Certain varieties of bacteria have been found associated with some forms of disease, and particular attention has been bestowed upon the study of these germs. The biological examination of water consists of pathological experiments on living animals, made by injecting a solution of the water-residue beneath the skins of rabbits, etc., and of experiments made by inoculating culture gelatine with the water. Of the latter methods of examination, that originally suggested by Dr. Koch, of Berlin, and described by Dr. Percy F. Frankland,[130] is well worthy of mention. In this process, the lower forms of life are cultivated in a solid medium, by means of which the growth of each colony is localised and rendered suitable for microscopic inspection. The medium employed by Dr. Frankland has the following composition: Lean meat 1 lb. Gelatine 150 grammes. Solid peptone 10 „ Sodium chloride 1 „ Distilled water 1 litre. The finely-cut meat is first infused in half a litre of cold water for two hours and strained; the gelatine is digested in the other half-litre of water, then mixed with the meat-extract, and the whole heated until the gelatine is completely dissolved, when the peptone and salt are added. The liquid is now cautiously neutralised with sodium carbonate, and clarified by beating it together with two or three eggs, boiling, straining through cloth, and filtering hot through bibulous paper; upon cooling it sets to a transparent jelly. Before setting, 7 c.c. of the liquid are introduced into a series of clean test-tubes, which are tightly plugged with cotton-wool and then sterilised by steaming them half-an-hour for three or four consecutive days. It is necessary to observe special precautions in the collection of the sample of water to be examined. Glass-stoppered bottles are well adapted for this purpose. These are to be very thoroughly washed with distilled water, then dried and finally sterilised by heating in an air-bath for three or four hours at a temperature of from 150° to 180°.