NEW CHEMICAL REACTIONS. 87 
emergent beam being therefore also vertical. As the light 
continued to act, a superb blue cloud, visible to both my 
assistant and myself, was slowly formed. But this cloud, 
so deep and rich when looked at from the positions men- 
tioned, utterly disappeared when looked at vertically down- 
ward or vertically upward. Reflection from the cloud 
was not possible in these directions. When the large 
Nicol was slowly turned round its axis, the eye of the 
observer being on the level of the beam, and the line of 
vision perpendicular to it, entire extinction of the light 
emitted horizontally occurred when the longer diagonal of 
the large Nicol was vertical. But now a vivid blue cloud 
was seen when looked at downward or upward. This 
truly fine experiment, which I contemplated making on my 
own account, was first definitely suggested by a remark in 
a letter addressed to me by Professor Stokes. 
As regards the polarization of skylight, the greatest 
stumbling-block has hitherto been, that, in accordance 
with the law of Brewster, which makes the index of 
refraction the tangent of the polarizing angle, the reflec- 
tion which produces perfect polarization would require to 
be made in air upon air; and indeed this led many of our 
most eminent men, Brewster himself among the number, 
to entertain the idea of aerial molecular reflection.* I 
have, however, operated upon substances of widely differ- 
*" The cause of the polarization is evidently a reflection of the 
sun's light upon something. The question is on what? Were the 
angle of maximum polarization 76, we should look to water or ice 
as the reflecting body, however inconceivable the existence in a 
cloudless atmosphere and a hot summer's day of unevaporated mole- 
cules (particles?) of water. But though we were once of this opinion, 
careful observation has satisfied us that 90, or thereabouts, is the 
correct angle, and that therefore whatever be the body on which the 
light has been reflected, if polarized by a single reflection, the polar- 
izing angle must be 45, and the index of refraction, which is the 
tangent of that angle, unity; in other words, the reflection would re- 
quire to be made in air upon air!" (Sir John Herschel, " Meteorologv, " 
par. 233.) 
Any particles, if small enough, will produce both the color and the 
polarization of the sky. But is the existence of small water-particles 
on a hot summer's day in the higher regions of our atmosphere incon- 
ceivable? It is to be remembered that the oxygen and nitrogen of 
the air behave as a vacuum to radiant heat, the exceedingly attenuated 
vapor of the higher atmosphere being therefore in practical contact 
with the cold of space. 
