told you that on boiling, the excess of carbonic acid holding chalk or
carbonate of lime in solution as bicarbonate, is decomposed and
carbonate of lime precipitated. You can at once imagine, then, what
takes place in your steam boilers when such water is used, and how
incrustations are formed. Let us now inquire as to the precise nature of
the waste and injury caused by hard and impure waters. Let us also take,
as an example, those most commonly occurring injurious constituents, the
magnesian and calcareous impurities. Hard water only produces a lather
with soap when that soap has effected the softening of the water, and
not till then. In that process the soap is entirely wasted, and the
fatty acids in it form, with the lime and magnesia, insoluble compounds
called lime and magnesia soaps, which are sticky, greasy, adhesive
bodies, that precipitate and fix some colouring matters like a mordant.
We have in such cases, then, a kind of double mischief--(i) waste of
soap, (ii) injury to colours and dyes on the fabrics. But this is not
all, for colours are precipitated as lakes, and mordants also are
precipitated, and thus wasted, in much the same sense as the soaps are.
Now by taking a soap solution, formed by dissolving a known weight of
soap in a known volume of water, and adding this gradually to hard water
until a permanent lather is just produced, we can directly determine the
consumption of soap by such a water, and ascertain the hardness. Such a
method is called Clark's process of determination or testing, or Clark's
soap test. We hear a great deal just now of soaps that will wash well in
hard water, and do wonders under any conditions; but mark this fact,
none of them will begin to perform effective duty until such hard water
has been rendered soft at the expense of the soap. Soaps made of some
oils, such as cocoa-nut oil, for example, are more soluble in water than
when made of tallow, etc., and so they more quickly soften a hard water
and yield lather, but they are wasted, as far as consumption is
concerned, to just the same extent as any other soaps. They do not,