178 TYPHOID FEVER. 



0.5 per cent, solution of phenolphthalein in 50 per cent, 

 alcohol. The acidity oi the juice should be such as to re- 

 quire 3 c.c. of a decinormal sodium hydroxide solution to 

 neutralize 10 c.c. of the juice. If the acidity is found to be 

 greater than this, which is usually the case, dilute with wafer 

 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 normally present 

 in the potato, and that it should not be artificially obtained 

 by the addition of other acids. Now add 10 per cent, of 

 gelatin (with no peptone and no sodium chloride present), 

 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 solu- 

 tion requisite to neutralize 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 potassium 

 iodide in the proportion of 1 per cent., decant into tubes, and 

 sterilize." 



IV. Stodard's and Hiss' Differentiation. By this method 

 use is made of the great motility of the Bacillus typhosus to 

 differentiate it from the coli communis. It is valuable at 

 times. Success in this procedure depends on the important 

 fact that in a semifluid mixture the Bacillus typhosus, on 

 account of its great motility, will diffuse much more rap- 

 idly from the point of inoculation to nearly all parts of the 

 medium, whereas the coli communis, having only a sluggish 

 or no motion at all, develops only at the place of immediate 

 inoculation. For detailed accounts of these methods the 

 reader is referred to larger treatises on bacteriology. 



V. The Bacillus fsecalis alkaligenes, described by Petruschky 

 in 1896, which is very nearly identical in appearance and 

 culture qualities, does not produce acids in media containing 

 dextrin, levulose, maltose, or mannit, and B. typhosus, on the 

 contrary, does produce acid in these media. 



