ISOLATING THE TYPHOID BACILLUS. 425 



normal sodium hydroxide solution to neutralize 10 e.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 be due to the acids nor- 

 mally present in the potato, and that it 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. 



Stoddarfs method. The principle upon which this 

 method is based may be briefly stated as follows : if the 

 typhoid bacillus be cultivated in a semi-solid nutrient 

 medium at about 35° C, its growth is not circumscribed 

 or confined to the immediate neighborhood of the point 

 of inoculation ; whereas if a less actively motile organ- 

 ism — the colon bacillus, for example — be cultivated 

 under similar circumstances there is no apparent ten- 

 dency for it to diflPuse itself through the medium ; its de- 

 velopment is confined to the point of inoculation. In the 

 former case the medium becomes clouded throughout ; 

 in the latter it remains clear, except at the immediate, 



