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 satisfied with
these explanations, and repeated the experiment very carefully. He
noted several new points, and hit on the capital idea of seeing what a
cold body did. From the cold body the descending current was just as
dark and dust-free as from a warm body. Combustion and evaporation
explanations suffered their death-blow. But he was unable to suggest
any other explanation in their room, and so the phenomenon remained
curious and unexplained.

In this state Mr. Clark and I took the matter up last summer, and
critically examined all sorts of hypotheses that suggested themselves,
Mr. Clark following up the phenomena experimentally with great
ingenuity and perseverance. One hypothesis after another suggested
itself, seemed hopeful for a time, but ultimately had to be discarded.
Some died quickly, others lingered long. In the examination of one
electrical hypothesis which suggested itself we came across various
curious phenomena which we hope still to follow up.[2] It was some
months before what we now believe to be the true explanation began to
dawn upon us. Meanwhile we had acquired various new facts, and first
and foremost we found that the dark plane rising from a warm body was