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DUST AND DISEASE. 305

varied and ingenious attack. On this point I will only say
that when feeling escapes‘from behind the intellect, where
it is a useful urging force, and places itself in front of the
intellect, it is liable to produce glamour and all manner
of delusions. Thus my censors, for the most part, have lev-
elled their remarks against positions which I never assumed,
and against claims which I never made. The simple his-
tory of the matter is this: During the autumn of 1868 I
was much occupied with the observations referred to at the
beginning of this discourse. For fifteen years I had ha-
bitually employed the electric light, making use of the
floating dust to reveal the paths of luminous beams; but
until 1868, when I was driven to it, 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, I thus described the observa-
tions which induced me to give more Special attention to
the question of spontaneous generation and the germ-
theory of epidemic disease.

The Floating Matter of the Air.

Prior to the discovery of the foregoing action (the chemical action of
light upon vapors), 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 empty within. Neither
condition is, however, easily attained.

For however well the tubes might be washed and polished, and how-
ever 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 pro-
cesses, 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 re-
vealedby the powerfully-condensed electric beam.

The floating motes resembled minute particles of liquid which had

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