ry one, based on
the number of grains of solids per gallon and waters may be classed on
such a basis as follows: 1-10 grain per gallon, soft water; 10-20 grain
per gallon, moderately hard water; above 25 grains per gallon, very hard
water.

_Alkalinity_ is a general term used for waters containing compounds with
the power of neutralizing acids.

_Causticity_, as used in water treatment, is a term coined by A. McGill,
indicating the presence of an excess of lime added during treatment.
Though such presence would also indicate alkalinity, the term is
arbitrarily used to apply to those hydrates whose presence is indicated
by phenolphthalein.

Of the chemical methods of water treatment, there are three general
processes:

1st. Lime Process. The lime process is used for waters containing
bicarbonates of lime and magnesia. Slacked lime in solution, as lime
water, is the reagent used. This combines with the carbonic acid which
is present, either free or as carbonates, to form an insoluble
monocarbonate of lime. The soluble bicarbonates of lime and magnesia,
losing their carbonic acid, thereby become insoluble and precipitate.

2nd. Soda Process. The soda process is used for waters containing
sulphates of lime and magnesia. Carbonate of soda and hydrate of soda
(caustic soda) are used either alone or together as the reagents.
Carbonate of soda, added to water containing little or no carbonic acid
or bicarbonates, decomposes the sulphates to form insoluble carbonate of
lime or magnesia which precipitate, the neutral soda remaining in
solution. If free carbonic acid or bicarbonates are present, bicarbonate
of lime is formed and remains in solution, though under the action of
heat, the carbon dioxide will be driven off and insoluble monocarbonates
will be formed. Caustic soda used in this process causes a more
energetic action, it being presumed that the caustic soda absorbs the
carbonic acid, becomes carbonate of soda and acts as above.

3rd. Lime and Soda Process. This process, which is the combination of
the first two, is by far the most generally used in water purification.
Such a method is used where sulphates of lime and magnesia are contained
in the water, together with such quantity of carbonic acid or
bicarbonates as to impair the action of the soda. Sufficient soda is
used to break down the sulphates of lime and magnesia and as much lime
added as is required to absorb the carbonic acid not taken up in the
soda re