The rapidity of oxidation which takes place in the septic tank is quite remarkable. In very favorable circumstances in the course of an hour or two the most objectionable sewage may be transformed into an effluent which is apparently not disagreeable either to the sense of smell or taste. For sentimental reasons it is not desirable to drink this effluent water, though a person drinking it might not be reminded of the fact that an hour before it had been nothing but most objectionable forms of sewage. The principle of the septic tank is found applied on a large scale by distributing the sewage over naturally sandy plains and thus depending upon the natural arrangement of the soil particles for the oxidizing purposes. A large part of the sewage of Paris is treated in this way, and the sandy plains thus irrigated not only produce an oxidation of the sewage, but utilize the fertilizing materials therein for the growth of crops. The principal objection to this method of purification of sewage is that in winter it acts slowly, so that it is difficult to purify the sewage in an open field at temperatures at or below freezing point. Naturally the efficiency of the septic tank depends upon the development therein of those organisms capable of rapidly destroying organic matter. These organisms exist in the sewage, and the arrangement of the layers of charcoal and sand in the septic tank afford them an opportunity of exerting their maximum activity. The surfaces of the charcoal especially become covered with these nitrifying organisms and they even penetrate into the interior of the porous substances, thus affording a maximum surface and a maximum intensity of action.