100 BACTERIOLOGY. 



through a folded paper filter and sterilized by steam. 

 Certain modifications of this method are of sufficient 

 value to justify mention. Most important is the neu- 

 tralization. 



In the exhaustive paper of Fuller l on the question of 

 reaction it was shown that the results obtained by tritating 

 the same culture-medium with the same alkaline solu- 

 tion differed very markedly with the indicator employed. 

 For instance, a litre of ordinary meat-infusion nutrient 

 agar-agar required 47 c.c. of a normal caustic alkali 

 solution to neutralize it when phenolphtalein was the 

 indicator used, 28 c.c. when blue litmus was employed, 

 and 5 c.c. when rosolic acid was substituted. It is 

 manifest from this that the actual reactions of media, 

 in the neutralization of which different indicators have 

 been used, may differ very widely from one another, 

 and that the results of cultivation on a medium neu- 

 tralized by one method are not fairly comparable with 

 those obtained when another indicator has been used. 

 For the sake of uniformity Fuller suggests that bac- 

 teriologists should agree upon some one trustworthy 

 method of neutralization and employ it to the exclusion 

 of other methods. He recommends, as the procedure 

 that has given the most satisfactory results in his hands, 

 a modification of Schultz's method, viz., 5 c.c. of the 

 culture-medium are to be mixed with 45 c.c. of distilled 

 water in a porcelain evaporating-dish and boiled for 

 three minutes, after which 1 c.c. of phenolphtalein 

 solution 2 is added and tit-ration with the one-twentieth 

 normal caustic alkali solution is quickly made. The 



1 Fuller : " On the Proper Reaction of Nutrient Media for Bacterial 

 Cultivation," Public Health (Journal of the American Public Health 

 Association), Quarterly Series, 1895, vol. i. p. 381. 



2 A 0.5 per cent, solution of the powder in 50 per cent, alcohol. 



