TECHNIC BACTERIOLOGICAL EXAMINATION OF SOILS. 



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followed in making the 1-400 bacterial dilutions and the bottles containing 

 1-40 bacterial dilutions are taken away. The 1-400 dilutions of the bacteria 

 in the field soil are shaken, in order, until each bottle has been shaken the 

 10 ten second periods. Following the same teehnic we make 1-4,000, 1-40,000 

 and 1-400,000 bacterial dilutions. The 1-40,000 and 1-400,000 bacterial 

 dilutions are retained for plating while all other dilutions are discarded. 

 The 1-400,000 bacterial dilutions are shaken exactly as though dilution were 

 to be made from them. 



Plate IV. 

 Ready to pour plates. 



Plating 



Thirty-six petri dishes are taken from an oven and spread out on the 

 laboratory table. Care is taken to prevent contamination from the lifting 

 up or sliding of the lids of the dishes. The dishes are arranged in rows of 

 three. All dishes are labeled and numbered, each set of three being num- 

 bered to correspond to one of the soil samples. The six dishes in excess of the 

 thirty actually needed for plating one dilution of the bacteria in the ten 

 samples of soil are utilized as follows; three for plating the water that has 

 been run as a check on the teehnic and three to plate the media alone. This 

 enables the investigator to trace contamination to the water, air, or media, 

 and classify the contamination accordingly. (Plate IV.) 



One of the dilution bottles is taken, the cotton plug flamed, a one cc. 

 pipette taken from a test tube case, the bottle shaken, the plug flamed and 



