Although it has never been claimed, and is further not open to doubt, that a septic tank alone and unaided by subsequent treatment in intermittent or continuous contact beds does not sufficiently purify the sewage, in these remarks the work of the septic tank only will be considered, as the treatment in contact beds will be dealt with separately. [Sidenote: Septic tank a suitable name.] (_a_) _Name of Septic Tank._—A good many names have been suggested by different observers—such as “anaerobic fermentation tank,” “putrefying tank,” “liquefying tank,” “cess-pit,” etc.—but there appears to be no reason why the name “septic tank” should not be adhered to, as it describes sufficiently correctly the work done by the tank, which is chiefly of a septic nature. (_b_) _Covered or Open Septic Tank._—Before dealing with the processes taking place in a septic tank, it will not be out of place to consider here, shortly, whether a closed septic tank confers advantages over an open septic tank sufficiently great to justify the considerably greater expenditure necessitated by its construction. It is well known, that at Exeter in the first experimental installation of this process, the septic tank was covered in by an arched roof; but subsequent experiments made elsewhere do not seem to support the theory then advanced, that such a tank should be a closed one. This is chiefly due to the thick skin which, after a few months’ work, forms on the surface of closed or open tanks, and which according to locality and season may reach a thickness of from 1 to 2 feet; it is maintained then that this cheap natural cover does away with the expensive artificial cover. In the report on the treatment of the Manchester sewage, by Messrs. Baldwin Latham, Percy F. Frankland and W. H. Perkin, it is stated on page 54, amongst the conclusions and recommendations, as follows:—“The anaerobic or septic process is found to take place as effectively in an open tank as in a closed one.” This conclusion does not appear to have been modified by the experiments made subsequent to the issue of this report. In the Leeds experiment a similar result was obtained. [Sidenote: Closed septic tanks possess generally speaking no advantages over open ones.] Whilst it would, therefore, appear to be correct to say, generally, that closed septic tanks afford no material advantages over open ones, so far as the purification of the sewage is concerned, they may become necessary in special cases, when the smells emanating from open ones might create nuisances in crowded neighbourhoods. The following remarks refer, therefore, equally to open as well as to closed septic tanks, and no distinction will be made between them. [Sidenote: The work in the septic tank is chiefly done by obligatory anaerobes.]