572 S. HENRY AYERS AND COURTLAND S. MTJDGE 



gen-ion concentration to pH 7.6. The medium with this pH, 

 however, does not give satisfactory results as to counts. 



By the addition of Na 2 HP0 4 + 2H 2 to the agar during 

 preparation, the calcium and magnesium salts are partly pre- 

 cipitated and can be removed by filtration. This agar when 

 used in the medium causes no precipitation upon sterilization 

 but cuts down the count. From our experiments it is believed 

 this reduction may be due to the increase in the phosphate 

 content of the medium. 



Washing the agar as described gives an agar which works 

 perfectly because the calcium and magnesium content is reduced 

 to a considerable extent and a readjustment of the medium to 

 a suitable pH is not necessary. 



We will consider the question of the special value of washed 

 agar in raising counts, in another paper. 



NO ADJUSTMENT OF REACTION NECESSARY 



It will be noted that no adjustment of reaction has been 

 mentioned. If the medium is made according to directions, 

 it will have a hydrogen-ion concentration of about pH 6.8 when 

 "Difco" or Parke-Davis peptone is used and about pH 6.7 with 

 Fairchild peptone. To be safe, it is desirable to check the 

 reaction of the milk-powder peptone-extract solution for its 

 hydrogen-ion concentration with indicators 2 for each new lot 

 of skimmed-milk powder, peptone, and meat extract used. This 

 can easily be determined in a sufficiently accurate manner by 

 adding 5 drops of phenol red indicator to a tube containing 5 cc. 

 of the double-strength milk-powder peptone-extract solution 

 diluted to 10 cc. with distilled water and 5 drops of brom-cresol 

 purple to another tube of the solution. The reaction is correct 

 when the tube with phenol red is yellow and the one with brom- 

 cresol purple is purple. If the tube with phenol red is red, add 

 ■& HC1, calculating the amount necessary to change it to yellow 



2 See paper by Clark, W. M., and Lubs, H. A. The colorimetric determina- 

 tion of hydrogen ion concentration and its application to bacteriology. Journ. 

 Bact., 2, p. 1, 1917. 



