water, hard from dissolved carbonate of lime, can be softened by merely
boiling the water, for the excess of carbonic acid is then expelled, and
the chalk is precipitated again. This would be too costly for the
softening of large quantities of water, the boiling process consuming
too much coal, and so another process is adopted. Quicklime, or milk of
lime, is added to the water in the proper quantity. This lime unites
with the excess of carbonic acid holding chalk in solution, and forms
with it insoluble chalk, and so all deposits together as chalk. By this
liming process, also, the iron of the water dissolved likewise in
ferruginous streams, etc., by carbonic acid, would be precipitated. To
show this deposition I will now add some clear lime-water to the
solution I made of chalk with the carbonic acid of my breath, and a
precipitate is at once formed, all the lime and carbonic acid together
depositing as insoluble chalk. Hence clear lime-water forms a good test
for the presence of bicarbonates of lime or iron in a water. But water
may be hard from the presence of other salts, other lime salts. For
example, certain parts of the earth contain a great deal of gypsum, or
natural sulphate of lime, and this is soluble to some extent in water.
Water thus hardened is not affected by boiling, or the addition of lime,
and is therefore termed permanently hard water, the water hardened with
dissolved chalk being termed temporarily hard water. I have said nothing
of solid or undissolved impurities in water, which are said to be in
suspension, for the separation of these is a merely mechanical matter of
settling, or filtration and settling combined. As a general rule, the
water of rivers contains the most suspended and vegetable matter and the
least amount of dissolved constituents, whereas spring and well waters
contain the most dissolved matters and the least suspended. Serious
damage may be done to the dyer by either of these classes of impurities,
and I may tell you that the dissolved calcareous and magnesian
impurities are the most frequent in occurrence and the most injurious. I