SPECIAL MEDIA. 119 



and for observing the variations of this power in closely 

 allied though not identical species. 



DUNHAM'S PEPTONE SOLUTION. The medium known 

 as Dunham's solution is prepared according to the follow- 

 ing formula : 



Dried peptone . 1 par t. 



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 metabolic products. It is essential for accu- 

 racy that the preparation of dried peptone employed 

 should be as nearly chemically pure as is possible, 

 and indeed the other ingredients should be corre- 

 spondingly free from impurities. Gorini 1 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, under such cir- 

 cumstances, 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. 



Peckham has also demonstrated that where bacteria 

 have the property of forming indol and also of fer- 



i Gorini : Centralblatt fur Bakteriologie und Parasitenkunde, 1893, 

 vol. xiii. p. 790. 



