SPECIAL MEDIA. 93 



by the addition to it of gelatin or agar-agar in the pro- 

 portions given for the preparation of the ordinary nutri- 

 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 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 reaction, 

 and neutralization is not, therefore, necessary. It is 

 filtered, decanted into tubes or flasks, and sterilized in 

 the steam sterilizer in the ordinary way. The most 

 common use to which this solution is put is in deter- 

 mining if the organism under consideration possesses 

 the property of producing indol as one of its products 

 of nutrition. It is essential for accuracy that the prep- 

 aration of dried peptone employed should be of as 

 nearly chemical purity as is possible, and indeed the 

 other ingredients should be correspondingly free from 

 impurities. Gorini [Centralblatt fur Bakteriologie und 

 Parasitenkande, 1893, Bd. xiii. p. 790) calls attention 

 to the fact that impurities in the peptone, particularly 

 the presence of carbohydrates, so interfere with the 

 production of indol by certain bacteria that otherwise 

 produce it that it is ofttimes impossible, when such prep- 



5* 



