579 
cance, though it is true that occasionally bacteria of certain diseases 
have been found floating in the air and these may be taken up by the 
water; still this is probably not frequent in rural districts at least. 
The mineral matter taken up from the soil, particularly the salts 
of lime and magnesia, make the water ^^hard,” and although this does 
not affect the health of those who take the water, unless the minerals 
be present in large amounts, it makes the water less suitable for pur- 
poses of cleanliness. The presence of sewage is of course an indi- 
cation at least that the water may be injurious to health; for as every- 
one knows outbreaks of typhoid fever and of disorders of the bowels 
have been frequently traced to water that was polluted with sewage. 
The substances other than the bacteria taken up by the water are 
not themselves injurious, but they may be more or less significant of 
pollution. Since the disease-producing property of polluted water is 
due to the bacteria which it contains, it would be obviously of the 
very greatest advantage to be able in any given case to detect the 
presence of the pathogenic organism concerned. It goes without 
saying that it would be very desirable to be able to state in a given 
case that the germ of typhoid fever or of scarlet fever or of dysentery 
were or were not present in the water. Unfortunately our knowl- 
edge has not yet been advanced to the point at which this is possible. 
There are many instances on record where epidemics of t3rphoid fever 
have been traced to certain water supplies, and yet the most pains- 
taking search has failed to show the presence of the bacterium which 
is generally regarded as the cause of typhoid fever. In fact the cases 
are comparatively rare in which this organism has been claimed to be 
detected in water which seemed beyond doubt to be the cause of 
typhoid fever. The most that can be accomplished by chemical 
analysis and bacteriological examination of water at present is to 
show with more or less certainty the presence of pollution with sew- 
age. It is not possible to state positively as a result of these tests 
that any given specimen of water, even though it contain sewage, 
will produce typhoid fever or other gastro-intestinal disorders when 
taken into the stomach. But it is quite possible by these means to 
show that water is more or less polluted and consequently a menace to 
health. This is after all probably as useful from a practical sanitary 
point of view as the ability to detect the specific organism of disease 
would be. If by any means it can be shown that a water supply is 
polluted or even suspicious this is reason sufficient for taking steps 
to pre^^ent the continuance of the pollution or failing in this to con- 
demn and close the source. 
It is rarely possible to form a just opinion simply by the examina- 
tion of samples of water sent from a distance, for there are other 
factors to be taken into consideration. The decision in regard to 
