446 BACTERIA PATHOGENIC TO MAN 



the 1 c.c. samples. Even in twenty-one stagnant pools he only found 

 colon bacilli in five in the 1 c.c. samples. 



The experience of all who have practically studied the subject is 

 that in delicacy the colon test surpasses chemical analysis; in constancy 

 and definiteness it also excells the quantitative bacterial test. 



Interpretation of the Quantitative Analysis. The older experimenters 

 attempted to establish arbitrary standards by which the sanitary 

 quality of water could be fixed automatically by the number of germs 

 alone. This has been largely given up. Dr. Sternberg considers 

 that a water containing less than 100 bacteria is presumably from a 

 deep source and uncoritaminated by surface drainage; that one with 500 

 bacteria is open to suspicion; and that one with over 1000 bacteria is 

 presumably contaminated by sewage or surface drainage. Even this 

 conservative opinion must be applied with caution. The source of the 

 sample is of vital importance in the interpretation; thus, a bacterial 

 count which would condemn a spring or well might be normal for a 

 river. In woodland springs and lakes several hundred bacteria are 

 frequently found. In lakes the point at which the sample is taken 

 is of great importance, as the bacterial count varies with the distance 

 and with the depth. The weather also is an influence, since the wind 

 causes waves which stirs up the bottom mud. Rains greatly influence 

 streams by flooding them with surface water. The season of the year 

 is an important factor. The counts are highest in the winter and spring 

 months, and lower from April to September. 



The following figures illustrate this point: 



Water. Observer. Year. Jan. Feb. March. April. May. June. 



New York City tap-water. Houghton 1904 890 1100 650 240 350 370 



Boston " Whipple 1892 135 211 102 52 53 86 



Merrimac River " Clark 1899 4900 5900 6300 2900 1900 3500 



The spring increase is not an exception to the rule that high num- 

 bers indicate danger but an indication of its truth, for it means a melt- 

 ing of the snow and a flow of surface water into the streams. A number 

 of severe epidemics of typhoid fever have been produced in this way. 

 Although, as a rule, a series of tests are necessary to pass judgment on 

 a water, a single test may be very important. A large increase in the 

 number in tap-water a day after a storm points unerringly to surface 

 pollution, and if towns exist in the water-shed, to street and sewer 

 pollution. The Croton water frequently jumps from hundreds to 

 thousands after such a storm. 



In a typhoid epidemic at Newport, Winslow reports that a test of 

 the water supply showed but 334 bacteria per cubic centimetre, but 

 one from a well showed 6100. The suspicion aroused was justified by 

 finding all the typhoid cases had gotten water from this well. 



The study of the bacterial effluent from municipal water filters is 

 the only way in which the efficiency of the filter and the accidents 

 which occur can be determined. In Germany these regular tests are 

 obligatory. Elaborate studies have recently been made of the exact 



