l6o SOIL BACTERIOLOGY 



and stopper, serve io remove any micro-organisms from the 

 air. If a liquid medium is used, the aeration tubes A and 

 B may be omitted. A' and B' are prepared in the same 

 way as A and B. The tubes A', B', D, and C are used to 

 carry water over from the flask to Woulfe's bottle. 



Prior to filling the Woulfe bottle with soil wrap some 

 glass-wool around the end of tube B, and cover the bottom 

 of the bottle with an inch layer of gravel. Now add the 

 soil and raise the water content to about half -saturation. 

 The special glass cylinder E which fits loosely around the 

 middle neck of Woulfe's bottle consists of a large glass 

 cylinder 4 to 5 cm. in diameter and 15 cm. in length. With- 

 in this cylinder there is a glass tube about ii to ij cm. 

 in diameter and 20 cm. long, which reaches to the wire net 

 (see Fig. 13). The top of the glass tube carries a cotton 

 plug. Between the two cylinders there is loose cotton 

 packing and three glass rods about 0.4 cm. in diameter and 

 15 cm. long (see Fig. 13). 



The entire apparatus should be connected as shown in 

 Fig. 12; the flask filled almost full of distilled water, a 

 screw-clamp fastened between C and D in order to prevent 

 the water in the flask from flowing over into the bottle, and 

 sterilized for two hours at 15 pounds' steam pressure for 

 two consecutive days. If carefully wrapped in paper, 

 heated and cooled slowly, there is not much danger of 

 cracking the glass. When cool, seal all stoppers with a 

 beeswax-rosin mixture. 



In view of the long time required for plant growth and 

 the danger of infection, it is well to keep the cultures in a 

 clean room as free from contamination as possible. 



To plant, remove the cotton plug from the inner glass 

 cylinder or tube (see Fig. 13), and by means of sterilized 

 forceps drop the seed down the inner tube on the wire net. 



