It may be convenient to point out in this place that bacterium coli can no longer be looked upon as a typical inhabitant of the human intestines after the very elaborate investigations carried out by Dr. Weissenfels, who arrived at the following conclusions: 1. The so-called bacterium coli can be cultivated from almost every kind of water, and its presence can be demonstrated in nearly every case, provided a sufficient volume of water is utilised. 2. It is not possible by the result of the experiments upon animals to decide whether the bacterium coli was cultivated from a pure or infected water, and the discovery of a virulent bacterium coli in any sample of water cannot, therefore, be regarded as a criterion that such water has been polluted with fæcal bacteria. After these remarks, it would seem quite possible that the bacterium coli discovered in eighteen cases by Dr. Korn in the Freiburg effluents was not derived from sewage at all but from the ordinary subsoil water of the land. [Sidenote: The possibility of further mischief by pathogenic micro-organisms on sewage farms is exceedingly remote, if it exists at all.] Bearing these observations in mind, it is quite clear, therefore, that neither theoretical investigations, as available up to now, nor practical results, support the theory that pathogenic micro-organisms may do mischief on sewage farms, and one is forced to conclude that this possibility—if it exists at all—after systematic treatment on land is an exceedingly remote one. [Sidenote: Sewage farms reduce the quantity of final effluent.] Before concluding these remarks on the natural purification of sewage it is necessary to draw attention to another considerable advantage which it possesses over artificial sewage treatments, and that is the reduction in quantity of the effluent, which at times is very considerable, whereas in the artificial methods such a reduction is comparatively small. [Sidenote: Loss of liquid by evaporation and by plant life.] Spread over a large area of land, well cropped, evaporation is very active—especially during the summer months, when the flow of water in the brook that takes the effluent is as a rule at its lowest; and, in addition to this, the growing plants further abstract a considerable amount of the liquid that finds its way into the soil, so that the quantity of the effluent may not be more than from 30 to 50 per cent. of the total quantity that was poured over the land. In the artificial treatment the evaporation is considerably smaller, and as plants are altogether absent the quantity of the effluent is probably about 90 per cent. and more of the total quantity of the raw sewage. This is a point of very considerable importance so far as the influence of the effluent upon the water in the stream that takes the same is concerned. Although the subject of natural purification is by no means exhausted, it is now time to direct attention to artificial methods.