hem is soluble in the medium, then
chemical action commences; but as most pigments are somewhat soluble,
there is always some danger in mixing them. The best we can do is, as
I said before, to try to have on the palette, as far as possible,
only colors which are friendly to each other.

As a student you should not be much occupied, however, with all this.
You must expect that all color will change somewhat. But you need not
use those which change immediately or markedly, and you may use them
in a way which will tend to make them change as little as may be.
Colors have stood for years, and what is practical permanence, not
perfect permanence, is all you need look for. If you think too much of
the permanence of your colors, it will interfere with the directness
of your study. Therefore, decide on a palette which is as complete and
safe as you can make it, excluding the notably bad pigments, and think
no more about it.

When you need to add a new color to your palette, choose it with
reference to those already on it, and go ahead. This is what the whole
subject resolves itself to, practically, for you as a student.

=Opaque and Transparent Colors.=--Some colors, like the madders, have
a jelly-like consistency when mixed with oil, others, the earths among
them, are dense and opaque. We speak of them respectively as
"transparent" and "solid" colors. These qualities, which divide the
paints into two classes, have no relation to their permanency. As far
as that is concerned you use them in the same way, as some
transparent colors are safe and some fugitive; and the same with the
opaque colors.

The only difference is in the fact that, as a rule, the solid colors
are better dryers. But you will notice that while you may mix these
colors together as though this difference between them did not exist,
in certain processes you use them differently. So you will see,
farther on, that for a "glaze" you can use only the transparent or
semi-opaque colors, for a scumble you naturally use the solid ones.
You should know, however, for the sake of clearness, just what is
meant when "solid" or "body" or "opaque" color is spoken of, and what
is meant by "transparent" color.

=Safe and Unsafe Colors.=--Beyond what has been said of the causes of
change in colors it is not necessary that you should know the chemical
constituents of them. If you want to look into the matter further
there are books, such as "Field's Chromatography," which tr