SPECIAL MEDIA. 105 



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 prep- 

 arations before using them, by means of the reaction 

 that they exhibit when acted upon by Fehling's alka- 

 line 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 precipitate, 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 (partic- 

 ularly the latter) should be relatively dilute in order 

 for the reaction to be successful. 



Peptone rosolic acid solution. Peptone solution con- 

 taining rosolic acid serves well for the detection of alter- 

 ations in reaction. It consists of the peptone solution 

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

 following solution are added : 



Rosolic 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 disappears 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 



