CULTURE-MEDIA. 79 



The coagulation of milk, which is accomplished by cer- 

 tain bacteria, is a very valuable differential 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 

 afterwards cultivated in the milk. 



Dunham's Peptone Solution. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Water i liter. 



Boil, filter, sterilize in the usual manner. 



Dunham's solution is valuable to test the development of 

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

 ment of acids may be detected after the addition of 2 per 

 cent, of rosolic acid solution (.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 sepa- 

 rated from the clot and may be drawn off with a siphon 

 into tubes. These tubes are sterilized for the first time in 

 a slanting position as the first sterilization coagulates the 

 serum. The coagulation may be done advantageously, as 

 advised by Councilman and Mallory, in the hot-air sterilizer 

 at a temperature below the boiling-point. After coagulation, 

 sterilize as usual. This serum makes an opaque medium of 

 a cream color. Blood-serum may be sterilized in the special 

 form of sterilizer devised for it. A clear blood-serum is to 

 be obtained by sterilization at a temperature of 58 C. for 

 one hour, on each of six days, if a fluid medium is desired, 



