66 Color of the Air and of Deep Woters. 
which is also observed in some other siliceous stones, and which is 
still more obvious in opaline glass. A weak solution of soap, is 
slightly blue; the jelly of ichthyocolla is more so, and an infu- 
sion of the bark of the large chesnut tree, (maronnier) which is per- 
fectly opaline, still more. Newton speaks of a wood which he calls ne- 
phritic, the infusion of which is opaline. In the Sicilian’ sea, at the 
mouth of the Giaretta, (the ancient Simethus) specimens of amber 
are found, which are in great request on account of their highly opa- 
line properties. 
A blue reflexion is also observed in certain bodies which are opake- 
white when reduced to plates thin enough to transmit light. <A fa- 
miliar example occurs in the skin covering the veins which trans- 
mits a blue, although neither the skin nor the blood is of that color. 
The mixture of white with black and with transparent colors gives, 
in painting, numerous examples of opaline blue. 
This blue color is the only one which can be explained on the 
theory of thin plates, by supposing that the particles of opaline bod- 
ies have just the dimensions requisite to reflect the blue ray. This 
explanation derives some probability from observing that the color 
transmitted by these bodies is the complementary yellow of the re- 
flected blue. This theory however presents great difficulties, and it 
is not intended absolutely to admit it in this essay. 
The analogy between the colors of opaline substances and those 
observed in the air and waters, will become obvious by an examina- 
tion of their action on reflected and transmitted light, proving that 
the phenomena are owing to the same cause. 
Opaline glass is produced by mingling in the common metal of 
white glass, a portion of calcined bones, which gives a blue shade 
without much impairing the transparency. The bone powder ap- 
pears to be in a state of extreme division or in a kind of demi-solu- 
tion which does not disperse the transmitted light. 
The color of the light transmitted by opaline bodies varies accord- 
ing to the volume of the mass; it is yellow if the body is thin, and 
becomes successively orange and red in proportion to the increase of 
thickness. ‘The analogy of the air, with opaline substances is not 
only manifest, in the blue reflexion, but also in its action on trans- 
mitted light which becomes successively yellow, orange and red, ac- 
cording to the volume of air and the kind of aqueous vapors with 
which it is impregnated. When the sun is high and his light crosses 
only the purest and thinnest portions of the atmosphere to reach the 
clouds, they are white with a slight tinge of yellow; they become some- 
