EXAMINATION OP WATER. 441 



ble number of these bacteria. Consequently, the air is 

 purified to a marked extent by washing, as it were, with 

 rain or snow. 



The melted water from freshly fallen snow, collected at 

 the ordinary low altitude, may contain from a few to as 

 many as 500 bacteria per c.c. In the higher altitudes, the 

 air contains only a few organisms and hence the precipita- 

 tion in such places will contain a smaller number than in 

 the previous case. The snow deposited at an altitude of 

 6,000 feet is not sterile, but very nearly so. Usually less 

 than five bacteria per c.c. of melted snow are found. The 

 rain-water at Paris has been found to contain from 5 to 20 

 bacteria per c.c. 



It follows therefore that all surface waters, beginning 

 even with the glacier brook, will contain bacteria. As the 

 temperature of the water and the amount of organic matter 

 increases, multiplication of the bacteria will take place. 

 The number is increased by contact with the soil, dust- 

 laden winds and above all by animal excreta and waste- 

 matter. 



The lakes in mountainous countries, as in Switzerland, 

 because of their high altitude and the source of their water 

 contain relatively very few bacteria. The surface water of 

 these lakes usually contains less than 50 and only occasion- 

 ally over 100 bacteria per c.c. Water, however, taken at 

 some depth below the surface of these lakes may contain 

 as many as 600 bacteria per c.c. This is undoubtedly due 

 to a gradual sedimentation of the suspended organisms. 



In some lakes, especially at low altitudes, this is not 

 always the case and, indeed, the conditions may be reversed. 

 .Thus, the surface water may contain thousands of bacteria 

 per c.c., whereas that near the bottom may contain but a 

 few hundred. This difference may be due to the fact that 

 these lakes are fed by deep springs; and, hence, while the 

 water at the bottom is very cold, that on the surface is 

 warmed by the sun and offers conditions which are favor- 



