94 BACTERIOLOGY. 



arations have been employed, to obtain the characteristic 

 color reaction of this body, and where it is obtained it 

 is always after a much longer time than is the case 

 where peptone free from these substances has been used. 

 He suggests the advisability of testing the purity of all 

 peptone preparations before using them, by means of 

 the reaction that they exhibit when acted upon by 

 Fehling's alkaline copper solution. Under the influence 

 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 

 experience 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 to 

 which rosalic acid is added serves well for the detection 

 of alterations in reaction. It consists of the peptone 

 solution of Dunham, to each 100 c.c. of which 2 c.c. of 

 the following solution is added : 



Rosalie acid (coralline) .... .0.5 gramme. 

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



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

 sterilized tesi^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 



