DISTRIBUTION AND HABITAT OF THE BACTERIA. 707 



distinguish, in the first place, a superficial zone of evaporation 

 in which the degree of moisture of the soil is very variable, 

 and oscillates between complete saturation and marked dry- 

 ness ; in this zone, when of great breadth as the result of the 

 summer heat, the whole of the rain in late summer and 

 harvest often finds a place, and the filling of the capillary 

 pores does not reach to its lower border ; in this case there is 

 therefore always a dry layer between the most external part 

 which is temporarily wetted by the rains, and the deeper 

 layers of the soil in which the water lies. And under these 

 conditions all the impurities which reach the soil remain in 

 the upper dry zone. 



Under this layer lies a zone which never dries, but in 

 which the capillary pores are always full of water. If this 

 zone receives water from above (when the upper dry layer 

 has been completely filled with rain water) the amount of 

 water in it remains the same, for the excess runs downwards 

 into the third zone, that of the ground water. 



It is evident that, when a drying zone exists, the con- Favouring 

 ditions are the most favourable for the detachment and a^g^one. 

 carrying away of the bacteria of the soil by currents of 

 air. It is only in that case that this most important 

 mode of transport comes into play. During the whole 

 of the winter and a great part of the spring there is 

 usually no dry zone in our climate, and hence no possi- 

 bility for such a passage of germs into the air. In the 

 latter end of summer and in autumn, on the other hand, 

 this possibility is present, and is only absent from time 

 to time when the occurrence of rain renders the outer 

 surface moist. 



As the existence of a dry zone always results in the The variations 

 cessation of addition to the ground water from above, and ^ San^ 



thus leads to sinking of the level of the ground water, index of 



, . . , .... the moisture 



we have in the variations in the level of the ground water of the soil, 



a fairly useful index of the possibility of a transport of 

 the bacteria of the soil by the wind. This index is not, 

 however, quite correct, because the temporary moisture of 

 the surface of the soil (sometimes lasting even for weeks 

 and months) and the cessation in the transport thus 

 occasioned find no expression in the variations of the 

 level of the ground water. 



The existence of a dry zone has a further favourable 



