rent blue liquid, in which, at various
elevations, clouds are suspended, those clouds being themselves only
particular visible spaces of a substance with which the whole mass of
this liquid is more or less impregnated. Now, we all know this perfectly
well, and yet we so far forget it in practice, that we little notice the
constant connection kept up by nature between her blue and her clouds,
and we are not offended by the constant habit of the old masters, of
considering the blue sky as totally distinct in its nature, and far
separated from the vapors which float in it. With them, cloud is cloud,
and blue is blue, and no kind of connection between them is ever hinted
at. The sky is thought of as a clear, high material dome, the clouds as
separate bodies, suspended beneath it, and in consequence, however
delicate and exquisitely removed in tone their skies may be, you always
look _at_ them, not _through_ them. Now, if there be one characteristic
of the sky more valuable or necessary to be rendered than another, it is
that which Wordsworth has given in the second book of the Excursion:--

"The chasm of sky above my head
Is Heaven's profoundest azure. No domain
For fickle, short-lived clouds, to occupy,
Or to pass through;--but rather an _abyss_
In which the everlasting stars abide,
And whose soft gloom, and boundless depth, might tempt
The curious eye to look for them by day."

And, in his American Notes, I remember Dickens notices the same truth,
describing himself as lying drowsily on the barge deck, looking not at,
but _through_ the sky. And if you look intensely at the pure blue of a
serene sky, you will see that there is a variety and fulness in its very
repose. It is not flat dead color, but a deep, quivering, transparent
body of penetrable air, in which you trace or imagine short, falling
spots of deceiving light, and dim shades, faint, veiled vestiges of dark
vapor; and it is this trembling transparency which our great modern
master has especially aimed at and given. His blue is never laid on in
smooth coats, but in breaking, mingling, melting hues, a quarter of an
inch of which, cut off from all the rest of the picture, is still
_spacious_, still infinite and immeasurable in depth. It is a painting
of the air, something into which you can see, through the parts which
are near you into those which are far off; something which h