372 PA THOGENIC BA CTERIA. upon plates, no bacteria develop well except the colon bacillus and the typhoid bacillus. These two bacteria, however, differ very markedly in their appearance upon the medium, for the colon bacillus appears as usual in twenty-four hours, while at that time, if present, the typhoid bacillus will have produced no colonies discoverable by the microscope. It is only after forty-eight hours, long after the colon colonies have attained considerable size and are conspic- uous, that the little colonies of the typhoid bacillus appear as small, round, shining, dew-like points, which are finely granular and in marked contrast to their coarsely granular predecessors. Unfortunately, many of the small colonies that develop in Eisner's medium sub- sequently prove to be those of the colon bacillus. Kashida1 prefers to make the differential diagnosis by observing the marked acid production of the Bacillus coli upon a medium consisting of bouillon containing i y2 per cent, of agar, 2 per cent, of milk-sugar, i.o per cent, of urea, and 30.0 per cent, of tincture of litmus. The cul- ture-medium should be blue. When liquefied and inocu- lated with the colon bacillus, poured into Petri dishes, and stood for sixteen to eighteen hours in the incubator, the blue color passes off and the culture-medium becomes red. If a glass rod dipped in hydrochloric acid be held over the dish, vapor of ammonium chlorid is given off The typhoid bacillus produces no acid in this medium, and there is consequently no change in its color. For the differentiation of the typhoid bacillus from the allied bacillary forms, Hiss2 recommends the use of two special media. The first consists of 5 grams of agar-agar, 80 grams of gelatin, 5 grams of Liebig's beef-extract, 5 grams of sodium chlorid, and 10 grams of glucose to the liter. The agar is dissolved in the 1000 c.cm. of water, to which have been added the beef-extract and sodium chlorid. When the agar is completely melted the gelatin is added 1 CentralbLf. Bakt. u. Paristenk., Bd. xxi.,Nos. 20 and 21, June 24, 1897. 2 Journal of Experimental Medicine, Nov., 1897, vol. ii., No. 6.