198 



MICROBIOLOGY OF WATER AND SEWAGE. 



than those of typhoid fever, owing to the "explosive" character of the 

 disease. At the time of the outbreak of cholera in Hamburg, in 1892, the 

 cholera vibrios were frequently isolated from the water of the river Elbe, 

 which was used to furnish the regular supply of the city. The adjoining 

 city of Altona also obtained its water from the same river, after it had 

 received some of the Hamburg sewage; yet it remained practically free 

 from the scourge, owing to the efficiency of sand niters which were 

 used to purify the water (Fig. 57). In times of epidemic, the organism 

 has been isolated from rivers, wells, and reservoirs in India, a country 

 in which the disease is endemic. 



THE NUMBER or BACTERIA IN RAIN, SNOW, HAIL, ETC., AND IN WATER 

 FROM WELLS, UPLAND SURFACE WATERS, RIVERS, AND LAKES. 



RAIN. The number of bacteria found in rain depends upon the- 

 month of the year and the dryness of the air. When considerable dust is 

 present in the air, the first rain beats it back to the soil; and at such time 

 rain water contains more organisms than usual. Rain falling in densely 

 inhabited cities always contains more microbes than rain falling on open 

 farm land or upland pastures. A few figures will be sufficient to illus- 

 trate: 



NUMBER OF BACTERIA PER LITER OF RAIN WATER. 

 Figures for Montsouris Park, Paris, France, and the average for two years. 



Yearly average 5,300 per liters per month. 



The average for the interior of Paris corresponds with the larger 

 amount of dust in the air, and reaches a total of 19,000 organisms per 1. 



