APPENDIX I4I 
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 7, c.c., and 3 solutions are required. 
1. A..5% solution of commercial phenolphthalein in 50 % alcohol. 
2. A n/2o solution of sodium hydroxid. 
3. A n/2o 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. 1) 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/zo 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 
