8 BACTERIOLOGY 



after the method of Buchner potato cultures are prepared, 

 and the mass of bacteria is then scraped from them, and 

 rubbed up in a mortar with a little water, mixed with 

 fifty times its volume of a half per cent, solution of caustic 

 potash, and then digested in a water-bath until the greatest 

 possible degree of fluidity is reached, after which it is 

 filtered through several small filters. Dilute acetic or 

 hydrochloric acid is next added to the filtrate until, while 

 avoiding any excess of acidity, the reaction becomes dis- 

 tinctly acid. The protein precipitated in this way is 

 collected on a filter, washed, and dissolved in water which 

 is feebly alkaline. 



Roemer procured his extracts of the Bacillus pyocyaneus 

 and pneumobacillus in the following manner : The masses of 

 bacteria are carefully scraped from well- developed cultures 

 on potato and rubbed to a fine emulsion with ten times 

 their bulk of distilled water. The emulsion, having been 

 sterilised by boiling for several hours, is left for about four 

 weeks in the incubator, during which time it must fre- 

 quently be boiled for an hour or two, so that in the course 

 of the process it is boiled for from thirty to forty hours in 

 all. When the four weeks have expired the emulsion is 

 filtered through a tubular filter made of kaolin (Chamber- 

 land's candle), or through one of infusorial earth, and the 

 resulting filtrate is a clear brownish or yellowish fluid con- 

 taining albuminoid substances. 



Koch obtained his tuberculin by extraction from pure 

 cultures of tubercle bacilli, which he grew upon a feebly 

 alkaline infusion of veal containing an addition of one per 

 cent, peptone and four to five per cent, glycerine. The 

 culture-vessels are inoculated by floating a fairly large 

 piece of the seed-culture on the surface of the fluid, and are 

 then kept at a temperature of 38 C. In from three to four 

 weeks the surface is covered with a tolerably thick mem- 



