WATER. 397 



permitted to escape. Of the specimen obtained, i, j, and 

 y cu. cm. are removed by means of sterilized pipets, and 

 introduced into liquefied gelatin, and plates are made. 

 Greatly polluted water should be diluted with sterilized 

 water from another source, and no more than -fa, yi^, and 

 ToVo cu. cm. should be employed. Before introducing the 

 pipets the specimen is vigorously agitated, as the bacteria, 

 by reason of their weight, quickly settle to the bottom. In 

 the process of water-examination Koch's old method of 

 making plates is .still frequently followed, because these 

 permit better than the Petri dishes the distribution of the 

 gelatin in as uniform a layer as possible. When the plates 

 have developed, the colonies are examined and counted, if 

 present in large number by means of a special enumerator 

 (a glass plate with etched squares). An estimate is made 

 of the number of germs to the cubic centimeter of water, 

 and the number as well as the identity of the varieties 

 present is determined. 



The Number of Bacteria Present in Water. Spring- 

 water, as well as subsoil-water, is free from germs at the 

 point where it escapes from the earth. It has been men- 

 tioned that the earth at the level of the subsoil-water no 

 longer contains bacteria. According to the most trust- 

 worthy investigations, pure tap-water and spring-water con- 

 tain, on the average, from 2 to 50 bacteria per cubic centi- 

 meter ; pure pump-water, from 100 or 200 to 500 germs ; 

 unfiltered water from streams kept unpolluted, from 6000 to 

 20,000 ; filtered river-water, from 50 to 200 ; polluted 

 wells, as many as 100,000 germs ; and river tap-water, 

 when the filtering apparatus is out of order, the same num- 

 ber. Drain-water and greatly contaminated streams contain 

 from 2,000,000 to 40,000,000 germs to the cubic centimeter 

 (Flugge). In the summer and after a heavy rainfall the 

 number of bacteria in water is increased. 



The Bacteria of Water. The microorganisms living in 

 water are mainly bacilli. A large number of them liquefy 

 gelatin ; others generate offensive gases, and still others 

 beautiful pigments. Of great interest are the so-called 

 typhoid-like water-bacteria (p. 402) and the water-vibrios, 

 which in a number of points resemble the comma-bacilli 

 of Asiatic cholera (p. 404). 



Among pathogenic bacteria typhoid-bacilli and cholera- 

 vibrios have been repeatedly found in water. The method 



