56 ESSENTIALS OF BACTERIOLOGY 



with hot-water filter, and very small quantities passed through 

 at a time, changing the filter-paper often. 



Cotton can be used instead of filter-paper, or filtering entirely 

 dispensed with, simply decanting. 



As agar is seldom clear, a little more or less opaqueness is 

 permissible. The test-tubes are filled as with the gelatin, and 

 sterilized in the same manner. While cooling, some of the 

 tubes can be placed in a slanting position, so as to obtain a larger 

 surface to work upon. 



Water of condensation will usually separate and settle at the 

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

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

 obstacle. 



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

 5 per cent, acetic acid and clear water again, to rid it of impuri- 

 ties. If agar is boiled thoroughly over a hot flame or in an 

 autoclave, 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- 

 dric 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 one and one -half 

 hours. Filtered, chopped agar is then added, and the mixture 

 cooked one hour longer; peptone and salt added next. Neutral- 

 ization. After the mixture has cooled to about 50 C., whites 

 of two 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 one and one-half to two 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. (Water must be added before filtering 

 to supply loss from evaporation.) 



The agar may be boiled separately in some of the water 



