MEDIA FOR QUALITATIVE ANALYSIS. 269 



These materials are to be dissolved at a temperature, pref- 

 erably not over 140. After thorough solution the mixture 

 is to be neutralized by the use of NaOH as described on page 

 254, and after neutralization the reaction is to be brought to 

 1.5% by adding 15 c.c. normal HC1 for each liter of solution. 

 The white of egg is added and the mixture boiled for half an 

 hour and then filtered through absorbent cotton. The solu- 

 tion thus made is like an ordinary gelatin culture medium, 

 except that it contains milk sugar and has slightly larger 

 proportions of the various ingredients. This medium, after 

 filtering, is to be filled into sterilized test-tubes, exactly 8 c.c. 

 of the medium being placed in each tube. These tubes are 

 then to be sterilized by steaming for fifteen minutes on three 

 successive days as usual. 



2. Litmus Solution. Weigh out 50 grams of dry litmus 

 cubes and add to it 300 c.c. of water. Allow the mixture to 

 steep for a few hours in warm water (four hours at 160), 

 or allow it to soak twenty-four hours in water at the 

 ordinary temperature of a room. This dissolves the active 

 material from the litmus. The whole is then to be filtered, 

 giving a deep blue solution. This solution is commonly alka- 

 line to litmus but acid to phenolphthaleiri. The next step is to 

 determine its grade of alkalinity. 5 c.c. is placed in an evapor- 

 ating dish, diluted with 45 c.c. of water, and one tenth normal 

 HC1 is added from a burette as explained on page 256, until 

 the litmus neutral point is reached. The neutral point is rec- 

 ognized by the turning of the blue solution into a faint red. 

 The HC1 is added, drop by drop, and the neutral point is 

 reached, when there is the first trace of change in the blue 

 color toward a reddish. The reading upon the burette will 

 show how much HC1 must be added to the solution to bring 

 it to the litmus neutral point. The calculation is made by the 

 method already described and the litmus solution is neutral- 

 ized. This solution is now too strongly acid for bacteria 



