f solids may exist in the body. For certain
kidney diseases, for example, pure water is prescribed, not merely as a
means of preventing further accretions, but for the purpose of
dissolving and removing the undesirable accumulations already existing.

Practically, considerable latitude is possible in the matter of the
purity of drinking water, and no particular harm is to be apprehended by
the constant use of either a water containing as little as ten parts per
million of total solids or of water containing as much as three hundred
parts per million of total solids. The human body, in this as in so many
other ways, is so constituted as to be able to adjust itself to varying
conditions of food, and, until an excessive amount of ingredients are
absorbed, no great harm is done. There are, however, certain definite
substances--animal, vegetable, and mineral--which, when found in water,
are decidedly objectionable, and it is not the amount of foreign matter
in a water-supply, but its character, which is of importance in a water
to be used for drinking.

_Mineral matter in water._

The mineral matter is the least objectionable as it is also the most
common, since all water is forced to partake, more or less, of the
nature of the rocks and soil over which it passes. Good waters contain
from twenty to one hundred grains per gallon of mineral salts; that is,
of various chemical substances which are able to be dissolved by water.
If the amount is much in excess of one hundred parts, the water is
noticeably "hard," and this may increase to a point where the water
cannot be used. For example, the writer once superintended the locating
and drilling of a well which passed through a bed of sodium sulphate or
gypsum, just before reaching the water, so that as the latter rose in
the well it dissolved and carried with itself a large amount of this
salt, so much that the water was useless. Water containing more than one
hundred grains per gallon of such salts as magnesium sulphate or sodium
phosphate is a mineral water rather than a good drinking water, and
while an occasional glass may do no harm or may even have desirable
medicinal effects, such a water is not fit for constant drinking.

It is worth noting that many attempts have been made to show the
relative effect of various hard waters on the health. A French
commissioner reported that apparently people in hard-water districts had
a better physique than in soft-water districts. A