SERUM WATER MEDIUM OF HISS 143 



must be done carefully, to avoid contamination of the tubes 

 during manipulation. It is better not to add the litmus 

 tincture before sterilizing the tubes, as its color-character- 

 istics 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. 



Lbffler's Blood-serum Mixture. Loffler's blood-serum mix- 

 ture consists of 1 part of neutral meat-infusion bouillon, 

 containing 1 per cent, of grape sugar, and 3 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 tempera- 

 ture 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 Medium of Hiss. A medium which 

 has been found very serviceable in the differentiation 

 between closely related bacteria is prepared by mixing 1 

 part of blood serum (either horse or bovine) and 3 parts of 

 distilled water. This is neutralized, and heated in a water- 

 bath or an Arnold steam sterilizer until it becomes opales- 

 cent. A 5 per cent, aqueous solution of litmus is then 

 added in the proportion of 1 per cent. Any one of the 

 carbohydrates, as dextrose, lactose, saccharose, levulose, 

 mannite, etc., is then added in the proportion of 1 per cent. 

 The finished medium is then placed in test-tubes. The 

 medium must be sterilized in an Arnold steam sterilizer, and 

 it is advisable to allow the sterilizer to remain uncovered 

 during the process of sterilization to avoid excessive heating 

 of the medium. 



The relative degree of acidity produced, with or without 

 coagulation, with or without gas-production, and with or 



