be found always yet each
found but once; it is through these that the lesson of devotion is
chiefly taught, and the blessing of beauty given. These are what the
artist of highest aim must study; it is these, by the combination of
which his ideal is to be created; these, of which so little notice is
ordinarily taken by common observers, that I fully believe, little as
people in general are concerned with art, more of their ideas of sky are
derived from pictures than from reality, and that if we could examine
the conception formed in the minds of most educated persons when we talk
of clouds, it would frequently be found composed of fragments of blue
and white reminiscences of the old masters.

I shall enter upon the examination of what is true in sky at greater
length, because it is the only part of a picture of which all, if they
will, may be competent judges. What I may have to assert respecting the
rocks of Salvator, or the boughs of Claude, I can scarcely prove, except
to those whom I can immure for a month or two in the fastnesses of the
Apennines, or guide in their summer walks again and again through the
ravines of Sorrento. But what I say of the sky can be brought to an
immediate test by all, and I write the more decisively, in the hope that
it may be so.

Sec. 5. Nature and essential qualities of the open blue.

Sec. 6. Its connection with clouds.

Sec. 7. Its exceeding depth.

Sec. 8. These qualities are especially given by modern masters.

Sec. 9. And by Claude.

Sec. 10. Total absence of them in Poussin. Physical errors in his general
treatment of open sky.

Sec. 11. Errors of Cuyp in graduation of color.

Let us begin then with the simple open blue of the sky. This is of
course the color of the pure atmospheric air, not the aqueous vapor, but
the pure azote and oxygen, and it is the total color of the whole mass
of that air between us and the void of space. It is modified by the
varying quantity of aqueous vapor suspended in it, whose color, in its
most imperfect, and therefore most visible, state of solution, is pure
white, (as in steam,) which receives, like any other white, the warm
hues of the rays of the sun, and, according to its quantity and
imperfect solution, makes the sky paler, and at the same time more or
less gray, by mixing warm tones with its blue. This gray aqueous vapor,
when very decided, becomes mist, and when local, cloud. Hence the sky is
to be considered as a transpa