CUL TIVA TION OF BA CTERIA. 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.