1915] C. H. Crahill and H. S. Reed 43 



agar is then given triple sterilization in an Arnold sterilizer. Most 

 of tlie carbonate settles to the bottom of the tubes, but a small 

 amount of finely divided material remains in Suspension in the 

 agar and renders it distinctly turbid. After the final sterilization, 

 the snpernatant, turbid, agar is poured into sterile Petri dishes, care 

 being taken to prevent carbonate in the bottom of the test tube from 

 passing into the dishes. 



The turbid agar was inoculated with Bactcrinin lactis acidi and 

 placed in the incubator for three days. At the end of that time the 

 colonies showed distinct halos due to the solvent action of acids 

 upon the calcium carbonate. (Plate i, Fig. 6.) 



Another convenient method of demonstrating the presence of 

 acids is the familiär one of adding litmus Solution to medium con- 

 taining lactose. The method is so well known to all bacteriologists 

 that it is unnecessary to repeat it here. 



Another indicator which is useful in this connection is rosolic 

 acid. A few drops of 0.5 percent Solution are added to tubes of 

 sterile lactose-agar before melting and pouring it into Petri dishes. 

 The results are shown in Table 1 1 . 



TABLE II 



Data pcrtaining to cffccts of organisms upon the color of rosolic acid 



Organism After 13 days After 28 days 



Glomcrclla riifomaculans Deep pink Beginning to fade 



Fusarium lycopcrsici Unchanged Colorless 



Stcrigmatocystis niger Slight change Deep pink 



Bacillus subtilis No growth 



B. pyocyaneus No growth 



The alkaline conditions observed were probably due to the for- 

 mation of ammonia from protein constituents. 



Numerous other applications of these various methods will sug- 

 gest themselves to students of these problems. When only quali- 

 tative results are required, it is believed these methods will be found 

 highly satisfactory. An added advantage lies in the ease with 

 which permanent records may be made for the student's note book. 

 The Petri dishes may Ije set on Photographie or blue-print paper and 

 exposed to light. The paper, after development, may be inserted 

 in the note book as a part of the record. 



