1 86 PA THOGENIC BA CTERIA . 



added, and by titration with the required reagent (No. 2 or No. 3) 

 the reaction is determined. In the majority of instances the re- 

 action will be found to be acid, so that the — sodium hydroxid 



is the reagent most frequently required. This determination 

 should be made not less than three times and the average of the 

 results obtained taken as the degree of the reaction. 



One of the most difficult things to determine in this process is 

 exactly when the neutral point is reached as shown by the color 

 developed, and to be able in every instance to obtain the same 

 shade of color. To aid in this regard, it may be remarked that in 

 bright daylight the first change that can be seen on the addition 

 of alkali is a very faint darkening of the fluid which, on the addi- 

 tion of more alkali, becomes a more evident color and develops 

 into what might be described as an Italian pink. A still further 

 addition of alkali suddenly develops a clear and bright pink color, 

 and this is the reaction always to be obtained. All titrations 

 should be made quickly and in the hot solutions to avoid compli- 

 cations arising from the presence of carbon dioxid." 



The next step in the process is to add to the bulk of the medium 

 the calculated amount of the reagent, either alkali or acid, as may 

 be determined. For the purpose of neutralization a normal solu- 

 tion of sodium hydroxid or of hydric chlorid is used, and after 

 being thoroughly stirred the fluid thus neutralized is again tested 

 in the same manner as at first, to insure the proper reaction of the 

 medium being attained. When neutralization is to be effected by 

 the addition of an alkali, it not infrequently happens that after 

 the calculated amount of normal solution of sodium hydroxid has 

 been added, the second test will show that the medium is acid to 

 phenolphthalein, to the extent sometimes of 0.5 to 1 per cent. 

 This discrepancy is perhaps due to side reactions which are not 

 understood. The reaction of the medium, however, must be 

 brought to the desired point by the further addition of sodium 

 hydroxid, and the titrations and additions of alkali must be re- 

 peated until the medium has the desired reaction (z. e., 0.0 per 

 cent. — 0.005 P er cent., see below). 



After the prescribed period of heating, it is frequently found 

 that the medium is again slightly acid, usually about 0.5 per 

 cent. Without correcting this the fluid is to be filtered and the 

 calculated amount of acid or alkali is to be added to change the 

 reaction to the one desired. A still further change in reaction is 

 not infrequently to be observed after sterilization, the degree of 

 acidity varying apparently with the composition of the media 

 and the degree and continuance of the heat." 



" Manner of expressing the reaction : Since at the time the re- 



