ELSNER'S MEDIUM. 491 



through absorbent cotton according to the procedure de- 

 scribed on p. 237. When this filter clogs the surface layer 

 of muslin should be carefully removed or scraped when the 

 filtration will recommence. 



10 per cent, of gelatin and 1 per cent, of KI are added 

 to the dark liquid which is gently warmed at about 40 

 till the former has dissolved. Portions of 10 c.c. of the 

 liquid are then titrated with & NaOH (p. 155). 10 c.c. of 

 the potato extract usually requires about 1.6 c.c. of & NaOH 

 for neutralization. It may, however, require as much as 

 3.2 c.c. The addition of gelatin still further increases the 

 acidity. The gelatin should be acid, but not too much so. ( 

 The acidity of 10 c.c. of the gelatin should not require more 

 than 2.0 c.c. of TTJ NaOH. If it requires more, the excess 

 over 2 c.c. (p. 354) should be neutralized by adding the cor- 

 responding amount of N NaOH to the remaining gelatin. 

 For example, if 10 c.c. of the gelatin requires 3.2 c.c. T* 

 NaOH, then the excess acidity, above 2.0, corresponds to 

 1.2 c.c T N TT NaOH. Now, if the volume of the gelatin meas- 

 ures 350 c.c. this amount will require 42 c.c. of & NaOH = 

 4.2 c.c. of N NaOH, in order to reduce the acidity to the 

 proper point. 



10 : 1.2 :: 350 : x x = 42 c.c. of & NaOH. 



The gelatin, having now the proper degree of acidity; 

 is immersed in a boiling water-bath for about ^ of an hour. 

 The soluble proteins coagulate and clarify the liquid. This 

 is then filtered, filled into tubes, and sterilized by steaming 

 for 15 minutes on each of three successive days. As a rule, 

 a slight precipitate forms in the tubes, during sterilization, 

 but this does not interfere with the usefulness of the 

 medium. 



On this medium the Eberth bacillus yields very finely 

 granular, small, bright droplets resembling water. On the 

 other hand, the colon bacillus gives rise to colonies which 

 are large, spreading, more strongly granular and brown in 

 appearance. A constant temperature of 15-18 should be 



