For a similar purpose electrolysis has been employed. In this process the sewage is made to flow between two iron plates which are connected up to a source of electric current so that they form electrodes, while the sewage is the electrolyte. The current decomposes the liquid sewage, causing chlorine and oxygen to be deposited upon that plate which forms the anode. This deodorises and purifies the sewage, in addition to which iron salts are formed on the iron plates, the effect of which is to precipitate the solid particles. Thus the same result is achieved as when chemicals are used, the main difference being that instead of chemicals being added, they are produced by the passage of the current. But, from the scientific point of view, the most interesting process of all is that in which bacteria or microbes are brought into the service. The fact is familiar to most people that there are certain minute organisms which cause terrible diseases. It is not so well known that there are still more of them whose action is extremely beneficent. The writer has seen these minute living things described in a popular book as "insects," but they really belong to a low order of plant life, and, as has been said in an earlier chapter, in spite of the lowliness of their status in the order of creation, they are able to accomplish certain chemical processes which baffle the cleverest men. They are particularly good, or some of them are at any rate, at forming compounds in which nitrogen forms a part. Further, they can be divided into two classes, the aerobic and the anaerobic. The former work best in air, while the latter need an absence of air while they perform their functions. After which preliminary explanation we can proceed to describe how they are induced to carry on this valuable work for mankind. The sewage flows first of all into a tank from which light and air are excluded as far as possible. There the anaerobic microbes flourish and multiply, and in the course of their life work they convert the sewage into an inoffensive liquid. After an appropriate interval the liquid passes to filter-beds, where it trickles over and through beds of coke, the effect of which is to aerate it very thoroughly, whereby the aerobic microbes come into action, completing the good work, so that nothing is left except a clean, colourless and odourless liquid. Indeed it is more than that, for the microbes have turned the offensive matter into nitrogenous compounds which, as we have seen in a previous chapter, are the best fertilisers. Hence this effluent, if placed upon the soil, is of great value. The advantage of this to towns which are not blessed, like London, with a broad river and the sea near at hand needs no explanation.