BACTERIA IN WATER 1017 



In regard to the less clearly understood diarrheal diseases, dysen- 

 tery, cholera infantum, etc., the direct relation to water supply has 

 not been so definitely proven, and can be deduced only from the 

 diminution of such infections after the substitution of pure water 

 for the previously used impure supply. It is thus seen that water 

 bacteriology is one of the most important branches of the science 

 of hygiene, and has led, and is constantly leading, to enormous 

 diminution of the death rate in all communities where an intelligent 

 study of the conditions has been made. 



The bacterial purity of natural waters, although dependent upon 

 special and local conditions in relation to possible contamination, 

 differs widely, according to the source from which such waters are 

 derived. 



Rain water and snow water are usually contaminated with bac- 

 teria by the dust which they gather on their way to the ground, 

 and are especially rich in bacteria when taken during the first few 

 hours of a rain or snow storm when the air is still dusty and filled 

 with floating particles. During the later hours of prolonged storms, 

 rain water and snow water may be comparatively sterile. Miquel, 1 

 who made extensive experiments in France on the bacterial contents 

 of rain water, found that in country districts, where the air is less 

 dusty, rain water contained an average of about 4.3 bacteria to the 

 cubic centimeter. 



The bacterial counts of snow water are usually somewhat higher 

 than those of rain. 



The waters of streams, ponds, and lakes are usually spoken of as 

 surface waters, and these of all natural supplies contain the largest 

 number of bacteria. In each case, of course, the quantitative and 

 qualitative bacterial flora of such waters is intimately dependent 

 upon the conditions of the surrounding country, the density of the 

 population, and the relation of these waters to sewage. It is also, 

 and to no less important degree, dependent upon weather conditions, 

 the influence of light and temperature, and the food supply contained 

 within the waters in the form of decayed vegetation. In all such 

 surface waters there is constantly going on a process of self-purifica- 

 tion. The chief factor in this process is sedimentation. In stagnant 

 ponds and lakes with but sluggish currents there is a constant 

 sedimentation of the heavier particles, which gradually but steadily 



1 Miquel, Revue d'hyg., viii, 1886. 



