DISTRIBUTION AND HABITAT OF THE BACTERIA. 691 



the air in the spread of saprophytic and infective germs, danger of air 

 By the experience obtained in bacteriological work and in 

 surgical practice, it has become evident that bacteria but 

 seldom enter nutrient substrata from quiescent air, that 

 even a simple cover which keeps off the vertically falling 

 particles of dust forms even in impure air an extremely 

 efficient protection, and that the entrance of bacteria 

 occurs far more frequently from unclean objects, unin- 

 tentional contaminations, &c., than from air germs. On 

 the other hand, dusty air in active movement offers an 

 excellent opportunity for the spread of bacteria ; and it 

 is remarkable in what numbers they may be deposited 

 on a cool object on nutrient materials, &c., placed in 

 ice along with the water vapour condensed at the 

 same time. But even then the pathogenic bacteria 

 form only a minute fraction as compared with the sapro- 

 phytes. In fact in the open air the dilution of the 

 pathogenic germs soon becomes so very great that a 

 direct infection is a rare occurrence ; it is only from the 

 air within dwellings and in the neighbourhood of the sick 

 that infection occurs at all frequently. 



Neither the points of view nor the methods for the in- Defects in the 

 vestigation of the air are at the present time so precise that investigations 

 it is possible to make definite statements as to the local on 

 and seasonal differences in the number of the air germs, 

 or that well-founded conclusions can be drawn as to the 

 part played by the air in the spread of one or other of 

 the infective diseases. Miquel's attempts to draw a 

 parallel between the results which he has obtained in 

 his air investigations, and the mortality from the various 

 infective diseases, are at least premature, and only show 

 how long-suffering the statistical method is, and hov* 

 easily it can be misused in order to prove a deceptive 

 causal connection. 



The distribution and behaviour of bacteria in the soil Occurrence 

 is of very special hygienic interest, because for a long of 

 time, and especially since Pettenkofer's convincing -de- the soil - 

 ductions, the soil has been regarded as a very important 



