198 PATHOGENIC BACTERIA. 



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

 the potato, which does not reach the bottom of the 

 rounded tube. 



Potatoes differ considerably in reaction, and so give us 

 very variable results. If the work done is to be accu- 

 rate, it may be necessary to correct this reaction if the 

 acids have not been sufficiently removed by the washing 

 in running water already described. 



To do this the cut cylinders are placed in a measured 

 quantity of distilled water and steamed for about an hour. 

 The reaction of the water is then determined by titration 

 and the desired amount of sodium hydroxid added, after 

 which the potatoes are again steamed in the corrected 

 solution for about thirty minutes, and then placed in 

 tubes. 



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, titrated if desired, 

 and receives 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 useful as a culture-medium. When the 

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

 of inconvenience, so that it is best to secure from a dairy 

 fresh milk from which the cream has been removed by 

 a centrifugal machine. It is given the desired degree of 

 alkalinity, placed in sterile tubes, and sterilized by steam 

 by the intermittent method. The opaque nature of this 

 culture-medium often permits the undetected develop- 

 ment 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- 



