NEW CHEMICAL REACTIONS. 89 
the question, the following facts bearing upon it may be 
submitted. 
The parallel beam employed in these experiments 
tracked its way through the laboratory air, exactly as sun- 
beams are seen to do in the dusty air of London. I have 
reason to believe that a great portion of the matter thus 
floating in the laboratory air consists of organic particles, 
which are capable of imparting a perceptibly bluish tint to 
the air. These also showed, though far less vividly, all 
the effects of polarization obtained with the incipient clouds. 
The light discharged laterally from the track of the illu- 
minating beam polarized, though not perfectly, the direction 
of maximum polarization being at right angles to the beam. 
At all points of the beam, moreover, throughout its entire 
length, the light emitted normally was in the same state of 
polarization. Keeping the positions of the Nicol and the 
selenite constant, the same colors were observed through- 
out the entire beam, when the line of vision was perpendic- 
ular to its length. 
The horizontal column of air, thus illuminated, was 18 
feet long, and could therefore be looked at very obliquely. 
I placed myself near the end of the beam, as it issued from 
the electric lamp, and, looking through the Nicol and 
selenite more and more obliquely at the beam, observed 
the colors fading until they disappeared. Augmenting 
the obliquity the colors appeared once more, but they were 
now complementary to the former ones. 
Hence this beam, like the sky, exhibited a neutral point, 
on opposiue sides of which the light was polarized in 
planes at right angles to each other. 
Thinking that the action observed in the laboratory 
might be caused, in some way, by the vaporous fumes 
diffused in its air, I had the light removed to a room at the 
top of the Royal Institution. The track of the beam was 
seen very finely in the air of this room, a length of 14 or 15 
foet being attainable. This beam exhibited all the effects 
observed with the beam in the laboratory. Even the un- 
condensed electric light falling on the floating matter 
showed, though faintly, the effects of polarization. 
When the air was so sifted as to entirely remove the visible 
floating matter, it no longer exerted any sensible action upon 
the light, but behaved like a vacuum. The light is scattered 
and polarized by particles, not by molecules or atoms* 
