doned towers of old cathedrals, and at the bottom of the
hypogea in ancient Egypt. Futile pains! It is not the
dust which falls and is deposited that interests us! You
will find therein only the heaviest parts of what the wind
carries, mineral particles, grains of starch or of pollen,
the spores of cryptogams or even bits of down, of cotton,
of wool from the living sheep or from our garments. It
is not these particles which we must study, but rather
those which we see dancing without repose in a ray of
sunlight, and which the air contains in the state of a
permanent suspension."

"Furthermore, your study of the dust of cathedrals
gives you no indication of quantity. What is the volume

FIG. 9. --Dust of the air caught by aspiration in the meshes of gun-
cotton.

of air which has deposited the little mass which you have
studied, and subjected to microscopic examination?
You do not know, and consequently your experiments
may well open the question, but they do nothing to solve
it."

"Nevertheless how easy the thing is! Let us take
the cotton filter of Schroeder and Dusch, and replace
it only by gun-cotton, and when by it we have arrested
in its passage the dust in a determined volume of air,
let us throw the gun-cotton into a mixture of alcohol and
ether in which it is soluble. All the weft of the filter
is dissolved. The particles of dust which have been
caught in the meshes are set at liberty and fall to the

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