136 PATHOGENIC BACTERIA. Petruschky's Whey.—In order to differentiate be- tween acid and alkaline producers among the bacteria, Petruschky has recommended a neutral whey colored with litmus. It is made as follows: To a liter of fresh skimmed milk i liter of water is added. The mixture is violently shaken. About 10 c.cm. are now taken out as a sample to determine how much hydrochloric acid must be added to produce coagulation of the milk, and, having determined the least quantity required for the whole bulk, it is added. After coagulation the whey is filtered off, exactly neutralized and boiled. After boiling it is generally found clouded and acid in reaction. It is therefore filtered again, and again neu- tralized. Litmus is finally added to the neutral liquid, so that it has a violet color, which can readily be changed to blue or red by alkalies or acids. The medium is a very useful aid in differentiating; the typhoid and colon bacilli, showing well the alkaline formation of the typhoid bacillus. Peptone Solution, or Dunham's solution, is very use- ful for the detection of certain faint colors. It is a per- fectly clear, colorless solution, made as follows: Sodium chlorid, 0.5*) Boil until the ingredients Witte's dried peptone, i. V dissolve; then filter, fill Water, 100. j into tubes, and sterilize. It is one of the best media for the detection of iudol. In it the bacillus pyocyaneus produces its blue color. A very important fact in regard to peptone has been pointed out by Garini,1 who found that many of the peptones upon the market were impure, and on this account failed to show the indol reaction for bacteria known to produce indol. He recommends the use of the biuret reaction for testing the peptone to be employed. The reagent jised is Fehling's copper solution, with which pure pep- tone strikes a violet color not destroyed upon boiling-, 1 Centralbl.f. Bakt. u. Parasitenk., 1893, xiii., p. 790.