TEXT-BOOK OF BACTERIOLOGY. 357 



dred germs in the cubic centimeter is the conventional limit which 

 is not to fee exceeded in filtered aqueduct water. 



Bacteriological investigation can, therefore, in everj^ case surely 

 determine whether a purified water can be admitted for use or not. 

 It would be vain to dispute this commanding position of such an in- 

 vestigation in favor of an older competitor. The time was when 

 the exclusion of infectious matter from water was unknown. It 

 had been merely ascertained that water was injurious whenever it 

 came from localities harboring garbage of human or animal origin 

 and exhibiting processes of decomposition, and these processes were 

 looked upon as the direct cause of the unwholesome nature of the 

 water. When certain chemical bodies began to appear under the 

 same conditions, they were made use of (as long as the infectious 

 substances themselves could not be obtained) as indicators of the 

 occurrence of those substances. There was no danger in the quan- 

 tities of chlorine, ammonia, nitrites, or organic substance as such, 

 which were declared inadmissible by chemical examination of water; 

 but experience had shown that a transgression of the chemical 

 limits still permitted (established by numerous comparative obser- 

 vations), frequently coincided with a hygienically defective quality 

 of the water. 



We need no longer content ourselves with such shifts and indi- 

 rect conclusions. Quite a number of infectious substances formerlj'^ 

 looked for in vain are now quite well known, and it is probable 

 that such as may yet be unknown pertain to the lowest class of 

 micro-organisms. We m ay, then, disregard chlorine, ammon ia, etc., 

 and deal with the infectious substances themselves or with their 

 nearest relatives. Chemical investigation has, therefore, forfeited 

 its claim as to the determination of the purity of water; it is re- 

 tained and practised only on the ground of thoughtless tradition. 



Chemical investigation can never, and bacteriological examina- 

 tion only exceptionally, ascertain infectious substances as such. 



Common surface water, from creeks, rivers, and lakes, is regu- 

 larly liable to be contaminated, especially by human refuse and ex- 

 crements. It is, therefore, regarded, by recent hygiene, as abso- 

 lutely suspicious of infection and excluded from use. It does not 

 matter whether bacteriological or chemical examination results 

 satisfactorily, and appearance, taste, etc., are ever so inviting, 

 germs of typhoid and cholera may have found access to the water 

 but a few hours before and have rendered it positively dangerous. 



Surface water should, therefore, be always purified before use, 

 unless it has just sprung from the earth and could not have received 

 any contaminating substances. But such aqueducts do not contain 



