fact the presence of a certain amount of such matter is
advantageous, supplying the mineral constituents necessary for the
formation of the solid tissues of the body. The presence of organic
matter, on the other hand, must always be regarded with suspicion. This
organic matter may consist not only of the products of animal and
vegetable life but also of certain microscopic forms of living organisms
which are likely to accompany such products. Contagious diseases are
known to be due to the presence in the body of minute living organisms
or germs. Each disease is caused by its own particular kind of germ.
Through sewage these germs may find their way from persons afflicted
with disease into the water supply, and it is principally through the
drinking water that certain of these diseases, especially typhoid fever,
are spread. It becomes of great importance, therefore, to be able to
detect such matter when present in drinking water as well as to devise
methods whereby it can be removed or at least rendered harmless.

~Analysis of water.~ The mineral analysis of a water is, as the
name suggests, simply the determination of the mineral matter
present. Sanitary analysis, on the other hand, is the
determination of the organic matter present. The physical
properties of a water give no conclusive evidence as to its
purity, since a water may be unfit for drinking purposes and
yet be perfectly clear and odorless. Neither can any reliance
be placed on the simple methods often given for testing the
purity of water. Only the trained chemist can carry out such
methods of analysis as can be relied upon.

[Illustration: Fig. 24]

~Purification of water.~ Three general methods are used for the
purification of water, namely, _distillation_, _filtration_, and
_boiling_.

1. _Distillation._ The most effective way of purifying natural waters is
by the process of distillation. This consists in boiling the water and
condensing the steam. Fig. 24 illustrates the process of distillation,
as commonly conducted in the laboratory. Ordinary water is poured into
the flask A and boiled. The steam is conducted through the condenser
B, which consists essentially of a narrow glass tube sealed within a
larger one, the space between the two being filled with cold water,
which is admitted at C and escapes at D. The inner tube is thus kept
cool and the steam in passing through it is condensed. The water fo