EFFECT OF BUFFER MIXTURES UPON GROWTH 307 



portance, such as the hydrogen ion concentration, must be kept 

 constant while the unknown factors are made the variables. 



EXPERIMENTAL 



In the first set of comparisons made by the writers 6 on malt 

 extract regulated with these two buffer mixtures, both media 

 were excellent, but the phosphate-potassium acid phthalate mix- 

 ture seemed to give the better growth. Mr. Hopfield then very 

 carefully repeated the tests and found the better growth with 

 acid potassium phthalate. In order further to verify the results, 

 Mr. Hopfield ran two other sets of tests, side by side, using the 

 same media given above and also media made up in the same way 

 excepting that corn meal gruel was substituted for the 2.5 per 

 cent of malt extract. The corn meal gruel was prepared by cook- 

 ing 7.5 grams of yellow corn meal and about 200 cc. of water in 

 a water bath at 60°C. for one hour. After allowing the meal to 

 settle the liquid was decanted and fresh water mixed with the 

 meal. After allowing the meal to settle again the water was de- 

 canted and added to the liquid first decanted. This liquid was 

 used in making up 250 cc. of each of the two media after adding 

 the proper amounts of K 2 HP0 4 and ac.etic acid or acid potassium 

 phthalate, and agar. 



The average diametric growth, in millimeters, of four Petri 

 dishes of each medium is given below: 



Diametric 

 Medium growth in mm. 



Meaium i n to days 



at !5°C. 



2.5 per cent of malt extract, M/50 K 2 HP0 4 and M/50 acetic acid. . 64 

 2.5 per cent of malt extract, M/50 K2HPO4 and M/50 acid potassium 



phthalate 80 



Corn meal with M/50 K 2 HPO< and M/50 acetic acid 55 



Corn meal with M/50 K2HPO4 and M/50 potassium acid phthalate 71 



Thus it is very evident that the potassium acid phthalate allows 

 more rapid growth of E. parasitica, although both media are 

 excellent. 



6 The data given here were obtained during the summer of 1918. 



