[Illustration]

The three impurities of water which most interest the photographer are lime
or magnesia salts, which give the so-called hardness; chlorides (as, for
example, chloride of sodium or common salt), which throw down silver salts;
and organic matter, which may overturn the balance of photographic
operations by causing premature reduction of the sensitive silver
compounds. To test for them is easy. Hardness is easily recognizable by
washing one's hands in the water, the soap being curdled; but in many cases
one must rather seek for a hard water than avoid it, as the tendency of
gelatine plates to frill is far less in hard water than in soft water. It
is, indeed, a common and useful practice to harden the water used for
washing by adding half an ounce or an ounce of Epsom salts (sulphate of
magnesia) to each bucket of water. Chlorides--chloride of sodium or common
salt being that usually met with--may be detected by adding a drop or two
of nitrate of silver to half a wineglassful of the water, a few drops of
nitric acid being then added. A slight cloudiness indicates a trace of
chlorides, and a decided milkiness shows the presence of a larger quantity.
If it is wished to get a somewhat more definite idea of the amount, it is
easy to make up a series of standards for comparison, by dissolving known
weights of common salt in distilled or rain water, and testing samples of
them side by side with the water to be examined.

Organic matters may be detected by adding a little nitrate of silver to the
water, filtering off from any precipitate of chloride of silver, and
exposing the clear liquid to sunlight; a clean stoppered bottle being the
most convenient vessel to use. The extent to which a blackening takes place
may be regarded as approximately proportionate to the amount of organic
matter present.