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