SUBSTANCES USED FOR COAGULATION. Mr. Fuller[33] has given a very full account of the substances which can be used for the clarification of waters. Without taking up all of the unusual substances which have been suggested, the most important of the coagulants will be briefly described below. _Lime._—Lime has been extensively used in connection with the purification of sewage, and also for softening water. Lime is first slaked and converted into calcium hydrate, which is afterwards dissolved in water, and applied to the water under treatment. The amount of lime to be used is fixed by the amount of carbonic acid in the water. So much lime is always used as will exactly convert the whole of the carbonic acid of the water into normal carbonate of lime. This substance is but slightly soluble in water and it precipitates. The precipitate is crystalline rather than flocculent, and is not as well adapted to aid in the removal of clayey matters as some other substances, although its action in this respect is considerable. The precipitate is quite heavy, and is largely removed by sedimentation, although filtration must be used to complete the process. Water which has been treated with lime is slightly caustic; that is to say, there is a deficiency of carbonic acid in it, and it deposits lime in the pipes, in pumps, etc.; and although the precipitated calcium carbonate is much softer than steel, it rapidly destroys pumps used for lifting it. Principally for these reasons it is necessary to supply carbonic acid to water which has been treated in this way, and this is done by bringing it in contact with flue-gases, or by the direct addition of carbonic acid. The use of lime for softening waters is known as Clark’s process. It was patented in England many years ago, and the patent has now expired. Various ingenious devices have been constructed for facilitating various parts of the operation. The process has hardly been used in the United States, but there is a large field for it in connection with the softening of very hard waters, and where such waters also contain iron or clay, these substances will be incidentally removed by the process. Larger quantities of lime have an action upon the suspended matters which is entirely different from that secured in Clark’s process, and the action upon bacteria is particularly noteworthy. This action was noted in experiments at Lawrence,[34] where it was found that sewage was almost completely sterilized by the application of considerable quantities of lime. An extremely interesting series of experiments upon the application of large quantities of lime to water was made by Mr. Fuller in 1899.[35] The bacterial results were extremely favorable, although the necessity for removing the excess of lime afterward is a somewhat serious matter, and in these experiments it was not entirely accomplished.