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 eacli 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 1-i or 15 

 foet being attainable. This beam exhibited all the effects 

 observed with the beam in the laboratory. Even the mi- 

 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. 



