ISOLATING THE TYPHOID BACILLUS. 355 



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 J per cent, solution 

 of phenol phtalein in 50 per cent, alcohol. The acidity 

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

 normal sodium hydroxide solution to neutralize 10 c.c. of 

 the juice (Potter). If the acidity is found to be greater 

 than this, which is usually the case, dilute with water 

 until the proper degree is reached. If less than this, 

 the juice may be concentrated by evaporation. It is 

 desirable that this acidity should arise from the acids 

 normally present in the potato, and should not be arti- 

 ficially obtained by the addition of other acids. Now 

 add 10 per cent, of gelatin (no peptone and no sodium 

 chloride), dissolve by boiling and again test the acidity, 

 using 10 c.c. of the mixture and phenolphtalein as 

 before. Deduct 3 c.c. (the acidity of the potato juice, 

 that is to be maintained) from the number of c.c. of the 

 decinormal sodium hydroxide solution required to neu- 

 tralize the 10 c. c. of the gelatin mixture, and from the 

 resulting figure calculate the amount of normal solution 

 of sodium hydroxide needed for the entire volume, and 

 add it. Boil, clarify with an egg, and filter through 

 paper in the usual manner. To the filtrate add potas- 

 sium iodide in the proportion of 1 per cent. Decant 

 into tubes and sterilize. 



The spleen of a patient dead of typhoid fever is the 

 safest source from which to obtain cultures of the 

 typhoid bacillus for study. But it must always be re- 

 membered that the same channels through which the 



