104 BACTERIOLOGY. 



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

 the disadvantage of not being transparent, and can 

 therefore 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 after pre- 

 cipitation of the casein. 



Dunham's peptone solution. The medium usually 

 known as Dunham's solution is prepared according to 

 the following formula: 



Dried peptone 1 part. 



Sodium chloride 0.5 " 



Distilled water 100 parts. 



It is usually of a neutral or slightly alkaline reac- 

 tion, and neutralization is not, therefore, necessary. 

 It is filtered, decanted into tubes or flasks, and ster- 

 ilized in the steam sterilizer in the ordinary way. 

 The most common use to which this solution is put 

 is in determining if the organism under considera- 

 tion possesses the property of producing indol as one 

 of its products of nutrition. It is essential for accu- 

 racy that the preparation of dried peptone employed 

 should be of as nearly chemical purity as is possi- 

 ble, and indeed the other ingredients should be 

 correspondingly free from impurities. Gorini {Central- 

 blait fur Bakteriologie und Parasitenkunde, 1893, Bd. 

 xiii. p. 790) calls attention to the fact that impurities 

 in the peptone, particularly the presence of carbohy- 

 drates, so interfere with the production of indol by 

 certain bacteria that otherwise produce it, that it is 

 ofttimes impossible, when such preparations have been 

 employed, to obtain the characteristic color-reaction of 



