EXAMINATION OF SOIL 173 



5. Transfer some of the colonies to slanted agar 

 prepared as described on p. 38. 



6. Study the colonies by stains, hanging drop, etc., 

 and identify the organism. 



Experiment 2. — Silica jelly method — 



1. Prepare some tubes with silica jelly as described 

 on p. 38. 



2. Melt these tubes, inoculate several with a soil 

 suspension, others with the contents of the flasks 

 prepared in Exercise i, others with colonies obtained 

 by the washed agar method. 



3. Incubate the plates at room temperature and 

 study the colbnies in the usual manner. 



Collodion sacs for the dialysis of silica are made in 

 the following manner: 



Depending on the size desired a culture tube or an 

 Erlenmeyer flask or any other suitable glass vessel can 

 be used. The finished sac is always much smaller 

 than the vessel used and this shrinkage must be con- 

 sidered in choosing the right size of vessel. Celloidin 

 is dissolved in a mixture consisting half of ether and 

 half of alcohol in such quantity that the solution con- 

 tains 6 per cent celloidin. Celloidin usually is covered 

 with water. This must be washed off with alcohol 

 before the celloidin is dissolved. A quantity of the 

 celloidin solution is then poured into the flask and the 

 flask rotated until the whole inside surface is covered 

 with the solution. The flask is then inverted and the 

 celloidin allowed to drain off so that a thin coat re- 

 mains on the sides of the flask. The flask must then 

 be rotated and air blown into it until the celloidin is 

 almost dry. The process is then repeated so that the 



