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to dry it, and to remove its carbonic acid, carried into the
experimental tube a considerable amount of mechanically
suspended matter, which was illuminated when the beam
passed through the tube. The effect was substantially the
same when the air was permitted to bubble through the
liquid acid and through the solution of potash.

Thus, on October 5, 1868, successive charges of air were
admitted through the potash and sulphuric acid into the ex-
hausted experimental tube. Prior to the admission of the
air the tube was optically empty ; it contained nothing
competent to scatter the light. After the air had entered
the tube, the conical track of the electric beam was in all
cases clearly revealed. This, indeed, was a daily observa-
tion at the time to which I now refer.

I tried to intercept this floating matter in various ways;
and on the day just mentioned, prior to sending the air
through the drying apparatus, I carefully permitted it to
pass over the tip of a spirit-lamp flame. The floating mat-
ter no longer appeared, having been burnt up by the flame.
It was, therefore, of organic origin. I was by no means
prepared for this result; for I had thought that the dust of
our air Was, in great part, inorganic and non-combustible.

I had constructed a small gas-furnace, now much em-_
ployed by chemists, containing a platinum tube, which
could be heated to vivid redness.1 The tube contained a
roll of platinum gauze, which, while it permitted the air to
pass through it, insured 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. The rapidity of admission Was also varied.
In the first column of the following table the quantity of
air operated on is expressed by the number of inches which
the mercury gauge of the air-pump sank when the air en-

1 Pasteur was, I believe, the first to employ such a tube.

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