Petruschky's Whey 187 



liquid is filtered, titrated if desired, and receives an addition 

 of 4 per cent, of glycerin. Upon this medium the tubercle 

 bacillus grows well, especially when the reaction of the 

 medium is acid. 



Milk.- Milk is a useful culture medium. As the cream 

 which rises to the top is a source of inconvenience, it is best 

 to secure fresh milk from which the cream has been removed 

 by a centrifugal machine. It is given the desired degree of 

 alkalinity by titration, dispensed in sterile tubes, and ster- 

 ilized by steam by the intermittent method or in the auto- 

 clave. The opaque nature of this culture medium often 

 permits the undetected development of contaminating or- 

 ganisms. A careful watch should therefore be kept lest it 

 spoil. 



Litmus Milk. This is milk to which just enough of a 

 saturated watery solution of pulverized litmus is added to 

 give a distinct blue color after titration. Litmus milk is 

 probably the best reagent for determining acid and alkali 

 production by bacteria. 



The watery solution of litmus, being a vegetable infusion, 

 is likely to be spoiled by micro-organismal growth, hence 

 must be treated like the culture media and sterilized by 

 steam every time the receptacle in which it is kept is 

 opened. 



If litmus be added to the milk before sterilization, it is apt 

 to be browned or decolorized, so that it is better to sterilize 

 the two separately and pour them together subsequently. 

 It is said that lacmoid is never thus changed, and many 

 workers prefer it to litmus on that account. 



Petruschky's Whey. In order to differentiate between 

 acid and alkali producers among the bacteria, Petruschky 

 has recommended a neutral whey colored with litmus. It is 

 made as follows: 



To a liter of fresh skimmed milk i liter of water is added. 

 The mixture is violently shaken. About 10 c.c. are taken 

 out as a sample to determine how much hydrochloric acid 

 must be added to produce coagulation of the milk, and, 

 having determined the least quantity required for the whole 

 bulk, it is added. After coagulation the whey is filtered off, 

 exactly neutralized, and boiled. After boiling it is found 

 clouded and acid in reaction. It is therefore filtered again, 

 and again neutralized. Litmus is finally added to the neu- 

 tral liquid, so that it has a violet color, changed to blue or 

 red by alkalies or acids. 



