98 BACTERIOLOGY. 



sealed at the point and plugged with cotton can be kept 

 on hand just as are sterilized test-tubes. 



It is sometimes desirable to preserve blood-serum in a 

 fluid state. This can be done by the fractional method 

 of sterilization at low temperatures, already described, 

 or with much less effort, and without the use of heat by 

 a method that we have found to be very satisfactory. 

 In the studies of Kirchner, chloroform was shown to 

 possess decided disinfectant properties ; as it is quite 

 volatile it is easily removed when its disinfectant or an- 

 tiseptic functions are no longer required. If, therefore, 

 the serum to be preserved be placed in a closely- 

 stoppered flask and enough chloroform added to form a 

 thin layer, about 2 mm. on the bottom, the serum may 

 be kept indefinitely without contamination, so long as 

 the chloroform is not permitted to evaporate. When 

 required for use the serum is decanted into test-tubes, 

 which are then placed in a water-bath at about 50 C. 

 until all the chloroform has been driven off; this can be ' 

 determined by the disappearance of its characteristic 

 odor. The serum may then be solidified, sterilized by 

 heat, and employed for culture purposes. We have 

 found serum so preserved to answer all requirements 

 as a culture medium. 



SPECIAL MEDIA. The media just described bou- 

 illon, nutrient gelatin, nutrient agar-agar, potato, and 

 blood-serum are those in general use in the laboratory 

 for purposes of isolation and study of the ordinary forms 

 of bacteria. For the finer points of differentiation 

 special media have been suggested ; a few of them will 

 be mentioned. 



Milk. Fresh milk should be allowed to stand over 

 night in the ice-chest, the cream then removed, and the 



