690 BACTERIA IN AIR, SOIL, WATER, AND MILK 



been so definitely proven, and can be deduced only from the diminu- 

 tion of such infections after the substitution of pure water for the pre- 

 viously 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 bacteria 

 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 exten- 

 sive experiments in France on the bacterial contents of rain water, 

 found that in countiy 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 num- 

 ber of bacteria. In each case, of course, the quantitative and quali- 

 tative 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 -purification. 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 leads to a diminution of the 

 number of bacteria in the upper layers of the water. In rivers where 

 sedimentation is to a certain extent prevented by rapidity of current, 

 the effectiveness of such sedimentation is, of course, entirely dependent 

 upon the speed of the current. 



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



