The alchemists dreamed and talked of that universal solvent which they so
long and vainly endeavored to discover; still, for all this, not only the
alchemist of old, but his more immediate successor, the chemist of to-day,
has found no solvent so universal as water. No liquid has nearly so wide a
range of dissolving powers, and, taking things all round, no liquid
exercises so slight an action upon the bodies dissolved--evaporate the
water away, and the dissolved substance is obtained in an unchanged
condition; at any rate, this is the general rule.
The function of water in nature is essentially that of a solvent or a
medium of circulation; it is not, in any sense, a food, yet without it no
food can be assimilated by an animal. Without water the solid materials of
the globe would be unable to come together so closely as to interchange
their elements; and unless the temperatures were sufficiently high to
establish an igneous fluidity, such as undoubtedly exists in the sun, there
would be no circulation of matter to speak of, and the earth would be, as
it were, locked up or dead.
When we look upon water as the nearest approach to a universal solvent that
even the astute scientist of to-day has been able to discover, who can
wonder that it is never found absolutely pure in nature? For wherever it
accumulates it dissolves something from its surroundings. Still, in a
rain-drop just formed we have very nearly pure water; but even this
contains dissolved air to the extent of about one-fiftieth of its volume,
and as the drop falls downward it takes up such impurities as may be
floating in the atmosphere; so that if our rain-drop is falling
immediately after a long drought, it becomes charged with nitrate or
nitrite of ammonia and various organic matters--perhaps also the spores or
germs of disease. Thus it will be seen that rain tends to wonderfully clear
or wash the atmosphere, and we all know how much a first rain is
appreciated as an air purifier, and how it carries down with it valuable
food for plants. The rain-water, in percolating through or over the land,
flows mainly toward the rivers, and in doing so it becomes more or less
charged with mineral matter, lime salts and common salt being the chief of
them; while some of that water which has penetrated more deeply into the
earth takes up far more solid matter than is ordinarily found in river
water. The bulk of this more or less impure water tends toward the ocean,
taking with it its load of salt and lime. Constant evaporation, of course,
takes place from the surface of the sea, so that the salt and lime
accumulate, this latter being, however, ultimately deposited as shells,
coral, and chalk, while nearly pure or naturally distilled water once more
condenses in the form of clouds. This process, by which a constant supply
of purified water is kept up in the natural economy, is imitated on a small
scale when water is converted into steam by the action of heat, and this
vapor is cooled so as to reproduce liquid water, the operation in question
being known as distillation.
For this purpose an apparatus known as a still is required; and although by
law one must pay an annual license fee for the right to use a still, it is
not usual for the government authorities to enforce the law when a still is
merely used for purifying water.
One of the best forms of still for the photographer to employ consists of a
tin can or bottle in which the water is boiled, and to this a tin tube is
adapted by means of a cork, one end of this tin tube terminating in a coil
passing through a tub or other vessel of cold water. A gas burner, as
shown, is a convenient source of heat, and in order to insure a complete
condensation of the vapor, the water in the cooling tub must be changed now
and again.
[Illustration]
Sometimes the vapor is condensed by being allowed to play against the
inside of a conical cover which is adapted to a saucepan, and is kept cool
by the external application of cold water; and in this case the still takes
the form represented by the subjoined diagrams; such compact and portable
stills being largely employed in Ireland for the private manufacture of
whisky.
[Illustration]
It is scarcely necessary to say that the condensed water trickles down on
the inside of the cone, and flows out at the spout.
An extemporized arrangement of a similar character may be made by passing a
tobacco pipe through the side of a tin saucepan as shown below, and
inverting the lid of the saucepan; if the lid is now kept cool by frequent
changes of water inside it, and the pipe is properly adjusted so as to
catch the drippings from the convex side of the lid, a considerable
quantity of distilled water may be collected in an hour or so.
The proportion of solid impurities present in water as ordinarily met with
is extremely variable: rain water which has been collected toward the end
of a storm contains only a minute fraction of a grain per gallon, while
river or spring water may contain from less than thirty grains per gallon
or so and upward. Ordinary sea water generally contains from three to four
per cent. of saline matter, but that of the Dead Sea contains nearly
one-fourth of its weight of salts.
[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.
Filtration on a small scale is not altogether a satisfactory mode of
purifying water, as organic impurities often accumulate in the filter, and
enter into active putrefaction when hot weather sets in.--_Photo. News._
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