2o6 'Appendix. 



combination of 25 per cent of red (wave length approxi- 

 mately 658) with 75 per cent of white as shown by the 

 disks of the color top, described under Record of Tints 

 and Shades of Apparent Color. In practice titration 

 is continued until the pink color of alkaline phenol- 

 phthalein matches that of the fuseid disks. 



All reactions shall be expressed with reference to the 

 phenolphthalein neutral point and shall be stated in per- 

 centages of normal acid or alkaline solutions required to 

 neutralize them. Alkaline media shall be recorded with 

 the minus ( — ) sign before the percentage of normal' acid 

 •needed for their neutralization, and acid media with the 

 plus (+) sign before the percentage of normal alkaline 

 solution necessary for their neutralization. 



The standard reaction of culture media shall be + i.o 

 per cent. If it differs from i per cent by more than 0.2 

 per cent it should be readjusted. 



Wherever reactions other than the standard above 

 given are used it shall be clearly stated in all results of 

 bacterial work, and the reasons therefor also stated. 



STORAGE OF MEDIA. 



It is recognized by the committee that it is desirable 

 to prepare media in large quantities in order to guard 

 against discrepancies in composition; but, all things con- 

 sidered, the complications resulting from the varying 

 amounts of heating incident to withdrawing portions 

 from time to time and tubing it, are believed to more 



