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244 ILLUSTRATED WORLD
When Dust Is Present
Condensation of the water vapor takes place on cooling when dust particles
are there to furnish nuclei to the saturated air.
means of microscopical examination. Of
course in such an experiment, the plates
must be protected in some suitable way
from the dust that would settle on them
in the lower strata of atmosphere. Microscopic
examination of such plates has
shown the presence of pollen-grains,
vegetable fibers, hair particles, mineral
and rock fragments, and iron both free
and in combination with sulphur.
We are in part, if not altogether, indebted
to dust for rain. Nuclei are
necessary for condensation. Now, the
dust particles at great heights are cooled
quickly by radiation, and then serve as
excellent points for condensation. The
electric charges on these dust particles
also play a part in cloud formation.
These facts can be shown very simply
by the following experiment: Take two
bell-jars or large bottles and pump the air
out of them. Then fill one with dust free
air containing water vapor, and the other
with ordinary air containing water vapor.
As has been said before, all ordinary air
contains dust. Now if both jars are
cooled suddenly, a dense white cloud will
form in the bottle containing ordinary
air, while no cloud will be formed in the
second bottle although it contains the
same water vapor. But if a little air
from the room is pumped
into the bottle in which no
cloud formed and which was
dust free, a cloud will
quickly form. This experiment
shows that nuclei of
some kind are necessary to
start condensation from
vapor saturated air.
In dew formation, dust is
not necessary because the
sharp points and edges of
leaves and grasses which
have become cooled by radiation
serve as points of condensation.
Perhaps many of us have
wondered why the sky is
blue and the sunset and sunrise
red and golden, and we
would not at first think that
these phenomena are also due to dust.
The blue color of the sky is dependent
on the dust which very high up reflects
and refracts the short blue waves of
light to us.
The influence of dust on light can be
understood from a simple experiment.
Ordinary air which contains dust is first
passed over heated platinum coils, then
washed by bubbling through water, and
dried by bubbling a second time through
strong sulphuric acid. This process gives
air free from particles. The dust freed
air is then passed into a long cylinder.
When a ray of light is passed into the
cylinder from end to end in a dark
room, the cylinder remains dark when
viewed laterally. The path of the ray is
plainly visible in the outer air just before
it enters the cylinder and just after it
leaves it. If a small amount of ordinary
air is admitted gradually to the air
within the cylinder, a slight blue haze
slowly forms which gradually deepens
into blue, and on the admission of still
more air this blue fades into a light blue
and soon becomes the color of the light
from the source of illumination.
Ordinary air contains many particles
varying in size, wdiich reflect all the rays
of light and give white light, but in pure