SPECIAL MEDIA. 121 



medium. It is especially useful in the differentiation 

 of the bacillus of typhoid fever, which does not possess 

 the property of bringing about fermentation of lactose, 

 from other organisms that simulate it in many other 

 respects, but which do possess this property. 



Its preparation is as follows : to nutrient agar-agar 

 or gelatin, the alkalinity of which is such that 1 c.c. 

 will require 0.1 c.c. of a 1 : 20 normal sulphuric-acid 

 solution to neutralize it, lactose is added in the propor- 

 tion of 2 or 3 per cent. ; it is then decanted into test- 

 tubes and sterilized in the usual way. When ster- 

 ilization is complete enough sterilized litmus tincture 

 should be added to each tube to give a decided, though 

 not very intense, blue color. This must be done care- 

 fully, to avoid contamination of the tubes during ma- 

 nipulation. It is better not to add the litmus tincture 

 before sterilizing the tubes, as its color-characteristics 

 are altered by contact with organic matters under the 

 influence of heat. This medium is used for both test- 

 tube and plate cultivation, just as is ordinary agar-agar 

 and gelatin. 



LOFFLER'S BLOOD-SERUM MIXTURE. Loffler's 

 blood-serum mixture consists of one part of neutral 

 meat-infusion bouillon, containing 1 per cent, of grape- 

 sugar, and three parts of blood-serum. This mixture is 

 placed in test-tubes, sterilized, and solidified in exactly 

 the way given for blood-serum. It requires for its 

 solidification a somewhat higher temperature and a 

 longer exposure to this temperature than does blood- 

 serum to which no bouillon has been added. (See also 

 the Councilman-Mallory method.) 



THE SERUM-WATER MEDIA OF Hiss. A medium 

 which has been found very serviceable in the differen- 



