APPENDIX 127 



are more often acid than alkaline, it is proposed that acid media be 

 designated by the plus sign and alkaline media by the minus sign, 

 and that the degree of acidity or alkalinity be noted in parts per 

 hundred; thus a medium marked + 1.5 would indicate that the 

 medium was acid and that 1.5 % of n/i sodium hydroxid is required 

 to make it neutral to phenolphthalein, while —1.5 would indicate 

 that the medium was alkaline and that 1.5 "/„ of n/i acid must be 

 added to make it neutral to the indicator. 



Limits of accuracy of the proposed method for the control of the 

 reaction of media : 



The available data are as yet insufficient to warrant any conclu- 

 sions upon this point. The limits of accuracy seem to vary with 

 the ingredients employed iri preparing nutrient media, different 

 samples of meat infusion, pepton, and gelatin appearing to react 

 differently with the acids and alkalis and in a way which is not 

 understood. 



This method, nevertheless, when carefully carried out, and when 

 the media before titration are thoroughly mixed and are of the 

 prescribed volume, give fairly uniform results. 



Standard reaction of media (provisional) : 



Experience seems to vary somewhat as to the optimum degree 

 of reaction which shall be uniformly adopted in the preparation of 

 standard culture media. To what extent this is due to variation in 

 natural conditions as compared with variations of laboratory pro- 

 cedure, it seems impossible to state. Somewhat different degrees 

 of reaction for optimum growth are required, not only in or upon 

 the media of different composition and by bacteria of different 

 species, but also by bacteria of the same species when in different 

 stages of vitality. 



The bulk of available evidence from both Europe and America 

 points to a reaction of +1.5 as the optimum degree of reaction for 

 bacterial development in inoculated culture media ; and while this 

 experience is at variance with that in several of our own laboratories, 

 it has been deemed wise to adopt +1.5 as the provisional standard 

 reaction of media, but with the recommendation that the optimum 

 growth reaction be always recorded in species descriptions." 



