Color of the Air and of Deep Waters. 67 
times yellow and orange as the sun declines; and at length red and 
purple, when his light grazes the earth and is transmitted by the den- 
sest portion of the air and loaded with the vapors of the evening. 
But it often happens that the colors do not appear and that the 
sun sets without producing them. It is not therefore to the purity 
of the air alone that we must attribute the opaline property of the 
atmosphere, but tothe mixture of air and vapor mingled in a special 
manner, and producing an effect similar to bone dust in opaline glass, 
neither is it the quantity of water in the air which occasions the 
colors, for when the weather is very damp, it is more transparent 
than during a time of drought. Distant mountains are seen more 
distinctly, a well known prognostic of rain; the sun then sets with- 
out producing colors, it looks white through the fog and damp va- 
pors of the morning, but when the clouds are colored red by the 
setting sun, the phenomenon is generally deemed the signal of fine 
weather, because these colors are a proof of the dryness of the air 
when these contain only the peculiar diffused vapors which give it 
its opaline quality. In this state of things, the disc of the sun appears 
like a red fiery globe divested of rays. 
The blueness of the sky therefore varies according to the kind of 
vapor which is spread through the air; and what renders it uuquestiona- 
ble that its blue color is caused by these vapors is, that it appears black 
when seen from the highest points of the globe, above which there is 
not sufficient vapor to reflect the blue color. 
Limpid waters, when they have sufficient depth, reflect, like air, 
a blue color from below; it is of a deeper shade because it is not 
mixed with white light; very often it is not perceived at all; the re- 
flexion from the surface on which the sky and surrounding objects 
are painted as in a mirror, often occasions the disappearance of the 
internal reflexion or forms with it complex shades. 
We have seen that the property which air possesses of producing 
colors is derived from the presence of watery vapor; analogy leads 
us to presume that this property im water arises from a mixture of air 
which it always contains to a greater or less amount. 
Although the blue color of water is often masked by numerous 
causes it is sometimes exhibited in all its intensity; a fine example 
of it is witnessed in looking at the Rhone from the Bridge at Gene- 
va. The river seems to flow from an ultramarine* source. The 
spectator is in the most favorable situation for observing the internal 
® Having the blue color of the ultramarine paint. 
& Pp 
