below the surface above the level of the ground water, none of them
found their way into the water of the soil. This experiment shows the
folly of building a cesspool in the vicinity of a well when they both go
down to the same water level, since the contents of the cesspool will be
carried into the well if the underground stream flows in the proper
direction. A shallow cesspool, however, would not be open to the same
objection.

It is always difficult to detect the direction or flow of underground
water, and various technical and delicate methods have been selected to
make this determination. A very simple test, however, is to dig a hole
at the point where pollution is suspected, carrying the hole down to
where ground water is reached, and then to throw a gallon of kerosene
oil into the hole, and if the ground-water flow is toward the well, the
presence of kerosene in the well water will make the fact known. This
would not, however, prove that the actual contamination would produce
disease, since a liquid like kerosene can find its way through the pores
of the soil to much greater distances than bacteria can be carried. But,
to be on the safe side, water from such a well should not be used.

To make sure of the quality of the water proposed for a water-supply, it
is wise to have such water examined by a chemist. The chemist will make
certain determinations of ammonia and other chemical combinations, and
will report his findings with an interpretation or explanation of the
result. What he finds is not the presence or absence of disease or
disease germs, but substances that suggest or involve the presence of
organic pollution. A test is made for the number of bacteria, and a well
of spring water which contains more than about fifty in a cubic
centimeter is a suspicious water. Surface water, on the other hand, may
contain two or three hundred without being necessarily bad, the types of
bacteria being harmless. Generally, a chemist will also determine the
presence of the colon bacillus which is found in the intestinal tract of
man or warm-blooded animals. Wherever this is found, in even such a
small quantity as one cubic centimeter of water or less, there is strong
presumption that the water has been polluted by human wastes and is
therefore not fit to drink.

_Dangers of polluted water._

Since no evidence of the danger of drinking polluted water can be so
graphically expressed as by a direct reference to epidemics ca