IN the present volume — a small book upon a large subject — the question of the Purification of Sewage is dealt with chieﬂy from a chemical and biological point of view, and in the light of the experience gained in the discharge of my duties as the Medical Officer of Health of a large County. That office has cast upon me the constant duty of inspecting some hundreds of sewage works in actual operation, and of analyzing the efﬂuents. From such works. As the result, I have endeavoured to set out in these pages as succinctly as possible, and yet (it is hoped) with sufficient fulness for the purpose in view, the conditions which appear favourable for particular processes for the purification of sewage, and their necessary limitations. Until the passing of the Local Government Act, 1888, by which County Councils were constituted, the enforcement of the Rivers Pollution Prevention Act of 1876 was left in the hands of the Sanitary Authorities, who, being themselves the chief offenders against the provisions of that enactment, were, naturally enough, very loth to institute proceedings against one another.