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STRUCTURE AND LIGHT OF THE SKY. 2:39

waves of light. The mixed air- and vapor are sufficient to
depress the mercurial column one inch. I add to this mix-
ture air, which has been permitted to bubble through dilute
hydrochloric acid, until the column is depressed thirty inches:
in other words, until the tube is full. And now I permit
the electric beam to play upon the mixture. For some
time nothing is seen. The chemical action is doubtless
progressing, and condensation is going on; but the con-
densing molecules have not yet coalesced to particles suffi-
ciently large to reflect sensibly the waves of light. As
before stated—and the statement rests upon an experimental
basis—the particles here generated are at first so small that
their diameters would probably have to be expressed in
millionths of an inch ; while to form each of these particles
whole crowds of molecules are probably aggregated. Helped
by such considerations the intellectual vision plunges more
profoundly into atomic Nature, and shows us, among other
things, how far we are from the realization of Newton’s
hope that the molecules might one day be seen by micro-
scopes. While I am speaking, rou observe this delicate blue
color, forming and strengthening within the experimental
tube. N o sky-blue could exceed it in richness and purity ;
but the particles which produce this color lie wholly beyond
our microscopic range. A uniform color is here developed,
which has as little breach of continuity—which yields as
little evidence of the. particles concerned in its production,
as that yielded by a body whose color is due to true mo-
lecular absorption. This blue is at first as deep and dark
as the sky seen from the highest Alpine peaks, and for the
same reason. But it grows gradually brighter, still main-
taining its blueness, until at length a whitish tinge mingles
with the pure azure ; announcing that the particles are now
no longer of that infinitesimal size which mainly scatters
the shortest waves.1

1 Possibly a photographic impression might be taken long before the
blue becomes visible, for the ultra-blue rays are first reflected.

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