120 BACTERIOLOGY. 



the disadvantage of not being transparent, and can 

 therefore 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 after pre- 

 cipitation of the casein. 



Litmus-milk-whey. 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 sat- 

 isfactory. 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 ta 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 



