ctically duplications of
color-quality in pigments which are bad, and in pigments which are
good; so that you can use the good color instead of the bad one to do
the same work. The good color will cost more, but there is no way of
making the bad color good, so you must pay the difference due to the
cost of the better material, or put up with the result of using bad
colors.

=Chemical Changes.=--The causes of change of color in pigments are of
four kinds, all of them chemical effects. 1, the action of light; 2,
the action of the atmosphere; 3, the action of the medium; and 4, the
action of the pigments themselves on each other. The action of light
is to bring about or to assist in the decomposition of the pigment. It
is less marked in oil than in water color, because the oil forms a
sort of sheath for the color particles. The manner in which light does
its deteriorating work is somewhat similar to that of heat. The action
of light is very slow, but it seems to do the same thing in a long
time that heat would do in a short time.

Some colors are unaffected or little affected by light, and of course
you will use them in preference to all others. The atmosphere affects
the paint because of certain chemical elements contained in it, which
tend to cause new combinations with the materials which are already
in combination in the pigment. The action of the oxygen in the air is
the chief agent in affecting the pigment, and it is here particularly
that light, and especially sunlight, assists in decomposition. The air
of towns and cities generally contains sulphuric and sulphurous acids
and sulphuretted hydrogen. This latter gas is most effective in
changing oil paintings, because of its action in turning white lead
dark; and as white lead is the basis of many qualities in painting,
this gas may have a very general action.

Moisture in the atmosphere is also a cause of change, but there is
little to be dreaded from this, as the oil protects the colors.

Oil absorbs oxygen in drying, and so is apt to have an effect on
colors liable to change from that element, and many vehicles contain
materials to hasten the drying which further aid in the deterioration
of the pigment. Bad oil will tend to crack the picture also. The
greatest care should be used in this direction, as the most permanent
colors may be ruined by bad vehicles.

Pigments will not have a deteriorating effect on each other as long as
they are solid. But if one of t