Color of the Air and of Deep Waters. 75 
As transparent colors in oil lose almost wholly the color which they 
have in a _pniverulent state, and thus in mass approach to black, the 
mixture of them with white produces also opaline blue, which modi- 
fies the natural shade of the color. 
Every painter knows the striking difference there is between the 
color of a mixture of cochineal lacker with white, and that which 
the same lacker produces as a thin coating upon a white ground; the 
first is of a violet color, and the second has all the purity and splen- 
dor which is characteristic of this fine color. Thus artists who wish 
to obtain the beautiful red of cochineal or madder in their draperies, 
always employ these lackers in mixture (en glacis.) Opake re- 
flecting colors, such as Naples yellow, chromate of lead, yellow 
ochre, produce, as well as white lead, opaline blue, by a mixture with 
black and the effect is still more sensible. ‘These compounds, ac- 
cording to theory, ought to give only shades of yellow; and yet 
their tints are decidedly green, so that they are often used for paint- 
ing the deepest verdure of landscapes. In these cases it is the 
opake reflecting color which is opaline. 
Ihave stated the most remarkable instances of the siugular prop- 
erty which certain colors possess of producing opaline blue by mix- 
ture, but there is an infinite number of other modifications less appa- 
rent, resulting from mixtures of compound colors, which it would 
be impossible to describe, but which may always be pre-ascertained 
by the following rule: When white lead, or opake reflecting colors 
are mixed with black or with transparent colors, there is a produc- 
tion of blue, and a consequent modification of the primitive shade 
of the coloring matter. 
These modifications are often very slight, but they do not escape 
attentive observers. In the preceding observations, I have described 
effects, well known it is true, but which appear to have no analogy to 
each other, and which appear to me to depend wholly on the pecu- 
liar property which the blue ray possesses of being reflected, in pref- 
erence to other rays more or less refrangible, by the simple mechan- 
ical resistance of the molecules of bodies which transmit light. ‘This 
resistance takes place in large masses of transparent fluids, as in air 
mixed with watery vapor, and in water mixed with air. 
It takes place also in opake bodies which are less transparent, but 
under smaller dimensions. Lastly, it is observed in white opake or 
colored bodies, as in the fine skin which covers the veins and in mix- 
tures of colors. 
