the total mystery of
volatile power in substance; and of the visible states consequent
on sudden--and presumably, therefore, imperfect--vaporization; as
the smoke of frankincense, or the sacred fume of modern devotion
which now fills the inhabited world, as that of the rose and violet
its deserts. What,--it would be useful to know, is the actual bulk
of an atom of orange perfume?--what of one of vaporized tobacco, or
gunpowder?--and where do _these_ artificial vapors fall back in
beneficent rain? or through what areas of atmosphere exist, as
invisible, though perhaps not innocuous, cloud?

All these questions were put, closely and precisely,
four-and-twenty years ago, in the 1st chapter of the 7th part of
'Modern Painters,' paragraphs 4 to 9, of which I can here allow
space only for the last, which expresses the final difficulties of
the matter better than anything said in this lecture:--

"But farther: these questions of volatility, and visibility, and
hue, are all complicated with those of shape. How is a cloud
outlined? Granted whatever you choose to ask, concerning its
material, or its aspect, its loftiness and luminousness,--how of
its limitation? What hews it into a heap, or spins it into a web?
Cold is usually shapeless, I suppose, extending over large spaces
equally, or with gradual diminution. You cannot have in the open
air, angles, and wedges, and coils, and cliffs, of cold. Yet the
vapor stops suddenly, sharp and steep as a rock, or thrusts itself
across the gates of heaven in likeness of a brazen bar; or braids
itself in and out, and across and across, like a tissue of
tapestry; or falls into ripples, like sand; or into waving shreds
and tongues, as fire. On what anvils and wheels is the vapor
pointed, twisted, hammered, whirled, as the potter's clay? By what
hands is the incense of the sea built up into domes of marble?"]

[Footnote 9: The opposed conditions of the higher and lower orders
of cloud, with the balanced intermediate one, are beautifully seen
on mountain summits of rock or earth. On snowy ones they are far
more complex: but on rock summits there are three distinct forms of
attached cloud in serene weather; the first that of cloud veil
laid over them, and _falling_ in folds through their ravines,
(the obliquely descending clouds of the entering chorus in
Aristophanes); secondly, the ascending cloud, which develops itself
loosely and independently as it rises, and does not attach itself
to t