II. 



BACTERIA IN WATER. 



THE water of the ocean, of lakes, ponds, and running streams 

 necessarily contains bacteria, as they are constantly being carried 

 into it by currents of air passing over the neighboring land surfaces, 

 and by rain water which washes suspended microorganisms from 

 the atmosphere ; and, as such water contains more or less organic 

 material in solution, many of the saprophytic bacteria multiply in it 

 abundantly. It is only in the water of springs and wells which 

 comes from the deeper strata of the soil that they are absent. The 

 number and variety of species present in water from any given 

 source will depend upon conditions relating to the amount of organic 

 pabulum, the temperature, the depth of the water, the fact of its 

 being in motion or at rest, its pollution from various sources, etc. 

 The comparatively pure water of lakes and running streams contains 

 a considerable number of bacteria which find their normal habitat 

 in such waters and which multiply abundantly in them, notwith- 

 standing the small quantity of organic matter and salts which they 

 contain. The water of stagnant, shallow pools, and of sluggish 

 streams into which sewage is discharged, contains a far greater 

 number and a greater variety of species. 



The study of these bacteria in water has received much attention 

 on account of the sanitary questions involved, relating to the use of 

 water from various sources for drinking purposes. In the present 

 section we shall first give an account of the methods of bacteriologi- 

 cal water analysis, and then a condensed statement of results ob- 

 tained in the very numerous investigations which have been made. 



A very important point to be kept in view is the fact that a great 

 increase in the number of bacteria present, in samples of water col- 

 lected for investigation, is likely to occur if these samples are kept 

 for some time. A water which, for example, contains only two 

 hundred to three hundred bacteria per cubic centimetre when the ex- 

 amination is made at once, may contain several thousand at the end 

 of twenty-four hours, and at the end of the second or third day 

 twenty thousand or more may be present in the same quantity. 



