ACIDITY OF MEDIUM 



121 



stant composition and in the variable quantity of alkali 

 required for the adjustment of the acidity. 



Clark 3 has pointed out that the method of adjusting the 

 acidity of media, as recommended in the standard methods of 

 analysis, is not scientific in principle and that it does not ensure 

 a constant hydrogen ion concentration. Various batches of 

 media prepared by different workers and adjusted to an acidity 

 of +1 per cent by the standard method (titration of the boiling 

 medium with alkali using phenolphthalein) were found to have 

 very different H ion potentials when tested by the electrical 

 method. No results are given by Clark as to the effect of this 

 variation on the bacterial reproduction in these media but the 

 comparative experiments of a group of New York bacteriolo- 

 gists indicate that any variation due to this cause is insignificant 

 and can safely be ignored. In these experiments media were 

 prepared by four laboratories and supplied to Dr. Conn, of 

 Middletown, Conn., who plated out two samples of milk on 

 each medium in triplicate. The results were as follows: 



Three of the above media gave an acidity of +1.0 per cent, 

 as determined by Conn, and the fourth +0.9 per cent. These 

 results show that media prepared in various laboratories accord- 

 ing to standard methods give results as close as can be expected 

 from a consideration of the technique. 



The technique of bacterial enumeration in milk was care- 

 fully investigated by the New York group of bacteriologists 

 above referred to and the results summarised by Conn. 4 

 Samples of various grades of milk and cream were prepared by 

 Conn and duplicate samples forwarded to the various laborator- 

 ies partaking in the work. As the samples invariably included 

 duplicate samples under different numbers, each sample was 



