DISTRIBUTION AND HABITAT OF THE BACTERIA. 697 



that bacteria penetrate to a very limited degree into the 

 deeper layers of the soil, more especially as it has been 

 shown from Hofmann's investigations that the passage 

 even of fluids and of substances in solution only takes 

 place with extreme slowness, and usually months and 

 years elapse before the layers in the neighbourhood of 

 the ground water are reached. 



That a current of fluid passing upwards by capillary By upward 

 attraction can carry bacteria from the deeper layers of the c 

 soil to the more superficial has been recently asserted w ater. 

 by Soyka* as the result of a series of experiments. 

 Similar experiments, however, when repeated by 

 Pfeifferf and also in Koch's laboratory, have led to 

 entirely contrary results. However, even if such a 

 transport of bacteria by capillary currents were possible 

 to a slight degree, we could hardly regard this mode of 

 transport for bacteria as of any value under normal 

 conditions, for we have evidence that the deeper layers 

 of the soil are the poorest in bacteria, while the most 

 superficial layers have the largest numbers ; and, further, 

 with regard to man, the capillary current as a means of 

 transport for germs from the soil is scarcely of any im- 

 portance, because, as we shall see, it is only the com- 

 position of the actual surface of the ground which has 

 to be taken into consideration. 



Whether currents of air passing through the soil can By currents of 

 carry bacteria along with them was first experimentally air> 

 tested by Niigeli, and then by Kenk, Soyka, Pfeiffer, 

 and others ; all these observers obtained the uniform 

 result that even strong currents of air were unable to 

 carry a single bacterial germ through a layer of earth 

 a few centimetres in thickness ; the layer of earth even 

 in the completely dry state acts perfectly as a filter, 

 and in ordinary soil, which is always slightly moist, and 

 in which the movement of the ground air is very slight, 

 there is so much less possibility of detachment and 

 transport of bacteria. Lastly, we might also think that By continued 

 bacteria might spread by continued growth. The ^ 



* Prager med. Wochenschr., 1885, No. 28. 



t Rrpert. d. anal. Ck., 1886, No. 1. Zeitschrift f. Hygiene, vol. 1, Part 3. 



