578 
The water table follows the dip of the rock or clay layer and is 
consequently to be found at various depths or it may crop out on 
the surface. Where it crops up it forms a spring where it is tapped 
by a shaft it furnishes the water for a well. If this water table lies 
between two impervious layers, and if the point at which the well 
is sunk is at a lower level than any part of the water table, the water 
flows out and constitutes an artesian well. In* order that the water 
may come out at the top of the well it is of course essential that the 
water table should lie between two impervious layers. The upper 
of the two may, however, be at the surface of the ground. A sub- 
artesian well is one in which the water comes up near the top of the 
shaft. 
SOURCES OF POLLUTION. 
Water takes up some part of everything with which it comes in 
contact. Some things, like common salt and potash, as everyone 
knows, are readily dissolved in water, while many other substances 
are dissolved in very small traces; but the solvent action of water 
even on the hardest stone may be noticed in time. Not only solid 
substances, but gases and liquids, as well as living micro-organisms, 
microscopic plants and animals, and minute particles of dust are all 
taken up by water. From this fact it is evident that everything 
with which it comes in contact from the time it leaves the ocean as 
clouds to the time it returns to the ocean as rivers is taken up by the 
Avater to a greater or less extent. In other words everytliing Avhich 
is found either dissolved or floating in water except such substances 
as are introduced directly, either intentiona]ly or by accident, is 
derived from the air or soil through which the water passes on its Avay 
from the ocean and back again. On its way through the air the 
water takes up various gases — oxygen, nitrogen, carbon dioxide, 
ammonia, and other substances of this nature as AveJl as fine dust 
particles and bacteria. On its passage through the soil it dissoh^es 
various minerals from the rocks, such as lime and magnesia, and if 
the soil is polluted in any Avay, it takes up Avhatever it can dissoh^e 
of the pollution. In the upper layers of the soil the water also comes 
in contact with bacteria AA^hich cause its contamination. 
Many of the substances taken up by the water are harmless or even 
beneficial, others are undesirable, Avhile others again may be harmful 
in themselves or indicative of former pollution. The nitrogen and 
ammonia from the air are probably Avuthout significance from a sani- 
tary point of vieAV, though these may be of some value in a different 
direction, as a source of food for groAA’ing plants. The oxygen and 
carbon dioxide serve a useful purpose in giving life and sparkle to 
the AA^ater and in this Avay impart an agreeable taste. The bacteria 
which the Avater takes from the air are probably seldom of any signifi- 
