In spite of all the difficulties which beset such a task, an attempt will be made in the following observations to evolve some general theory concerning the processes at work in the artificial self-purification of sewage. Such a theory, it is quite clear, cannot be complete in the present state of our knowledge, and it is sincerely hoped that the many and serious gaps will be filled up by later investigations. For convenience of reference the different forms of the process, such as are now employed, shall be dealt with separately, commencing with contact or oxidation beds. 2. ARTIFICIAL SELF-PURIFICATION OF SEWAGE IN INTERMITTENT CONTACT BEDS. At the outset it may not be out of place to make a few remarks concerning the various names given to this form of application. The term “intermittent contact bed” is here used to distinguish this kind of bed from the “continuous contact bed,” frequently called “continuous filtration.” [Sidenote: Names of process misleading.] (_a_) _Name of Process._—This process has frequently been called “biological process,” “bacteriological process,” “contact bed system” or “oxidation bed system,” but all these terms do not appear to define it sufficiently, as they do not cover the whole, but only phases or stages in the same; hence, they do not seem appropriate. [Sidenote: Biological process.] The name “biological process” is decidedly misleading, for besides biological agencies there are also at work physical (mechanical) and chemical ones. [Sidenote: Bacteriological process.] The term “bacteriological or bacterial process” is likewise erroneous, for besides bacteria a number of other micro-organisms participate in it—such as yeast fungi, mould fungi, algæ, protozoa, and even higher forms of life, such as earthworms and insects. [Sidenote: Contact bed system.] [Sidenote: Oxidation bed system.] The expressions “contact bed system” or “oxidation bed system” are in so far inappropriate as they describe only portions of the process but not the whole. The term “contact bed” describes the first stage, and the term “oxidation bed” portion of the second stage only. [Sidenote: Term most suitable.] The term which seems most suitable of all is “artificial self-purification in contact beds,” as it includes every phase of this lengthy process applied in an artificial form; the term “natural self-purification” being applied to land treatment of sewage, as it is the only method in which the self-purifying powers are employed under natural conditions. [Sidenote: Working operations.] (_b_) _Explanation of Process._—The cycle of operations commences with the filling of the bed, and during the same the sewage comes gradually in contact with the filling material. When the bed is full, the inflow is stopped and the sewage allowed to remain in contact with the material for some time. The bed is then emptied, and a period of rest is given it before the filling is commenced again.