156 BACTEEIOLOGY. 



and one of red litmus paper. These should be immersed in 

 the liquid. Again, heat to boiling and set aside for one 

 minute. By means of a glass rod draw out the papers, side 

 by side, upon the wall of each tube and, when cold, compare 

 the colors of the papers, by holding the tubes before a win- 

 dow, over a white surface. What was originally the blue 

 paper in tube 1, will probably have a slight red color. 

 This shows that 2.5 c.c. of i^ NaOH is not siiffii-ient to neu- 

 tralize 10 C.C. of the gelatin solution. On the other hand, 

 in tube 4, both papers are blue, which indicates that 3.5 c.c. 

 of the alkali is more than is necessary to neutralize 10 c.c. 

 of the liquid. The neutral point, that is, where the blue 

 and red paper retain their color side by side, lies therefore 

 between these two extremes. Tube 2, apparently, shows a 

 slight acid reaction, whereas tube 3, is faintly alkaline. 

 The neutral point lies, therefore, between 2.8 and 3.2; it 

 probably is about 3.0. Hence, measure out into a test-tube 

 10 c.c. of the gelatin, and add 3.0 c-c. of the fs NaOH. 

 Then boil and add litmus paper, and examine as above. In 

 this way it is possible to determine the neutral point to 

 within 0.1 or 0.2 of a cc, and this corresponds to a 

 probable error of 1 or 2 c.c. of N NaOH per liter of 

 gelatin. 



The above experiment has shown that 10 c.c. of the 

 gelatin requires 3.0 cc. of ft NaOH for neutralization. In 

 order to ascertain the amount of alkali necessary for the 

 neutralization of all of the remaining gelatin, the latter 

 must be measured in a cylindrical graduate. The amount 

 left corresponds, for example, to 950 c.c. The amount of 

 ft alkali necessary to neutralize this quantity is ascertained 

 from the following proportion: 



10 :3 ::950 :x x = 285. 



That is to say, in order to neutralize the 950 cc. of ge- 

 Ipitin, it would be necessary to add 285 c.c of ft NaOH. 

 Since this corresponds to 28.5 c.c. of N NaOH, the latter is 



