424 BACTERIOLOGY. 



mistake them for typhoid colonies ; while typhoid col- 

 onies may present the characteristics of those due to the 

 growth of the colon bacillus. This is especially the 

 case in plates more than forty-eight hours old that have 

 been kept at ordinary room-temperature. 



In our experience the most serviceable feature of this 

 method is the elimination of many of the common sap- 

 rophytes usually present in mixtures containing typhoid 

 and colon bacilli. The majority of them do not grow 

 upon gelatin made by this method, which will now be 

 described. 



The description given by Eisner' of the mode of 

 preparation of the medium is so incomplete and unsat- 

 isfactory in most of the important details that prac- 

 tically all who use the method have been obliged 

 to develop their own technique from the general sug- 

 gestions made in his original communication. The 

 "Eisner medium" that has given satisfaction in our 

 hands is prepared as follows : grate one kilogramme of 

 peeled potatoes and allow to stand over night in a 

 refrigerator. Then press out all juice, using an ordi- 

 nary meat-press for the purpose. Filter this fresh 

 juice cold, to remove as much of the starch-gran- 

 ules as possible ; if this is not done, the starch when 

 heated swells to such an extent as to render filtration 

 almost impracticable. Boil the filtrate and filter again. 

 Test the filtrate for acidity by titrating 10 c.c. with a 

 decinormal solution of sodium hydroxide, the indicator 

 used being 6 drops of the ordinary |- per cent, solution 

 of phenolphtalein in 50 per cent, alcohol. The acidity 

 of the juice should be such as to require 3 c.c. of a deci- 



• Eisner : Zeitschrift fiir Hygiene und Infektionskrankheiten, 1896, 

 Bd. xxi. S. 25. 



