

SOLID TRANSPARENT MEDIA. 55 



bottom, or a little white sediment remain encysted in the centre ; 

 this cannot easily be avoided, nor does it form any serious obstacle. 



The crude agar should first be rinsed in water, and then in 

 5fc acetic acid and clear water again, to rid it of impurities. 

 If agar is boiled thoroughly over a hot flame or in an auto- 

 clave, it can be filtered much more readily. The main point 

 is to see that all the agar is dissolved. 



It has been suggested to pour the hot agar into high cylin- 

 drical glass vessels and allow it to cool slowly in the steam 

 oven, the flame having been gradually lowered and then turned 

 out. After a time the cloudy portion will form a sediment at 

 the bottom ; the agar can then be shaken out as a long cylinder 

 and the cloudy portion cut off. 



The Japanese Method. Yokote prepares agar as follows : the 

 meat is cooked in water over a sand bath 1 hours. Filtered, 

 chopped agar is then added and the mixture cooked 1 hour 

 longer ; peptone and salt added next. Neutralization. After the 

 mixture has cooled to about 50 C. whites of 2 eggs are added 

 and the mixture shaken thoroughly. 



Again the mixture is placed on the sand-bath and heated to 

 110 C. and over for lj to 2 hours, and then filtered through 

 ordinary filter-paper. Yokote claims that by this procedure 

 the agar can be filtered as easily as bouillon and without any 

 loss. (The water evaporated in boiling must be added before 

 filtering.) 



Glycerine Agar. The addition of 4 to 6% of glycerine to 

 nutrient agar greatly enhances its value as a culture medium. 



Gelatine-Agar. A mixture of 5 per cent, gelatine and 0.75 

 per cent, agar combines in it some of the virtues of both agents. 



Blood Serum. Blood serum being rich in albumen coagulates 

 very easily at 70 C., and if this temperature is not exceeded, 

 a transparent, solid substance is obtained upon which the ma- 

 jority of bacteria develop, and some with preference. 



Preparation of Nutrient Blood Serum. If the slaughter of 

 the animal can be supervised, it were best to have the site of 

 the wound and the knife sterilized, and sterile flasks to receive 

 the blood directly as it flows. 



It is placed on ice forty-eight hours, and the serum is drawn 



