708 DISTRIBUTION AND HABITAT OF THE BACTERIA. 



Further action effect on the number and variety of the bacteria trans- 

 zone. ryU1 ' ported by the wind, because all impure fluids, dejecta, 

 &c., and with them all bacteria, remain in the most 

 superficial dry zones. In this way the accumulation of 

 the most various kinds of saprophytes, and also of patho- 

 genic bacteria, takes place to a much greater extent than 

 in the case of a thoroughly moist soil where there is no 

 drying zone, because the continuous stream of water in 

 the latter case carries the bacteria to a certain though 

 moderate depth and removes them from the action of 

 the wind. 



The other modes of transport are only to a slight 

 degree subject to influences varying according to season. 

 Formerly a variation in infective power was ascribed to 

 the ground water according as its level was high or low, 

 usually, however, the number of bacteria present in it is 

 but little influenced by alterations of the level. 



The spread by food, man, and various materials can 

 perhaps be favoured by the presence of a drying zone, in 

 that when such a zone exists a large accumulation of bac- 

 teria occurs in the most superficial layers of the soil from 

 which the transport takes place ; nevertheless a constant 

 difference of this kind can only be made out in the case 

 of those soils which are exposed to continual contamina- 

 tion, as in the soil of streets, courts, &c. ; while the soil 

 of fields, for example, which are only impregnated at 

 considerable intervals with fluid containing bacteria, can 

 only show seasonal variations in the action of the mode 

 of transport as the result of a conjunction of accidents. 



On the whole, therefore, it is practically only the drying 

 of the surface of the soil which increases the danger of 

 distribution of pathogenic germs from the soil. 



influence of The existence of any influence on the transport of the 

 constitution bacteria of the soil as the result of the character of the soil 

 of the soil on in any given locality is not so evident. Here again it is 

 the bacteria . only a porous soil, and one which can take up large numbers 

 of bacteria, which could exert an influence on their dis- 

 tribution. Further, we may suppose that where the pores 

 are large the bacteria do not on the whole accumulate in 

 such numbers, but are more readily distributed over con- 



