EXAMINATION OF SOIL 167 



sterile water from one of the dilution flasks for one 

 minute. 



6. Add some more water, so that the final volume 

 is 100 c.c. The suspension should be perfectly uniform. 



7. Shake the suspension vigorously for three 

 minutes. 



8. Prepare dilutions from this suspension in the fol- 

 lowing manner: Add i c.c. suspension to a flask with 

 99 c.c. sterile water. Shake well. Remove 10 c.c. 

 water from another flask and add 10 c.c. of the dilu- 

 tion of 1 : 100 to this flask. This makes a dilution of 

 1:1,000. Then add i c.c. of the dilution of 1:100 to 

 another flask, making a dilution of i : 10,000. i c.c. 

 from the dilution i : 1,000 added to another flask makes 

 a dilution of i : 100,000. Finally add i c.c. of the dilu- 

 tion 1:10,000 to another flask, making a dilution of 

 1:1,000,000. 



9. Pour duphcate agar plates from all dilutions and 

 incubate for four days at room temperature. Plates 

 should be prepared in both agar and gelatin. 



10. Examine the gelatin plates daily, and count the 

 colonies earlier than four days if 'there are many 

 liquefiers present. 



11. After counting the colonies on both agar and 

 gelatin calculate the number present in one gram of 

 dry soil. 



12. Examine the colonies and determine as near as 

 possible the group to which they belong according to 

 Chester's Determinative Bacteriology. In most cases 

 stained preparations and hanging drops will give this 

 information. If not, cidtures on slant agar must be 

 prepared and the organisms studied in the usual 

 manner by making subcultures. 



