30 Citiciuiiali Sociefy of Xalioal History. 



tions — that of the bacteriological. If the air be full of 

 microbes the water is almost equally so. Many of them are 

 harmless, but it is a vehicle or source of nutrition for the 

 good and bad alike. If chemical analysis gives us a test of 

 organic impurities, and clinical experience locates these 

 impurities in well or river, bacteriological examinations dis- 

 cover the guilty individuals in certain microbes and their 

 to.xic products. The germ theory finds "confirmation" in 

 the precise connections of bacterial growths in water with 

 di.sease. The methods are so complete that the number of 

 microbes in each cubic centimeter have been counted. 



Miqual ( Bacteriological Analysis of Waters, p. 4.SS) gives 

 following figures as to bacteria in one cubic centimeter : 



MIOUKL'S ST.\NDARI) ok rURlTV IS .\S KOLI.UVVS. 



Bacteria to cubic centimeter. 

 17 minims. 



Water excee(lin.t(ly pure o to 10. 



Water very pure 10 to lofj. 



Water pure 100 to 1,000. 



Water mediocre 1,000 to 10,000. 



Water impure 10,000 to loo.ocx). 



Water very imjiurt- 100,000 and more. 



WATKRS OF THK KIVKR SKINK— I.X .WICRAOHS HV SKASONS. 



At Ivry. 



Winter 43i500 



vSpring 26,570 



Summer '3,7 10 



Autumn 46,340 



His numerous experiments have established that these 

 waters are most pure in vSummer — July, August and Septem- 

 ber. No relation then exists between the temperature of the 

 water and its wealth in micro-organisms. The sewer waters of 

 Paris, on an average, contain i3,Srx),(KK:>, demonstrating the 

 great immense bacterial contamination in those places, as 

 compared with outside waters. These waters, filtered through 

 the soil when returned to vSeine, are 1,800 times purer, v'^c^me 

 of them, filtered through an artificial filter of soil about six 

 feet thick, lose about 999 of every 1,000 bacteria — having 

 originally 2o,o<xD,c)r)o to the cubic centimeter (17 minims). 

 These few facts demonslrate the purifsing effect of filtration 



