136 BIOLOGY AND TECHNIQUE 



sary for routine laboratory media if the constitution of the tap 

 water is known and controlled from time to time.) 



2. The meat used should be fresh, lean beef (when veal or chicken 

 is substituted the change should be stated). 



3. The pepton used should be Witte's pepton, dry, made from 

 meat. 2 



4. Only C. P. NaCl should be used. 



5. For alkalinizing C. P. sodium hydrate should be used in normal 

 solutions. 



6. For acidification C. P. hydrochloric acid in normal solution 

 should be used. 



7. When glycerin is used, this should be of the neutral redistilled 

 variety. 



8. The agar-agar employed should be of the finest grade of 

 commercial thread agar. 



9. The gelatin should be the commercial sheet gelatin washed as 

 free as possible of acid and impurities. 



10. Chemicals and carbohydrates which are used should be as 

 nearly chemically pure as possible. 



Titration of Media. Next in importance to the actual composi- 

 tion of media is the adjustment of their reaction. Bacteria are 

 highly susceptible to varations in the acidity and alkalinity of media, 

 excessive degrees of either may completely inhibit development or 

 moderate varations may lead to marked modifications of cultural 

 characteristics. It is necessary, therefore, to adjust the reaction 

 both f or x the sake of favoring growth and in order to insure uni- 

 formity of growth characters. This is accomplished by titration 

 which is best carried out according to the recommendations of the 

 committee mentioned above. 



OLDER METHOD OF DIRECT TITRATION. The color indicator employed for 

 the titration is a five-tenths per cent solution of phenolphthalein in fifty per 

 cent alcohol.. The chief advantage of this indicator over others is due to 

 the fact that it indicates the presence of organic acid and acid compounds 

 in its reaction. For actual titration -^L (_!_ normal) solutions of sodium 

 hydrate or of hydrochloric acid are used. Since media in the process of 

 preparation are usually acid, the NaOH solution is the one most frequently 



- Since the Trar many domestic peptons have been introduced and can be used 

 with success. 



