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rises, and comes between the eye and the bright atmosphere, it at
once shows of an ashy grey colour; and this happens because it no
longer has darkness beyond it, but this bright and luminous space.
If the smoke is from young, green wood, it will not appear blue,
because, not being transparent and being full of superabundant
moisture, it has the effect of condensed clouds which take distinct
lights and shadows like a solid body. The same occurs with the
atmosphere, which, when overcharged with moisture appears white, and
the small amount of heated moisture makes it dark, of a dark blue
colour; and this will suffice us so far as concerns the colour of
the atmosphere; though it might be added that, if this transparent
blue were the natural colour of the atmosphere, it would follow that
wherever a larger mass air intervened between the eye and the
element of fire, the azure colour would be more intense; as we see
in blue glass and in sapphires, which are darker in proportion as
they are larger. But the atmosphere in such circumstances behaves in
an opposite manner, inasmuch as where a greater quantity of it lies
between the eye and the sphere of fire, it is seen much whiter. This
occurs towards the horizon. And the less the extent of atmosphere
between the eye and the sphere of fire, the deeper is the blue
colour, as may be seen even on low plains. Hence it follows, as I
say, that the atmosphere assumes this azure hue by reason of the
particles of moisture which catch the rays of the sun. Again, we may
note the difference in particles of dust, or particles of smoke, in
the sun beams admitted through holes into a dark chamber, when the
former will look ash grey and the thin smoke will appear of a most
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