CULTIVATION OF BACTERIA. 135 



adding a few drops of clean water to each tube before 

 sterilizing. It is not necessary to have a special small 

 chamber blown in the tube to contain this water ; only 

 a small quantity need be added, and this will not touch 

 the potato, which does not reach the bottom of the 

 rounded tube. 



A potato-juice has also been suggested, and is of some 

 value. It is made thus : To 300 c.cm. of water 100 grams 

 of grated potato are added, and allowed to stand on ice 

 over night. Of the pulp 300 c. cm. are expressed through 

 a cloth and cooked for an hour on a water-bath. After 

 cooking, the liquid is filtered and receives 4 per cent, of 

 glycerin. It may or may not need neutralization. Upon 

 this medium the tubercle bacillus grows well, especially 

 when the reaction of the medium is acid, but loses its 

 virulence. 



Milk. Milk is useful as a culture-medium. As when 

 the milk stands the cream which rises to the top is a 

 source of inconvenience, it is best to secure from a dairy 

 fresh milk from which the cream has been removed by 

 a centrifugal machine. It is placed in sterile tubes and 

 sterilized by steam by the intermittent method. The 

 opaque nature of this culture-medium often permits the 

 undetected development of contaminating organisms. 

 A careful watch should therefore be kept upon it 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. Cow's milk is inclined to 

 be acid in reaction, and a small amount of sodium car- 

 bonate may be necessary to give it a distinct blue. The 

 use of litmus is probably the best method of determining 

 whether bacteria by their growth produce acids or alka- 

 lies. 



The watery solution of litmus, being a vegetable in- 

 fusion, is likely to spoil; hence it should always be treated 

 like the culture-media and sterilized by steam every time 

 the receptacle in which it is kept is opened. 



