306 Journal of Applied Microscopy. 



ever claimed that the end-points were alike ; on the contrary, the use of phe- 

 nolphtalein is recommended for the reasons given in the foregoing paragraph, 

 which would lose weight if the end-points were the same. 



Phenolphtalein is not the only indicator which shows a different end-point 

 from litmus ; not only do different lots of litmus differ from one another in this 

 respect, but all the customary indicators have different end-points according to 

 circumstances. For example, methyl orange is not affected by carbon dioxid, be- 

 cause it is itself a stronger acid and therefore is not affected by the weaker acid. 

 Phenolphtalein and turmeric are, by common consent, the best indicators for 

 weak acids, with which latter we have to deal in media preparation. (Journ. 

 Am. Pub. Health Assn., October, 1895, p. 389.) The following table illustrates 

 this point, showing the reaction of various media and of their ingredients to 

 different indicators : 



Media. 



Meat infusion, 



Merck's peptone, 



Witte's peptone, 



Sargent's peptone, 



Gelatin, 



Agar, 



Nutrient gelatin. 



Nutrient agar. 



The above was condensed from an article by G. W. Fuller in Journ. Am. 

 Pub. Health Assn., October, 1895, p. 389. 



A casual inspection of the above table will show that, of the two, the litmus 

 neutral end-point is much the more variable, and this variance is further in- 

 creased because of the impossibility of making and keeping litmus paper which 

 will give constant results. Therefore, when solutions of the character used are 

 brought to the litmus neutral point only a partial and extremely variable 

 neutralization is affected. 



Dr. Novy concludes his comparison of the two indicators by implying that it 

 is easier to determine the reaction of. media and to duplicate the same in suc- 

 cessive batches of media when using litmus, than when using phenolphtalein as 

 an indicator. 



The writer's experience in a number of sanitary laboratories leads him to 

 believe that if the recommendations of the Bacteriological Committee are 

 followed out, no difficulty is experienced, from a practical standpoint, in making 

 different lots of media of the same reaction. It seems therefore that the 

 evidence is easily in favor of the use of phenolphtalein as an indicator, since 

 the reaction passes beyond accurate control when litmus and the other acid 

 indicators are used. 



In the preparation of the specific media, Dr. Novy does not specify the 

 optimum reaction for general work. He overlooks also the advantage of having 

 the three principal artificial media analogous in the sense of being broth, broth 

 plus gelatin, and broth plus agar-agar. 



