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air there are no particles to
reflect light, so in the cylinder
there is at first a dark
space. When a little air
which has not been completely
purified is allowed to
enter, a few very small
motes of dust are introduced.
These very small
pieces reflect the blue light
waves, and then the space
within the cylinder takes on
the blue haze. On the other
hand, when a little more
ordinary air is admitted,
larger particles are added
which reflect nearly all rays,
giving white or yellow light.
When the sun is near
the horizon, either at sun­
DUSTY RAINSTORMS AND SUNSETS 245
rise or sunset, we have the beautiful
red, orange, and golden colors. We
look through dense strata of atmosphere
near the earth wdiich are filled with
the larger particles of dust. These reflect
the longer rays of light to us. Blue is
first reflected by the smaller particles,
leaving yellow; then the coarser dust
reflects green, leaving orange; then still
coarser pieces reflect orange and yellow,
leaving red. There are various combinations
of these colors, often intensified
by banks of clouds wdiich aid in the reflection
from their under surfaces.
The absence of dust from our atmosphere
might cause more serious troubles
than depriving us of the main colors
due to it. Without the minute particles
above, the sky would appear black just
IB
I
A
An Experiment in Refraction
The cvlinder A (I will glow when filled with ordinary air, containing dust particles.
hut will rema n black when containing dust-free air. B and (1 are stop-cocks. E
contains water lot washing, D holds sulphuric acid for drying the air
When There Is No Dust
Altli ..h then- is just as much moisture in the air contents of this flask as
then- is m tin llavk on the facing page, no cloud forms. This is because no
dust is present to aid condensation.
as our cylinder did when light was passed
through its dustless air. In a dustless
world, rain would seldom fall in sufficient
quantities to do plant life good.
and evaporation would be rapid, aiding
the formation of moisture-laden air and
arid tracts of land.
While dust is beneficial, its presence
may also be dangerous. Very fine dust
of any kind mixed with the right proportion
of air is explosive, and many of the
explosions in our flour mills, grain elevators,
and coal mines are due to this
cause. Sometimes the dust-laden air of
a