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
behave very differently. Dusty air itself is only a kind of smoke, and
it looks bright, and the thicker the smoke the brighter it looks; the
blackness is simply the utter absence of smoke; there is nothing at
all for the light to illuminate, accordingly we have the blankness of
sheer invisibility. Here is a flame burning under the beam, and, to
show what real smoke looks like, I will burn also this spirit lamp
filled with turpentine instead of alcohol. _Why_ the convention
currents were free from dust was unknown; Tyndall thought the dust was
burnt and consumed; Dr. Frankland thought it was simply evaporated.

In 1881 Lord Rayleigh took the matter up, not feeling sa