e use of floating dust to reveal the paths of
luminous beams through the air; but until 1868 I did not intentionally
reverse the process, and employ a luminous beam to reveal and examine
the dust. In a paper presented to the Royal Society in December, 1869,
the observations which induced me to give more special attention to
the question of spontaneous generation, and the germ theory of
epidemic disease, are thus described:

The Floating Matter of the Air.

Prior to the discovery of the foregoing action (the chemical action of
light upon vapours, Fragment IV.), and also during the experiments
just referred to, the nature of my work compelled me to aim at
obtaining experimental tubes absolutely clean upon the surface, and
absolutely free within from suspended matter. Neither condition is,
however, easily attained.

For however well the tubes might be washed and polished, and however
bright and pure they might appear in ordinary daylight, the electric
beam infallibly revealed signs and tokens of dirt. The air was always
present, and it was sure to deposit some impurity. All chemical
processes, not conducted in a vacuum, are open to this disturbance.
When the experimental tube was exhausted, it exhibited no trace of
floating matter, but on admitting the air through the U-tubes
(containing caustic potash and sulphuric acid), a _dust-cone_ more or
less distinct was always revealed by the powerfully condensed electric
beam.

The floating motes resembled minute particles of liquid which had been
carried mechanically from the U-tubes into the experimental tube.
Precautions were therefore taken to prevent any such transfer. They
produced little or no mitigation. I did not imagine, at the time,
that the dust of the external air could find such free passage through
the caustic potash and sulphuric acid. This, however, was the case;
the motes really came from without. They also passed with freedom
through a variety of aethers and alcohols. In fact, it requires
long-continued action on the part of an acid first to wet the motes
and afterwards to destroy them. By carefully passing the air through
the flame of a spirit lamp, or through a platinum tube heated to
bright redness, the floating matter was sensibly destroyed. It was
therefore combustible, in other words, organic, matter. I tried to
intercept it by a large respirator of cotton-wool. Close pressure was
necessary to render the wool effective. A plug of the woo