356 TEXT-BOOK OF BACTERIOLOGY. 



to accomplish the end; and even then we may not succeed in sepa- 

 rating- a few typhus or cholera colonies from among- countless other 

 bacteria. 



Let us take a case of typhoid fever. It has not been noticed, per- 

 haps hardly diagnosticated. The next few days bring an accumu- 

 lation of such cases. An epidemic has made its appearance. The 

 examination of the drinking- water is urgently demanded. But 

 typhoid fever takes some time for incubation. From the moment 

 of the reception of the fatal germ by the first individual to that 

 moment when the bacteriologist completes his examination, days 

 and even weeks and months will have elapsed. This accounts for 

 the fact that investigations almost always come too late and that 

 the results are usually negative. 



It may now be asked how the bacteriological results can be 

 utilized, if the number of bacteria is not decisive and the establish- 

 ment of their nature is but rarely satisfacto^. 



Entire communities as well as individuals are frequently obliged 

 to use bad and suspicious water, though previously improved and 

 cleaned by proper measures, i.e., freed from infectious matter. 

 This is generally done by sand-filtration on a large scale, and for 

 individual purposes by domestic filters, boiling, etc. But bacteriolog- 

 ical investigation here proves its great power, for it alone can decide 

 whether water has been deprived of its injurious elements. If any 

 method of purifying water is certain of removing infectious matter 

 from it, it must be able to remove all the existing micro-organisms- 

 both pathogenic and non-pathogenic, because the former can be 

 recognized as such only with difficulty and the latter may serve as 

 objects of comparison, and because we can trust to the measures 

 employed only when they also prove efficacious as to the harmless- 

 bacteria. 



In other words, water having gone through such a purifying 

 process must prove free from germs through bacteriological inves- 

 tigation. This demand is, indeed, imperative only in reference to 

 individual cases, for instance, in the use of domestic filters. The water 

 supplied must in this case be completely sterile under all circum- 

 stances a claim, by the way, not yet satisfied by a-ny of the small 

 filters (which, indeed, deteriorate the water instead of improving it). 

 But water free from germs proves to be unattainable in large 

 quantities, be cause its control is more difiicult and it is more liable 

 to contamination. 



It has been agreed to regard a certain quantity of micro-organ- 

 isms as an unavoidable evil, due in part to defects of apparatus, and 

 partly to subsequent pollution. One hundred and fifty to two hun- 



