SPECIAL MEDIA. 101 



of this agent, pure peptone in solution gives a violet 

 color (the biuret reaction), which remains permanent 

 even after boiling for five minutes. If instead of a 

 violet color there appears a red or reddish-yellow pre- 

 cipitate the peptone should be discarded, as in his ex- 

 perience no indol is produced from peptone giving this 

 reaction. Both the peptone solution and that of the 

 copper (particularly the latter) should be relatively 

 dilute in order for the reaction to be successful. 



Peptone rosalic acid solution. Peptone solution con- 

 taining rosalic acid serves well for the detection of altera- 

 tions in reaction. It consists of the peptone solution 

 of Dunham, to each 100 c.c. of which 2 c.c. of the fol- 

 lowing solution is added : 



Rosalie acid (coralline) 0.5 gramme. 



Alcohol (80 per cent.) 100 c.c. 



This is to be boiled, filtered, and decanted into clean, 

 sterilized test-tubes, about 8 to 10 c.c. to each tube. The 

 tubes are then to be sterilized in the usual way by steam. 

 When sterilization is completed and the tubes cooled, the 

 solution will be of a very pale rose color, which disap- 

 pears entirely under the action of acids, and becomes 

 much more intense when alkalies are produced. We 

 have used this solution for some time for the study of 

 the reactions produced by different organisms, and find 

 it a valuable addition to our means of differentiating 

 bacteria. 



Rosalic acid cannot be used with safety in solutions 

 containing glucose, as the reducing action of the latter 

 deprives it of its color. 



Lactose-litmus-agar, or gelatin of Wurtz. A medium 

 of much use in the differentiation of bacteria is that 



