TEXT-BOOK OF BACTERIOLOGY. 355 



for instance, cholera vibrios in an Indian tank and typhoid bacilli 

 in the drinking-water of several small cities. 



The bacteriological investigation of water is of great practical 

 importance as compared with that of the soil and air. Soil and 

 air are but rarely examined, owing to the many difSculties met 

 with. The complicated apparatus required for it will not be em- 

 ployed very often because the results are, at best, not decisive and 

 surely not in proportion to the means employed. It having been 

 ascertained, in general, that but few micro-organisms exist in the 

 air, very many in the superficial strata of the earth, and none at 

 all in the deeper strata, we have, for the present, about reached 

 the limits of investigation. Bacteriological examination cannot, at 

 this date, furnish any valuable conclusions (as to whether this or 

 that soil is objectionable or this or that air healthier, from a 

 sanitary point of view) and investigation may never be able to 

 accomplish it. 



But not so with water. The question as to whether it satisfies hy- 

 gienic requirements and can be used without hesitation is, in many 

 cases and under definite conditions, decided only b^' the result of 

 bacteriological examination. We repeat "under definite condi- 

 tions " in order to oppose, from the beginning, the great mischief 

 often done by bacteriological investigations and calculated to dis- 

 credit them. The danger of overestimating our ability in this very 

 sphere is not inconsiderable, and it may not be amiss to briefly 

 state the principles of procedure. 



Hygiene has for many years repeatedly declared and demanded 

 that available drinking-water must, above all, be free from infec- 

 tious matter. So long as the nature of these suspicious admixtures 

 was unknown, their occurrence was surmised rather than really 

 and positively ascertained. But it was discovered that these infec- 

 tious substances were living organisms which should be classed 

 among the bacteria. The more infectious substances water con- 

 tained, the more replete it was with micro-organisms. 



The number of bacteria found is of far less importance than 

 their nature. Water containing 5,000 germs of the hay-bacillus or 

 the fluorescent bacillus, etc., in 1 c.cm. is altogether harmless; but 

 water with only ten germs, two of which are cholera vibrios and two 

 typhoid bacilli, is exceedingly dangerous. It may be concluded from 

 all this, that the single colonies must be carefully examined as to 

 their natui'e before we can decide whether some drinking-water is 

 to be considered as the cause of the outbreak of some typhoid epi- 

 demic. But such an investigation is very difHcult. Only great 

 experience, a skilled eye, and well-disciplined technique will be able 



