7iTY J 



SPECIAL MEDIA. 133 



off and the clot hung up to drain in a piece of muslin. 

 The whey, which is somewhat turbid and yellow, is 

 then cautiously neutralized with a 4 per cent, citric acid, 

 solution, neutral litmus solution being used as the indi- 

 cator. It is then heated upon a water-bath to 100 C. 

 for about half an hour ; thereby nearly the whole of the 

 proteid is coagulated. It is then filtered clear and neu- 

 tral litmus solution is added until it is of a distinct pur- 

 ple color. If the filtered whey is cloudy, let it stand in 

 a cold place for a day or two and decant off the clear 

 supernatant portion or pass it through a Berkfeld filter. 

 The whey should never be heated above 100 C. or 

 neutralized with mineral acids, otherwise there is a 

 danger of so modifying the milk-sugar present as seri- 

 ously to impair the usefulness of the medium. When 

 properly prepared, the medium is free from proteid, and 

 contains only water, lactose, the salts of the milk, and 

 a small quantity of a body suggestive of dextrose or 

 galactose. The medium is of great utility in detecting 

 the power of bacteria to cause acid fermentation in a 

 non-proteid medium containing a fermentable sugar; 

 and for observing the variations of this power in closely 

 allied though not identical species. 



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- 



