580 
j 
the potability of water in any given case is a matter often requiring j 
very careful consideration of the results of analysis, along vdth a j 
sanitary survey and a knowledge of the normal standards for pure i 
water in the section from which the sample is obtained. In forming j 
a judgment the inspection of the premises and the normal standard ■* 
of other waters in the same region coming from demonstrably unpol- 
luted sources must be taken into account. Injustice and hardship 
have unquestionably resulted from the condemnation of water sup- 
plies on chemical and bacteriological examinations alone, vdthout 
regard to local conditions. The standards of purity should be estab- 
lished for each neighborhood for itself by the analysis of samples 
from several supplies of unpolluted water in the locahty. It by no 
means follows that what has been found as a standard for Massa- 
chusetts or ^lichigan can be used as a standard for Maryland and 
Virginia, nor that the standard for even one section in Virginia or 
Maryland can be used as a standard for another part of the same 
States. In fact, of some 150 water supplies on farms in Virginia 
recently examined, it was very apparent that certain substances 
regarded as indications of pollution were present in unquestionably 
uncontaminated waters in some localities in greatly larger amounts 
than pure water in other localities in the same section. It does not 
seem justifiable in the light of the data obtained in the examination i 
of these supplies to take the standards laid down in the books as a 
mechanical and inelastic measure of the purity of these waters. In 
the statements just made the chemical examination of water was 
specially had in mind, but the statements apply equally well to bac- 
teriological examination, particularly to bacteriological examination 
made of water shipped from a distance where judgment is pro- 
nounced without reference to local conditions or other modif;\nng 
circumstances. 
As in the case of the chemical examination, the bacteriological 
examination as a rule merely indicates the probabilities in regard to 
pollution; it does not give in most cases at least positive information 
as to the presence or absence of organisms which would cause this, 
that, or the other disease. It is, moreover, a matter of experience 
that the results of a bacteriological examination made at one time 
may differ vers^ decidedly from that made at another time of the 
same water under apparently the same conditions. In regard to 
the detection of Bacillus coli communis, which is at present regarded 
by many as a more or less trustworthy indication of contamination, 
recent examination of the water supplies in Virginia has shown that 
this organism was present at one time and not present at another in 
the same water supply. 
The significance of the colon bacillus in the dairy water supplies . 
will be discussed in a different paper. It is merely referred to here 
