no Journal of Agricultural Research voi.x.No.3 



Observations were also made with a strain of the anthrax bacillus fur- 

 nished by Dr. R. A. Kelser, of the Bureau of Animal Industry, and a 

 strain of B. abortus given by Miss Alice C. Evans, of the Dairy Division. 



Observations were made at 20, 26, 48, and 72 hours after inoculation 

 and from then on at various periods for a month. Incubation tempera- 

 tures were appropriate to the organisms studied. 



It is almost impossible to describe the color of indicator solutions by 

 means of Ridgway's color charts,^ and there are no standards which may 

 be used satisfactorily with milk for determining colorimetrically Pg 

 values. The writers must therefore be content with sa>ing that no change 

 was observed in the litmus-milk cultures which could not be seen so well 

 with milk colored with bromcresol purple. In a few instances a more 

 rapid change was observed in one case than in another, but this may be 

 ascribed either to the peculiarity of an individual culture or to a more 

 favorable initial Pg in the one case or the other. 



A noteworthy example of the higher value of bromcresol purple was 

 observ^ed when comparing cultures of B. coli, streptococci, and B. hul- 

 garicus. As the senior writer noted in a former paper,^ B. coli cultures do 

 not attain the same hydrogen-ion concentration in milk that they attain in 

 other media. The writers now observe that even when they bring milk to 

 the point of coagulation the bromcresol purple is left with a tinge of glau- 

 cous gray. Streptococcus cultures which in milk arrive at a higher 

 hydrogen-ion concentration give to the bromcresol purple a clear cream 

 color. Cultures of B. hulgaricus produce so much higher reaction that 

 the cream color of a streptococcus culture gives place to a maize-yellow.^ 

 So beautiful a graduation of color change is entirely lost in litmus 

 cultures. 



In those cases in which a digestion of the milk occurs there is a very 

 marked change in the quality of the color, owing to the fact that as 

 turpidity is removed greater depth of the solution is observed, and, in- 

 stead of the transmitted blue of the indicator being dominant, as it is 

 in thin layers of solution, the transmitted red becomes more noticeable. 

 This change may prove confusing to one who is unfamiliar with this 

 indicator, but one who is familiar with its colors in various solutions and 

 in the colorless standards of known Pg may still follow approximately 

 the degree of alkali or acid production. 



The most noteworthy advantage of bromcresol purple observed in this 

 series of comparative tests with litmus was found to be the resistance of 

 the new indicator to reduction. In very few instances was any serious 

 reduction or destruction detected. While litmus seemed to be decom- 

 posed in a variety of ways with the production of mere muddy colors, in 

 many instances bromcresol purple continued to indicate changes in Ph, 



1 RiDGWAY, Robert. Op. cit. 



2 Clark, W. M. the pinai. hydrogen-ion concentration op cultures of bacillus coli. In Jour. 

 Biol. Chem., v. 22, no. i, p. 87-9S, i fig. 1913. 



^ RiDGWAY, Robert. Op. cit., pi. 4. 



