SPECIAL MEDIA. 75 



ent gelatin or agar-agar. It has, however, in this form 

 the disadvantage of not being transparent, and can there- 

 fore best be used for the study of those organisms which 

 grow upon the surface of the medium without causing 

 liquefaction. 



Nutrient gelatin and agar-agar can also be prepared 

 from neutral milk whey, obtained from milk by pre- 

 cipitation of the casein. 



Dunham's solution and peptone-rosalic-acid solution. 

 Peptone solution, to which rosalic acid has been added, 

 also serves very well for the detection of alterations in 

 reaction. The peptone solution of Dunham is the form 

 that we have usually employed. It consists of 



Distilled water 100 parts. 



Dried peptone 1 part. 



Sodium chloride 0.5 " 



and 4 c.c. of the following solution : 



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 differentiation of 

 bacteria. 



Ldffler's blood-serum mixture. Loffler's blood-scrum 

 mixture consists of one part of neutral meat-infusion 



