358 TEXT-BOOK OF BACTERIOLOGY. 



surface, but underground water now appearing as a spring and 

 formerly raised artificially by wells. 



The underground water as such is, as a rule, free from germs. 

 The filtering power of the upper strata of the earth is so great 

 that all micro-organisms are prevented from penetrating deeply. 

 If there should be a change in these conditions, such water must be 

 regarded with suspicion, if not actually infected. It often happens 

 that water, faultless per se, is contaminated subsequently by be- 

 ing received in improperly constructed reservoirs accessible to pol- 

 lution. It is thus changed into surface-water and just as much in 

 need of purification as river or lake water. 



There remains only underground water unobjectionably ob- 

 tained, raised by tube-wells or rising naturally. It may also (as 

 before mentioned) become infected. The ground above that water 

 may in the first place, be wanting in filtering power owing to physi- 

 cal properties, coarseness of grain, etc., or because the water is too 

 near to the surface, or a source of pollution may flow at a depth. 



Can chemical or bacteriological investigation discover this fact 

 with certainty ? This question should be answered with great cau- 

 tion. The bacteriological results can hardly be utilized, because 

 germs (harmless water bacteria) are almost always reproduced 

 within the conduit-pipes, etc., and because, therefore, the number 

 of micro-organisms does not afford any actual proof of the ori- 

 ginal condition. The difficulty of judging the species found has 

 already been discussed. We are justified in assuming a direct 

 connection of water with some focus of putrefaction and decom- 

 position only when very different bacteria appear side by side, or 

 when germs are discovered which we know come from human in- 

 testines, as, for instance, Emmerich's bacillus. 



But the result of chemical examination may here be relied upon 

 more safely. If great quantities of organic substances, of chlorine, 

 etc., are detected in a certain water, it must be regarded with posi- 

 tive distrust, and its surroundings should be carefully examined. 

 The circumstances Avill be determined still more clearly and hardly 

 admit of any doubt, wheneverbacteriological and chemicals inves- 

 tigations agree in condemning such water. 



