placed in
succession in the path of the air, before it entered the liquid whose
vapour was to be carried into the experimental tube. One of the
U-tubes contained fragments of marble wetted with a strong solution of
caustic potash; the other, fragments of glass wetted with concentrated
sulphuric acid which, while yielding no vapour of its own, powerfully
absorbs the aqueous vapour of the air. [Footnote: The apparatus is
figured in Fig. 3.] To my astonishment, the air of the Royal
Institution, sent through these tubes at a rate sufficiently slow to
dry it, and to remove its carbonic acid, carried into the experimental
tube a considerable amounts 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.

I tried to intercept this floating matter in various ways; and on
October 5, 1868, prior to sending the air through the drying
apparatus, it was carefully permitted to pass over the tip of a
spirit-lamp flame. The floating matter no longer appeared, having
been burnt up by the flame. It was therefore _organic matter_. I was
by no means prepared for this result; having previously thought that
the dust of our air was, in great part, inorganic and non-combustible.
[Footnote: According to an analysis kindly furnished to me by Dr.
Percy, the dust collected _from the walls_ of the British Museum
contains fully 50 per cent. of inorganic matter. I have every
confidence in the results of this distinguished chemist; they show
that the _floating_ dust of our rooms is, as it were, winnowed from the
heavier matter. As bearing directly upon this point I may quote the
following passage from Pasteur: 'Mais ici se presente une remarque: la
poussiere que Pon trouve a la surface de tous les corps est soumise
constamment a des courants d'air, qui doivent soulever des particules
les plus legeres, au nombre desquelles se trouvent, sans doute, de
preference les corpuscules organises, oeufs ou spores, moins lourds
generalement que les particules minerales.']

I had constructed a small gas-furnace, now much employed by chemists,
containing a platinum tube, which could be heated to vivid redness.
[Footnote: Pasteur was, I believe, the first to employ such a tube.]
The tube contained a roll of platinum gauze, which, while it permitted
the air to pass through it, ensur