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tered. In the second column the condition of the platinum
tube is mentioned, and in the third the state of the air
which had entered the experimental tube.

Quantity of Air. State of Platinum Tube. State of Experimental Tube.
15 inches .............. Cold .............. Full of particles.
15 “ .............. Red hot ........... Optically empty.

The phrase “ optically empty ” shows that when the con-
ditions of perfect combustion were present, the floating
matter totally disappeared. It was wholly burnt up, leav-
ing no sensible residue. The experiment was repeated
many times, with the same invariable result.

The whole of the visible particles floating in the air of
London rooms being thus proved to be of organic origin,l
I sought to burn them up at the focus of a concave re-
flector. One of the powerfully convergent mirrors em-
ployed in my experiments on combustion by dark rays was
here made use of, but I failed in the attempt. Doubtless
the floating particles are in part transparent to radiant heat,
and are" so far incombustible by such heat. Their rapid
motion through the focus also aids their escape. They do
not linger there sufficiently long to be consumed. A flame
it was evident would burn them up, but I at first thought
the presenceof the flame would mask its own action among
the particles.

.1 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 dis-
tinguished 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
présente une remarque: 1a poussiere que l’on trouve a la surface de tous
les corps est soumise constamment a des courants d’air, qui doivent sou-
lever ses particules les plus légeres, au nombre desquelles se trouvent, sans
doute, de preference les corpuscules organisés, oeufs ou spores, moins
lourds généralement que les “particules minérales.”

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