130 PATHOGENIC BACTERIA. 



Experience shows that 1000 c.cm. of agar-agar rarely go 

 through one paper, and I always expect when beginning 

 the filtration to be compelled to boil the material which 

 remains on the paper again, and pour it through a new 

 filter. 



The formerly much-employed hot-water and gas-jet 

 filters seem unnecessary. If properly prepared, the whole 

 quantity will filter in from fifteen to thirty minutes. 



If made from beef-extract, the agar-agar almost always 

 precipitates a considerable amount of meat-salts as it 

 cools. This should be anticipated, but, so far as I can 

 determine, cannot always be prevented. The amount is 

 certainly lessened by making the bouillon first, filtering 

 it cold, then adding the agar-agar, and dissolving and 

 filtering it. 



The difficulty of filtering the agar-agar has led Fliigge 

 and others to adopt a method of sedimentation. An in- 

 genious apparatus for this purpose has lately been devised 

 by Bleisch. The methods can be simplified by using a 

 small pharmaceutical percolator, the bottom of which is 

 closed by a rubber cork containing a tube which extends 

 nearly to the top of the percolator and is attached to 

 a rubber tube with a pinchcock below. The melted agar- 

 agar is poured into this, and kept in the steam apparatus 

 until the sedimentation is sufficient to allow clear fluid to 

 be drawn from the top. As the clear agar-agar is drawn 

 off the tube is pulled down through the rubber cork, and 

 more drawn off until only the sediment is left. 



Agar-agar is dispensed in tubes like the gelatin and 

 bouillon, sterilized by steam by the intermittent process, 

 and after the last sterilization, before cooling, each tube 

 is inclined against a slight elevation, so as to offer an ex- 

 tensive flat surface for the culture. 



After the agar-agar jelly solidifies its contraction causes 

 some water to collect at the lower part of the tube. This 

 should not be removed, as it keeps the material moist, 

 and also because it has a distinct influence upon the cha- 

 racter of the growth of the bacteria. 



