APPENDIX 125 



The question of the proper reaction of media for the cultivation 

 of bacteria, and the method of obtaining this reaction, have been 

 discussed in a valuable paper by Mr. George W. Fuller, published 

 in the Journal of the American Public Health Association, Vol. 20, 

 Oct., 1895, p. 321. Some of the main results there given have been 

 mentioned above. 



Method of determining the Degree of Reaction of Culture Media. — 

 For this most important part in the preparation of culture media, 

 burettes, graduated into -^ c.c, and 3 solutions are required. 



1 . A .5 % solution of commercial phenolphthalein in 50 % alcohol. 



2. A n/20 solution of sodium hydroxid. 



3. A n/20 solution of hydric chlorid. 



Solutions Nos. 2 and 3 must be accurately made up and must cor- 

 respond with the normal solutions soon to be referred to. Solutions 

 of sodium hydroxid are prone to deterioration from the absorption 

 of carbon dioxid and the consequent formation of sodium carbonate. 

 To prevent as much as possible this change, it is well to place in 

 the bottle containing the stock solution a small amount of calcium 

 hydroxid, while the air entering the burettes or the supply bottles 

 should be made to pass through a " U " tube containing caustic soda, 

 to extract from it the carbon dioxid. 



The medium to be tested, all ingredients being dissolved, is 

 brought to the prescribed volume by the addition of distilled water 

 to replace that lost by boiling, and after being thoroughly stirred, 

 5 c.c. are transferred to a 6-inch porcelain evaporating dish ; to this 

 45 c.c. of distilled water are added, and the 50 c.c. of fluid are 

 boiled for 3 minutes over a flame. One c.c. of the solution of 

 phenolphthalein (No. i) is then added, and by titration with the 

 required reagent (No. 2 or 3) the reaction is determined. In the 

 majority of instances the reaction will be found to be so that 

 the n/20 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 

 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 here be remarked 

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



