11 '(■(!///(■)-, Il'd/rr and Pisrase. 31 



throug^h soil. But the one all-important fact in regard to the 

 purification of water is tliat boiling is absolutely necessary, 

 and then protection from further contact with impurities in 

 air and vessels. 



The rapid de])ositi()n, from the alm()S])liere of microbes, in 

 water, while being exi)osed to the air, re(|uires that, for pur- 

 poses of analysis especialh', the specimen be perfectly pro- 

 tected from the air, or that the analysis be made at the place 

 from which tlie water is taken. 



The waters so crowded ultimately become ])oisonous to the 

 l)acteria, or the food retpiired becomes exhausted, so that 

 there is a purifying process going on in that way. A Black 

 Hole of Calcutta sacrifice takes place. 



The water of the soil for variable depths below the surface, 

 or to a point where drainage is obstructed by impermeable 

 strata, is called the "Ground-Water." It is found that this 

 water furnishes a large proportion of supply to wells or super- 

 ficial basins, and is especially rich in bacteria and liable to 

 infection. The clo.ser the level of this ground water to the 

 surface the more rapidly the air above becomes impure with 

 carbonic acid and bacteria, which are present in the soil. If 

 the water level lies uniformly close to the surface, it forms an 

 unhealthy condition of dampness of soil, which is favorable 

 to the production of such diseases as consumption and typhoid 

 fever. If the water level is far below the surface it gives you 

 a dry soil, favorable to the activity of such bacteria as find 

 food and living there, and to the production of carbonic acid. 

 In the fall of the year, when the ground water is at its 

 lowest, typhoid fever develops. A rise again to its highest 

 level brings to the surface the carbonic-acid gas and the 

 bacteria to the atmosphere, and there is again chance for 

 infection, but the soil is not then so fit for farther growth of 

 carbonic-acid microbes — not until recession again occurs. 

 These statements have been illustrated in the histor}- of 

 epidemics of typhoid fever that have occurred in Berlin, 

 Munich and other places. This ground-water, in its varia- 

 tions of movement up and down, may, in its upward move- 

 ment, force impure air into houses. It has been shown by 

 exhaustive experiments by PettenkofFer and by Marker and 

 Shultze, that in a room with an external temperature of 32°, 



