It will not be needful to insist upon the obvious fact that bacteria abound in sewage. Such a large quantity of organic matter, in which decomposition is constantly taking place, will afford an almost ideal nidus for micro-organic life. There is indeed but one reason why such a medium is not absolutely ideal from the microbe's point of view, and that reason is, that in sewage the vast number of bacteria present make the struggle for existence exceptionally keen. Not only are the numbers incredibly large, but we also find a very extensive representation of species, including both saprophytes and parasites, non-pathogenic and pathogenic. Not infrequently it is from pollution by sewage that drinking water is contaminated with disease. A patient, we will say, suffers from typhoid fever. The specific organism has its habitat largely, though not exclusively, in the alimentary canal. It passes out in the excreta, and though sometimes partially disinfected, may escape without hindrance into the drains, and thus to the sewer or cesspool. How often, by means of direct connection or by percolation, sewage, from sewers or cesspools, gains access to drinking water, the history of typhoid outbreaks in this country only too fully records. It is impossible to lay down any exact standard of the chemical and bacteriological quality of sewage. The quality will differ according to the size of the community, the inclusion or otherwise of trade-waste effluents, the addition of rain-water, and other like physical conditions. Moreover, sewage itself when, so to speak, fully formed is liable to undergo rapid changes owing to fermentation and the competition of micro-organisms. It is clear that these latter are the chief agents in bringing about the change, because, if sewage be placed in hermetically sealed flasks and sterilised by heat, it is found that no change occurs. From facts such as the above it will be apparent that no exact standard of chemical or bacterial contents is possible. Respecting the chemical condition we may shortly say that the chief characteristic of sewage is its enormous amount of contained organic matter in suspension or solution; respecting the bacterial content we may say that the chief species of the very numerous organisms are those commonly concerned in fermentative putrefaction. London crude sewage contains on an average about four millions of micro-organisms per cc. Many of these are "liquefying" bacteria; that is to say, they have the power of liquefying gelatine, which is generally one of the features of putrefactive species. In considering the quality of the bacteria present in sewage, a still wider field of research opens before us. For though we can say that, roughly, all sewage will contain probably between four and eight millions of bacteria, we cannot even lay down a rough standard respecting the kinds of bacteria present more than we have done already in stating that a very large number indeed out of the