136 EXAMINATION OF AIR, WATER, AND SOIL. 



disk and the colonies are counted as they lie between the 

 white lines. 



Ordinary hydrant- water usually contains from 2 to 50 bac- 

 teria per cubic centimeter ; filtered river- water, from 50 to 

 200; unfiltered river-water, from 6000 to 20,000; ground- 

 water may contain as many as 130,000. 



An Esmarch roll culture may also be used, but is most ser- 

 viceable \vhen the sample of water contains but few bacteria. 



Of ^^ 



8 

 Pakes's apparatus for counting colonies (reduced one-third). (Abbott.) 



The surface of the tube (Fig. 47) is divided into squares, and 

 the colonies counted with a hand lens and estimated as in the 

 plate culture. 



Inasmuch as all water contains liquefying bacteria, cult- 

 ures are made on both gelatin and agar. the Bacillus coli 

 communis is frequently found in sewage-water and in the 

 water into which sewage empties. It is detected readily 



