696 DISTRIBUTION AND HABITAT OF TI1E BACTERIA. 



transport of bacteria in the soil. The bacteria in the 

 first place generally reach the most superficial layers 

 with refuse fluids, from the air, &c. ; and hence in these 

 layers we find by far the greatest number of bacteria. 

 Many bacteria also pass at once from cesspools, &c., 

 into somewhat deeper layers of the ground, 1 3 metres 

 under the surface, and impregnate these especially 

 strongly in the immediate neighbourhood of the cess- 

 pool. The question is whether bacteria can spread 

 from these points over considerable distances of the soil 

 in a horizontal and vertical direction. Currents of 

 water or air suggest themselves as the chief means of 

 transport. The former, when they soak through the 

 soil from the surface as far as the ground water, 

 ultimately carry the bacteria into the deeper layers and 

 into the ground water; or again, where there is great 

 evaporation from the surface, the water rising by capil- 

 lary action carries bacteria which have collected beneath 

 By downward into the upper layers. But on experimental investiga- 

 watet. tion neither mode of transport has proved to be prac- 



ticable. Numerous filtration experiments on a large 

 and small scale have shown most distinctly that a layer 

 Of earth | to 1 metre in thickness is an excellent filter 

 for bacteria, and hence the purification of fluids from 

 bacteria must be still more complete in cultivated and 

 especially in clay soil, and where the fluid moves with 

 extreme slowness. In harmony with this we have the 

 fact, first ascertained by Koch and later on repeated 

 occasions in the author's laboratory, that the deeper 

 layers of soil contain very much fewer or indeed no 

 bacteria in contrast to the superficial layers which are 

 almost always rich in them (apart naturally from soil which 

 has been artificially disturbed). Further, it has been 

 repeatedly observed that wells which are well protected 

 against contamination with bacteria from the surface 

 and from the sides of the well furnish a water almost 

 entirely free from bacteria; that, further, wells of water 

 containing bacteria become the purer the more water is 

 pumped up, and the more ground water comes in from 

 the deeper layers of the soil. Hence we can only assume 



