54 PRINCIPLES OF BACTERIOLOGY 



tents must balance when the volume is exactly one litre. A portion of 

 the medium brought to the exact volume is then taken and cooled to 

 room temperature (20 C.), or to a point a few degrees above solidifi- 

 cation, and 10 c.c. withdrawn, placed in a small beaker, 50 c.c. of dis- 

 tilled water and 1 c.c. of the phenolphthalein solution added. If the 

 medium is acid the -f$ NaOH solution is then run in cautiously until a 

 pale but decided pink color is obtained. The number of cubic centi- 

 metres of the solution used, multiplied by ten, will give the number 

 of cubic centimetres of normal sodium hydrate per litre necessary to 

 effect complete neutralization. The question as to what is the best 

 reaction of media for general work is not an easy one to settle, and 

 one on which bacteriologists differ. What is the proper reaction for one 

 variety of bacteria is often far from the best for some other variety. 

 Reactions are now commonly expressed by plus or minus signs, the 

 former representing an acid and the latter an alkaline condition, the 

 number following the sign representing the percentage of normal acid 

 or alkali present in the medium. Thus, +1.5 would indicate that the 

 medium contained 1.5 parts per 100 or 1.5 percent, of free normal acid, 



FIG. 23 FIG. 24 



Krlenmeyer flask. Pasteur flask. 



while 1.5 would indicate that the medium contained an equivalent 

 quantity of free alkali. The committee of the American Public Health 

 Association in 1898 adopted a medium whose litre was +1.5 as the 

 best for general work in water examinations. In 1905 this was changed 

 to +1.0 per cent. A medium whose reaction is +0.5 per cent, acid to 

 phenolphthalein is still better adapted for many bacteria. It cannot be 

 too strongly impressed upon the reader that whatever the reaction, its 

 measure should be stated in all descriptions of cultural characters. 

 The litmus solution is added in the same way as that of phenolphthalein. 

 Storage of Media. The nutrient media are stored in glass flasks (Fig. 

 23). From these, as needed, glass tubes are filled. When small amounts 

 of media are taken frequently from flasks, Pasteur's flasks (Fig. 24) are 

 of great convenience. They consist of a flask with a ground-glass neck, 

 over which fits a cap. This cap may or may not terminate, as desired, 

 in a narrow tube, which is plugged with cotton. The cap keeps the 

 edges of the flask free from bacteria and prevents the cotton from 



