CULTURE-MEDIA. 67 



About 7 to 10 c.c. are to be run into each test-tube. The tube 

 is plugged with cotton, and sterilized as usual. When milk is 

 contaminated with spores of the hay or potato bacillus it is 

 sometimes very difficult to sterilize, a fact of much importance 

 in connection with the feeding of children, where the fractional 

 method of sterilization and the use of the autoclave are im- 

 practicable. 



The coagulation of milk, which is accompHshed by certain 

 bacteria, is a very valuable diEferential point. A little litmus 

 tincture may be added to the tubes of milk before sterilizing, 

 until they acquire a blue color, to indicate whether or not acids 

 are formed by the bacteria which are afterward cultivated in 

 the milk. 



Dunham's Peptone Solution. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Water i liter. 



Boil, filter, sterilize in tlie usual manner. 



Dunham's solution is valuable to test the development of 

 indol by bacteria (see Part II., Chapter II.). The development 

 of acids may be detected after the addition of 2 per cent, of 

 rosoHc acid solution (0.5 per cent. solution in alcohol); alkaline 

 solutions give a clear rose-color which disappears in the presence 

 of acids. 



Blood-serum. — The blood of the ox or cow may be ob- 

 tained easily at the abattoir. It should be collected in a clean 

 jar. When it has coagulated, the clot should be separated 

 from the sides of the jar with a glass rod. It may be left on the 

 ice for from twenty-four to forty-eight hours. At the end of 

 that time the serum will have separated from the clot and may 

 be drawn off with a siphon into tubes. These tubes are ster- 

 ilized for the first time in a slanting position, as the first steril- 

 ization coagulates the serum. The coagulation may be done 

 advantageously, as advised by Councilman and MaUory, in the 



