our aether-waves untie the bond of chemical affinity, and
liberate a body--sulphur--which at ordinary temperatures is a solid,
and which therefore soon becomes an object of the senses. We have
first of all the free atoms of sulphur, which are incompetent to stir
the retina sensibly with scattered light. But these atoms gradually
coalesce and form _particles_, which grow larger by continual accretion,
until after a minute or two they appear as sky-matter. In this
condition they are individually invisible; but collectively they send
an amount of wave-motion to the retina, sufficient to produce the
firmamental blue. The particles continue, or may be caused to
continue, in this condition for a considerable time, during which no
microscope can cope with them. But they grow slowly larger, and pass
by insensible gradations into the state of _cloud_, when they can no
longer elude the armed eye. Thus, without solution of continuity, we
start with matter in the atom, and end with matter in the mass;
sky-matter being the middle term of the series of transformations.
Instead of sulphurous acid, we might choose a dozen other substances,
and produce the same effect with all of them. In the case of
some--probably in the case of all--it is possible to preserve matter
in the firmamental condition for fifteen or twenty minutes under the
continual operation of the light. During these fifteen or twenty
minutes the particles constantly grow larger, without ever exceeding
the size requisite to the production of the celestial blue.

Now when two vessels are placed before us, each containing sky-matter,
it is possible to state with great distinctness which vessel contains
the largest particles. The eye is very sensitive to differences of
light, when, as in our experiments, it is placed in comparative
darkness, and the wave-motion thrown against the retina is small. The
larger particles declare themselves by the greater whiteness of their
scattered light. Call now to mind the observation, or effort at
observation, made by our President, when he failed to distinguish the
particles of mastic in Bruecke's medium, and when you have done this,
please follow me.

A beam of light is permitted to act upon a certain vapour. In two
minutes the azure appears, but at the end of fifteen minutes it has
not ceased to be azure. After fifteen minutes its colour, and some
other phenomena, pronounce it to be a blue of distinctly smaller
particles t