. The principles of bacteriology: a practical manual for students and physicians. xture of bacteria. Many modifications of this methodexist; all, however, are based upon the same prin-ciples. The modifications have for their object theaccomplishment of the same end, but with a smallerarmamentarium of apparatus; according to individual 136 BACTERIOLOGY. preference, the one or the other of these modificationshas supplanted the original plate method as practisedand recommended by Koch. Petris Modification of the Plate Method.—The modification wliich approaches nearest to the orig-inal method, and
. The principles of bacteriology: a practical manual for students and physicians. xture of bacteria. Many modifications of this methodexist; all, however, are based upon the same prin-ciples. The modifications have for their object theaccomplishment of the same end, but with a smallerarmamentarium of apparatus; according to individual 136 BACTERIOLOGY. preference, the one or the other of these modificationshas supplanted the original plate method as practisedand recommended by Koch. Petris Modification of the Plate Method.—The modification wliich approaches nearest to the orig-inal method, and at the same time lessens very mate-rially the number of steps in the process, is that sug-gested by Petri. It consists in substituting for theplates small, round, double glass dishes, having aboutthe same surface-area as the plates (Fig. 27). Theliquid medium is poured directly into these little dishesand their covers replaced ; they are then set aside for ob-servation. In all other respects the process is the sameas Kochs original method. Petris dishes are about 8 Fig. Petri double dish, now generally used instead of plates. cm. in diameter and about to 2 cm. in height, thesides being vertical. They may readily be sterilizedby either hot-air or steam. They are very useful for thiswork, as they do away with the necessity for the cool-ing-stage and levelling-tripod, though in warm weatherthe cooling-stage may be used to hasten the solidifica-tion of gelatin. A cooling-stage of very convenientdesign for use with these dishes consists of a closed, flatmetal box, either of copper or block tin, and roundor square in shape, so arranged that it can be filledwith cold water, or that cold water can constantly be ESMABOH TUBES. 137 passed through it by means of a rubber tube connectedwith a spigot. The inlet for the water sliould be justabove the bottom of the box, and the outlet just beneaththe top and slightly turned upward and then downward,so as to insure filling the spac
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