100 BACTERIOLOGY. 



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 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 Balderiologie und 

 Parasitenkunde, 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- 

 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 



