compared with those which are able to act as condensers of moisture.
The fine dust of Mr. Aitken exists everywhere, even in the upper
regions of the atmosphere; many of its particles are of
ultra-microscopic fineness, one of them must exist in every raindrop,
nay, even in every spherule of a mist or cloud, but it is only
occasionally that one can find them with the microscope. It is to such
particles as these that we owe the blue of the sky, and yet they are
sufficiently gross and tangible to be capable of being filtered out of
the air by a packed mass of cotton-wool. Such dust as this, then, we
need never be afraid of being without. Without it there could be no
rain, and existence would be insupportable, perhaps impossible; but it
is not manufactured in towns; the sea makes it; trees and wind make
it; but the kind of dust made in towns rises only a few hundred yards
or so into the atmosphere, floating as a canopy or pall over those
unfortunate regions, and sinks and settles most of it as soon as the
air is quiet, but scarcely any of it ever rises into the upper regions
of the atmosphere at all.

Dust, then, being so universally prevalent, what do I mean by
dust-free spaces? How are such things possible? And where are they to
be found? In 1870 Dr. Tyndall was examining dusty air by means of a
beam of light in which a spirit-lamp happened to be burning, when he
noticed that from the flame there poured up torrents of apparently
thick black smoke. He could not think the flame was really smoky, but
to make sure he tried, first a Bunsen gas flame and then a hydrogen
flame. They all showed the same effect, and smoke was out of the
question. He then used a red-hot poker, a platinum wire ignited by an
electric current, and ultimately a flask of hot water, and he found
that from all warm bodies examined in dusty air by a beam of light the
upstreaming convection currents were dark. Now, of course smoke would