In spring we have a bluer sky and greater transparency of the
atmosphere; in autumn, even on very fine days, there is always a kind of
yellowish haze, resulting in a want of clearness in the air and purity
of colour in the sky. These phenomena are quite intelligible when we
consider that during winter less dust is formed, and more is brought
down to the earth by rain and snow, resulting in the transparent
atmosphere of spring, while exactly opposite conditions during summer
bring about the mellow autumnal light. Again, the well-known beneficial
effects of rain on vegetation, as compared with any amount of artificial
watering, though, no doubt, largely due to the minute quantity of
ammonia which the rain brings down with it from the air, must yet be
partly derived from the organic or mineral particles which serve as the
nuclei of every raindrop, and which, being so minute, are the more
readily dissolved in the soil and appropriated as nourishment by the
roots of plants.
It will be observed that all these beneficial effects of dust are due to
its presence in such quantities as are produced by natural causes, since
both gentle showers as well as ample rains and deep blue skies are
present throughout the vast equatorial forest districts, where
dust-forming agencies seem to be at a minimum.
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