An agar medium is conveniently prepared from broth by adding 1 or 1.5 per cent. of the finely divided agar to the broth and boiling until dissolved, standardizing, clearing, filtering, and sterilizing. The agar must be thoroughly boiled, usually for ten to fifteen minutes, and the water loss made up by the addition of distilled water before titration. Agar is practically neutral so that there is little difference between the titration of the dissolved agar and the original broth. The agar solution should be kept hot from the beginning to the end except the cooling down to below 60°, when the egg white for clearing is added. Though filtration through paper is possible as with gelatin, if the agar solution is thoroughly boiled and filtered boiling hot, it is more satisfactory for beginners to use absorbent cotton wet with boiling water and to pour the hot agar through the same filter if not clear the first time. The solidified agar medium is never perfectly clear, but always more or less opalescent. The agar medium may be sterilized in the autoclave for fifteen minutes at 15 pounds pressure as the high temperature does not injure the agar. =Potato Media.=--Potatoes furnish a natural culture medium which is very useful for the study of many bacteria. The simplest, and for most purposes the best, way to use potatoes is in culture tubes as “potato tube cultures” (No. 8, Fig. 119). These are prepared as follows: Large tubes are used. Large healthy potatoes are selected. Each end of the potato is sliced off so as to have parallel surfaces. With a cork-borer of a size to fit the tubes used, cylinders about one and one-half inches long are made. Each cylinder is cut diagonally from base to base. This furnishes two pieces each with a circular base and an oval, sloping surface. The pieces are then washed clean and dropped for a minute into boiling water to destroy the oxidizing enzyme on the surface which would otherwise cause a darkening of the potato. (The darkening may also be prevented by keeping the freshly cut potatoes covered with clean water until ready to sterilize.) A bit of cotton one-fourth to one-half inch in depth is put into each of the test-tubes to retain moisture and a piece of potato dropped in, circular base down. The tubes are then plugged with cotton and sterilized in the autoclave at 15 pounds pressure for not less than twenty-five minutes, since potatoes usually harbor very resistant spores, and it is not unusual for a few tubes to spoil even after this thorough heating.