The actual examination of the water is executed by first heating one of the test-tubes containing the sterilised gelatin medium in a water-bath to 30°, by which it is fused. The external portion of the cotton-wool is next burned, the tube opened, and a certain number of drops of the water to be tested (previously well shaken) are introduced by means of a sterilised pipette. The mixture is immediately poured out upon a clean and sterilised glass plate which rests in a perfectly horizontal position, and is covered by a glass shade. The plate is supported by a glass tripod which dips into a dish containing a two per cent. solution of mercuric chloride--thus forming an antiseptic protection from the external air. The tripods, dishes, etc., are sterilised by washing them with the mercuric chloride solution. As soon as the gelatine mixture has set, the glass plate (together with the cover) is introduced into an air-bath kept at a temperature of from 20°-25°, where it is allowed to remain for two to five days for incubation. The individual organisms and the progress of the formation of colonies are observed from time to time by inspecting the plate, which can be done without removing the glass cover. As soon as they have become easily visible to the naked eye, the plate is removed from the bath, and placed upon another glass plate, which is ruled in squares, and put over a black paper. The colonies are then counted by aid of a lens, or, if they are too numerous to admit of this, the number contained in a few of the squares is determined and multiplied accordingly. Dr. Frankland has applied the foregoing method to the examination of the London water supply (1885), with the following results:-- MICRO-ORGANISMS IN 1 C.C.