ed the practical contact of the dust
with the incandescent metal. The air of the laboratory was permitted
to enter the experimental tube, sometimes through the cold, and
sometimes through the heated, tube of platinum. In the first column
of the following fragment of a long table the quantity of air operated
on is expressed by the depression of the mercury gauge of the
air-pump. In the second column the condition of the platinum tube is
mentioned, and in the third the state of the air in the experimental
tube.

Quantity of air State of platinum tube State of experimental tube

15 inches Cold Full of particles.

30 inches Red-hot Optically empty.

The phrase 'optically empty' shows that when the conditions of perfect
combustion were present, the floating matter totally disappeared.

*****

In a cylindrical beam, which strongly illuminated the dust of the
laboratory, I placed an ignited spirit-lamp. Mingling with the flame,
and round its rim, were seen curious wreaths of darkness resembling an
intensely black smoke. On placing the flame at some distance below
the beam, the same dark masses stormed upwards. They were blacker
than the blackest smoke ever seen issuing from the funnel of a
steamer; and their resemblance to smoke was so perfect as to lead the
most practised observer to conclude that the apparently pure flame of
the alcohol lamp required but a beam of sufficient intensity to reveal
its clouds of liberated carbon. But is the blackness smoke? This
question presented itself in a moment and was thus answered: A red-hot
poker was placed underneath the beam: from it the black wreaths also
ascended. A large hydrogen flame was next employed, and it produced
those whirling masses of darkness, far more copiously than either the
spirit-flame or poker. Smoke was therefore out of the question.
[Footnote: In none of the public rooms of the United States where I
had the honour to lecture was this experiment made. The organic dust
was too scanty. Certain rooms in England--the Brighton Pavilion, for
example--also lack the necessary conditions.]

What, then, was the blackness? It was simply that of stellar space;
that is to say, blackness resulting from the absence from the track of
the beam of all matter competent to scatter its light. When the flame
was placed below the beam the floating matter was destroyed _in situ_;
and the air, freed from