BACTERIOLOGIC EXAMINATION OF WATER. 145 



an average estimation of a number of squares ruled upon 

 a glass background (Fig. 40), as suggested by Wolf hiigel, 

 is most convenient. In his apparatus a large plate of glass 

 is divided into small square di- 

 visions, the diagonals being spe- 

 cially indicated by color. The 

 plate or Petri dish is stood upon 

 the glass, and the number of 

 colonies in a number of small 

 squares is easily counted, and 

 the total number of colonies es- 

 timated. In counting the colo- 

 nies a lens is indispensable. 

 Special apparatuses have been 

 devised for counting the colo- 

 nies in Petri dishes (Fig. 4 l) , FlG * 42-Esmarch's instrument 

 ^ ~ ' for counting colonies of bactena 



and in Ksmarch tubes (Fig. 42). - m tUDe s. 



The majority of the water- 

 bacteria are rapid liquefiers of gelatin, for which reason 

 it seems better to employ agar-agar than gelatin for 

 making the cultures. 



In ordinary hydrant-water the bacteria number from 

 2-50 per cubic centimeter ; in good pump- water, 100-500 ; 

 in filtered water from rivers, according to Gunther, 50-200 

 are present ; in unfiltered river- water, 6000-20,000. Ac- 

 cording to the pollution of the water the number may 

 reach as many as 50,000,000. 



The waters of wells and springs are dependent for their 

 purity upon the character of the earth or rock through 

 which they filter, and the waters of deep wells are much 

 more pure than those of shallow wells, unless contamina- 

 tion takes place from the surface of the ground. 



Ice always contains bacteria if the water contained 

 them before it was frozen. In Hudson-River ice Prud- 

 den found an average of 398 colonies in a cubic centi- 

 meter. 



A sample of water when collected for examination 

 should be placed in a clean sterile bottle or in a her- 

 10 



