agent, tall cylinders were filled with water and a solution of alum was added, the whole well mixed, and allowed to stand. It was found that in varying lengths of time, depending on the amount of alum used, a gelatinous precipitate settled out, and the water above it became perfectly clear. On adding a relatively large amount of alum, and mixing, the coagulation and separation of the precipitate is at once visible, the water appearing by careful examination to be filled with gelatinous particles. The amount of alum necessary for the precipitation of a water will, of course, depend on the amount of impurity present, but in the present case, which may be taken as a typical one, we found that 0.02 gramme of alum to a litre of water (1.2 grains to a gallon) caused the separation and settling of the impurities, so that the supernatant water could be poured off. This amount of alum was shown by numerous experiments to be about the practical limit. The complete settling took place as a rule in not less, and usually more, than two days. It is evident that the amount of alum thus added is too slight to be perceptible to the taste, and can exert no physiological action. We were unable to detect the slightest taste or change in the water so treated.

"To determine if there was free alum in the water, a sample of the clear water, filtered off from the precipitate produced by the alum, was made slightly alkaline with ammonia and warmed for some time. Only the merest traces of an alumina reaction could be obtained, and, in fact, in some cases, it was doubtful if a reaction was observable. To prove that no more matter could be precipitated-by the addition of a greater amount of alum, samples of the clean filtered water were treated with more alum, but there was in no case any indication of further precipitation on standing. We consider it, then, established that by the addition of two grains of alum to the gallon, or half an ounce to one hundred gallons, water can be clarified by standing, and that neither taste nor physiological properties will be imparted to it by this treatment. By increasing the amount of alum, the time required for the separation and settling can be diminished; and vice versa, by diminishing the amount of alum added, a greater time will be required for the clarification. This method is particularly adapted to the clarification of large volumes of water, where filtration is not practical. The clear water can be racked off to as low a level as possible, after which the sediment should be washed out and the receptacle cleansed by a free use of water.

"In order to test the clarification of water by filtration after addition of alum, the water taken from the same source was again made the subject of our experiments. It was found that the suspended clayey matters were so fine that the best varieties of filtering papers were unable to remove them. This, however, is not surprising, since it is well known that the mineral matters suspended in water are of a remarkable degree of fineness. Thus the water of the river Rhine, near Bonn, cannot be clarified by simple filtration, and takes four months to settle. The addition of certain chemicals aids the filtration of suspended matters in some cases, but it does not always entirely remove them. Calcium chloride and other salts are recommended as effective agents in aiding the removal of suspended matters, but in case of some waters, at least, they have no apparent action. The following substances were found to have no effect in aiding the filtration of the water: sodium salts - chloride, carbonate, nitrate, acid carbonate, hydrogen phosphate, acid sulphite, ammonium phosphate, sulphate, biborate, tungstate, acetate; potassium salts - hydroxide, chloride, bromide, iodide, acetate, phosphate; ammonium salts - chloride, sulphate, nitrate, acetate; calcium salts - oxide, cloride, sulphate, nitrate. Zinc sulphate and ferrous sulphate (copperas) had no action. Acid sulphate of potassium and of sodium had a slight clearing action. Acetate and chloride of zinc had an apparent action. Ferric chloride (perchloride of iron) cleared perfectly, as also did the nitrate and sulphate of aluminium.