92 SOIL BACTERIOLOGY 



impurities. When this coagulum has settled to the bottom, 

 pour the cleared gelatin through the filter. 



6. If properly prepared, gelatin may be filtered through 

 filter-paper. Otherwise it will be necessary to use an ab- 

 sorbent cotton filter. 



Note. A cotton filter is prepared as follows: In the base of a large funnel 

 place a small amount of clean excelsior. In place of the excelsior a small 

 spiral of copper wire may be used. On top of this put two or three layers of 

 absorbent cotton. Split a piece of absorbent cotton, somewhat larger 

 than the top of the funnel, horizontally into two layers of equal thickness. 

 Place one layer of cotton above the other, so that the fibers are at right 

 angles. Pour the medium, slowly at first, on to the filter. (In order to 

 avoid breaking the filter use a glass rod to direct the fluid to the center of 

 the filter.) When the filtrate begins to come through the cotton, fill the 

 funnel. If the first filtrate is not clear, the turbid liquid should be refiltered 

 through the same cotton. 



7. Sterilize in the autoclave for ten minutes at 120 C. 



8. As soon as removed from the autoclave stand in cold 

 or ice-water. Gelatin is easily decomposed, and if heated, 

 too high or too long will not solidify. 



Medium 3 



Nutrient Agar 



Agar 15 g m . 



Liebig's meat extract 3 gm. 



Peptone 10 gm. 



Distilled water 1000 c.c. 



1. In a vessel containing 1000 c.c. of water add 15 grams 

 of thread agar. 



2. Heat in the steamer or double boiler until the agar is 

 dissolved. This requires at least one hour. 



3. Add 3 grams extract of meat and 10 grams of peptone. 



