TITRATION AND ADJUSTMENT OF CULTURE MEDIA 211 



c. The reaction of peptone solutions and the effects upon 

 them of prolonged heating. 



d. The question of indicators with consideration of the 

 significance and sensitiveness of their end points indifferent 

 media — also the method of choosing the one most suitable 

 with reference to the hydrogen electrode as a standard. 



MEAT INFUSIONS 



It is a well known fact that each time a medium is heated to 

 the boiling point, or above it, the reaction changes and be- 

 comes more and more acid, depending on the length of time 

 and the degree of heating. On this subject Eyre (1915) says: 



Meat extract [meat infusion] is acid in reaction owing to presence 

 of acid phosphates of potassium and sodium; weak acids of the gly- 

 colic series and organic compoimds in which an acid character pre- 

 dominates. 



Owing to the nature of the substances from which it derives its 

 reaction, the total acidity of meat extracts [infusion] can only be esti- 

 mated accurately when the solution is at the boiling point. Prolonged 

 boiling [as in media preparation] causes it to undergo hydrolytic changes 

 which increase the acidity. 



He states further that meat extract [infusion] becomes stable 

 in reaction after being heated at the boiling point for forty-five 

 minutes so that no additional increase of acidity occurs on 

 further heating. 



To procure more definite data as to the effect of heat on the 

 acidity of media the following work was carried out: 



Preparation of the meat infusions. Chopped lean veal was 

 soaked over night in tap^ water in the proportion of one pound 

 of meat to one liter of water. It was then heated at 45° to 55° 

 C. for one hour. At this point it was brought to a boil. Then 



3 Weekly analyses of the Croton water supply shows it to contain a negligible 

 amount of mineral matter (only about 40 parts per million, expressed as total 

 hardness.) Where the water supply is at all "hard," it is advisable to employ 

 distilled water exclusively. 



