POPULAR scmrrriFic LTXTUIIES. 



609 



nrisos from a fine opacity in the atmos- 

 phere, which never entirely disappears. If, 

 in. ft transparent medium, there are fino 

 transparent particles of varying density and 

 varying refrangibility, in so far as they aro 

 struck by it, they deflect the light passing 

 through such a medium, partly by reflection 

 and partly by refraction ; to use an optical 

 expression, they scatter it in all directions. If 

 the opaquo particles are sparsely distributed, 

 so that a great part of the light can pass 

 through them without being deflected, distant 

 objects are seen in sharp, well-defined outlines 

 throxigh such a medium, while at the same 

 time a portion of the light which is deflected 

 is distributed in the transparent medium as 

 an opaque halo. Water rendered turbid by 

 a few drops of milk shows this dispersion of 

 the light and cloudiness very distinctly. The 

 light in this case is deflected by the micro- 

 Bcopic globules of butter which are suspended 

 in the milk. 



In the ordinary air of our rooms this tur- 

 bidity is very apparent when the room is 

 closed, and a ray of sunlight is admitted 

 through a narrow aperture. We see then 

 some of these solar particles, large enough to 

 be distinguished by the naked eye, while 

 others form a fine homogeneous turbidity. 

 But even the latter must consist mainly of 

 suspended particles of organic substances, 

 for, according to an observation of Tyndall, 

 they can be burnt. If the flame of a spirit 

 lamp is placed directly below the path of 

 these rays, the air- rising from the flame 

 stands out quite dark in the surrounding 

 bright turbidity ; that is to say, the air ris- 

 ing from the flame has been quite freed 

 from dust. In the open air, besides dust 

 nnd occasional smoke, we must often also 

 take into account the turbidity arising from 

 incipient aqueous deposits, where the tem- 

 perature of moist air sinks so far that the 

 water retained in it can no longer exist as 

 invisible vapor. Part of the water settles 

 then in the form of fine drops, as a kind of 

 the very finest aqueous dust, and forms a 

 finer or denser fog ; that is to say, cloud. 

 The turbidity which forms in hot eunshino 

 and dry air may arise, partly from dust which 

 the ascending currents of warm air whirl 

 about ; and partly from the irregular mix- 

 ture of cold and warm layers of air of cliffer- 

 eat densitj', as is seen in the tremulous mo- 

 tion of the lower layers of air over surfaces 

 irradiated by the sun. But science can as 

 yet give no explanation ot' the turbidity in 

 the higher regions of the atmosphere which 

 produces the blue of the sky ; we do not 

 know whether it arises from suspended par- 

 ticles of foreign substances, or whether the 

 molecules of air themselves may not act as 

 turbid particles in the luminous ether. 



The color of the light reflected by the 

 opaque particles mainly depends on their 

 magnitude. When a block of wood floats on 

 water, and by a succession of falling drops 

 we produco small wave-rings near it, these 

 are repelled by the floating wood as if it 

 were a solid wall. But in the long waves of 



the *ea a block of wood wotld bo rocked 

 about without the waves being thereby ma- 

 terially disturbed in their progress. Now 

 light is well known to be an undulatory mo- 

 tion of the ether which fills all space. Tho 

 red and yellow rays have the longest waves, 

 the blue and violet the shortest. Very fine 

 particles, therefore, which disturb the uni- 

 formity of the ether, will, accordingly, reflect 

 the latter rays more markedly than the red 

 and yellow rays. The light of turbid media 

 is bluer, the finer are the opaque particles i 

 while tho larger particles of uniform light 

 reflect all colors, and therefore give a whitish 

 turbidity. Of this kind is tho celestial blue, 

 that is, tho color of the turbid atmosphere aV 

 seen against dark cosmical space. The pure* 

 and the more transparent the nir, the bluer 

 is the sky. In like manner it is bluer and 

 darker -when we ascend high rcountain.% 

 partly because tho air at great heights is free* 

 from turbidity, and partly because there in 

 less air above us. But the same blue, which 

 is seen against the dark celestial space, also 

 occurs against dark terrestrial objects ; for 

 instance, when a thick layer of illuminated 

 air is between us and masses of deeply 

 shaded or wooded hills. Tho same aerial 

 light makes the sky blue, ns well cs tho 

 mountains ; excepting that in tho forrart 

 case it is pure, while in the latter it is mixei? 

 with the light from objects behind ; an<? 

 moreover it belongs to the coarser turbidity 

 of the lower regions of the atmosphere, so 

 that it is whiter. In hot countries, and with 

 dry air, the aerial turbidity is also finer in 

 the lower regions of the air, nnd therefor* 

 the blue in front of distant terrestrial ob- 

 jects is more like that of the sky. The clear- 

 ness and the pure colors of Italian landscapes 

 depend mainly on this fact. On high moun- 

 tains, particularly in the morning, the aerial 

 turbidity is often no slight that the colors of 

 the most distant objects can scarcely be dis- 

 tinguished from those of the nearest. The 

 sky may then appear almost bluish-black. 



Conversely, the denser turbidity consists 

 mainly of coarser particles, and is therefore 

 whitish. As a rule, this is the case in thfr 

 lower layers cf nir, and in states of weather 

 in which the aqueous vapor in the air is near 

 its point of condensation. 



On the other hand, the liqht which reaches 

 the eye of the observer after having pnsscd 

 through a long layer of nir, has been robbed 

 of part of its violet and blue by scattered re 

 flections ; it therefore appears yellowish to 

 reddish yellow or red, the former when the 

 turbidity is fine, the latter when it is coarse. 

 Thus the sun and the moon t their rising 

 and setting, and ulso distiint brightly illu- 

 minated mountain-tops, especially snow- 

 mountains, appear colored. 



These colorations are moreover not pecul- 

 iar to the air, but occur in all cases in which 

 a transparent suVmtnnon is made turbid by 

 the admixture of another transparent HBb- 

 stsincc. Wo HOC it, as we have observed, in 

 diluted milk, and in water to which a few 

 drops of euu dc Cologne Lave been added, 



