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 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.
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