BACTERIA IN WATER . 81 



mention must be made of the effect of light, which has been proved 

 to have a germicidal effect on bacteria. In the case of rivers, however, 

 its contribution to the diminishing of the number of bacteria is small 

 in comparison with the other agencies. 



The number of bacteria in pools and other stationary waters is 

 dependent on the nature of the bed, the depth of the water, and 

 the presence or absence of germicidal substances in the water. A 

 surface water may contain many thousands of bacteria per cubic 

 centimetre, whereas deep well waters and spring waters contain very 

 few bacteria, for these waters have very little opportunity of becoming 

 contaminated. In reservoirs and similar waters, where there is no 

 motion, there are very few bacteria, for, however impure the water 

 may have been when carried into the reservoir, sedimentation soon 

 removes the particulate matter, and with it almost the whole of the 

 bacteria. Thus, the water supplied to London, which is stored in 

 reservoirs before being carried to its destination, contains, when it 

 is consumed, less than 20 bacteria per cubic centimetre. The water 

 supplied to Glasgow (from Loch Katrine), which also is allowed to 

 have the benefit of sedimentation in large subsidence reservoirs before 

 consumption, contains even less than this small number. The greater 

 facility for sedimentation results normally in the presence of a very 

 small number of bacteria in lakes, and, speaking generally, the deeper 

 the lake the smaller will be the number of bacteria per cubic centimetre. 

 Bacteria are also to be found in the sea, the number present in any 

 particular locality being dependent on the amount of organic matter 

 in the water. Consequently, we find that the greatest number is 

 to be found near the land, and that the number diminishes rapidly 

 further out to sea. As the sea bottom contains a large amount of 

 organic remains, bacteria are there present in considerable numbers, 

 and currents sometimes bring them up to the surface from a depth of 

 as much as 600 fathoms. They have been found in large numbers at 

 a depth of 100-200 fathoms. As a class such bacteria are generally 

 motile, and the majority belong to the Spirillaceae. 



To estimate the number of bacteria in any particular sample of 

 water, a small quantity, diluted if necessary, is added to liquefied 

 nutrient-gelatine or nutrient-agar contained in a Petri-dish. After 

 cooling, the Petri-dish is set aside until the colonies appear. These 

 are then counted : as each of the microbes placed on the Petri-dish 

 grows into a colony, the number of colonies gives the number of 

 bacteria that was present in the small quantity of the sample that was 

 transferred to the Petri-dish. 



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