88 BACTERIOLOGY 



vary for different lots of media. Another source of error and 

 misunderstanding arises from the fact that the reaction of a 

 medium changes somewhat after its neutralization, especially 

 during sterilization, but also upon standing afterward at ordinary 

 temperature. This change is toward decreased alkalinity and 

 increased acidity and its extent is not the same for different 

 media, being most marked, perhaps, in those rich in glucose. 

 Where particular importance is attached to the titre of a medium, 

 it is well, therefore, to determine this upon a sample of the medium 

 taken from the lot at the time it is used, rather than to quote 

 figures obtained before sterilization. The optimum reaction for 

 most microbes is very close to the neutral point for litmus and 

 preferably slightly alkaline to this indicator. 



Gelatin. Finely chopped meat, 450 to 500 grams, is mixed 

 with a liter of distilled water and digested on the water bath for 

 half an hour at 50-55, with stirring. It is then strained through 

 muslin, yielding a filtrate of deep red color, which should be made 

 to equal 1000 c.c. This filtrate is placed in the inner compart- 

 ment of a double boiler (rice cooker) and to it are added 10 grams 

 peptone, 5 grams sodium chloride and 100 to 150 grams of sheet 

 gelatin of the best quality ("gold label" gelatin). The larger 

 amount of gelatin should be used during warm weather if no low- 

 temperature incubator is at hand. These constituents are dissolved 

 by stirring at a temperature below 55 C, After complete solution, 

 the reaction is titrated as has been described for the titration of 

 broth. From 30 to 50 c.c. of normal alkali are usually required 

 to give the proper reaction to a liter of the medium. After this 

 has been ascertained, and the amount added, the medium is 

 thoroughly mixed and then left covered and undisturbed while 

 the water in the outer compartment of the cooker is boiled for 

 an hour. It is well to have boiling water at hand in another 

 receptacle so that the supply in the cooker may be replenished 

 if it gets low, without chilling the medium. The gelatin is now 

 filtered through paper wet with hot water, and should be kept 

 warm during filtration by means of a funnel-heater, or by a steam 



