118 BACTERIOLOGY. 



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. 



LITMUS-WHEY-MILK. An important differential 

 medium is milk-whey to which litmus tincture has been 

 added. A number of methods for its preparation are 

 in use, but the one employed by Durham seems to be 

 the most satisfactory. Briefly it is as follows : fresh 

 milk, free from antiseptic adulterations, is gently warmed 

 and clotted with essence of rennet. The whey is strained 

 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; 



