eat fully
of the subject, and which you may study.

But practically you should know which colors are to be depended on and
which not. Let us consider the principal colors in detail then, merely
as to their actual stability. I will speak of them in connection with
the plates of colors at the end of this book. I would like you to
compare what is said of each color with the corresponding color in the
plates. Those colors in the plates which are not spoken of here, you
may consider as useful in showing you the character of different
colors which are made, but which may or may not be used, according as
you may need them. I shall not attempt to mention all the pigments
that are in the market. You need never use more than fifteen or twenty
all told. Many painters use more, it is true; but if you know how to
make the best use of that number, you may safely wait till you "grow
to them" before you bother with more. And I shall speak only of those
which you will find essential or most generally useful, and those
which should be particularly avoided.

=Permanency.=--It should be stated what is meant by a permanent color.
There is no color which is not to be influenced in some way. The most
sound of pigments will change if the conditions favor the change. When
we speak of a permanent color, we mean only one which under the usual
conditions will stand for an indefinite time. By which is meant
ordinary diffused daylight, not direct sunlight, and the ordinary air
under normal conditions. If there be direct sunlight, you may expect
your picture to change sooner or later. But one does not hang his
pictures where the sun's rays will fall on them. If there is any
exceptional condition of moisture in the air, the picture may suffer.
Or if from any cause unusual gases are in the atmosphere, or if the
picture be too long in a dark, close place, the picture may smother
for lack of fresh air, just as any other thing, plant or animal, which
depends on normal conditions of atmosphere would do.

Let us say, then, that what we mean by a permanent color is one which
will stand unchanged for an indefinite length of time in a room which
is of the usual condition of temperature and freedom from moisture,
and where the light is diffused, and such that the direct rays of the
sun are not on the picture often, or to any great extent. Cold will
not hurt a picture if the canvas is not disturbed in that condition,
but to bend or roll it while it is very co