ARTIFICIAL SKY. 113 



composing action of solar light, but when placed in 

 contiguity with chlorophyl in the leaves of plants, has 

 its molecules shaken asunder. 



Dry air was permitted to bubble through the liquid 

 nitrite of butyl, until the experimental tube, which had 

 been previously exhausted, was filled with the mixed 

 air and vapour. The visible action of light upon the 

 mixture after fifteen minutes' exposure was slight. 

 The tube was afterwards filled with half an atmosphere 

 of the mixed air and vapour, and a second half-atmo- 

 sphere of air which had been permitted to bubble 

 through fresh commercial hydrochloric acid. On send- 

 ing the beam through this mixture, the tube, for a 

 moment, was optically empty. But the pause amounted 

 only to a small fraction of a second, a dense cloud being 

 immediately precipitated upon the beam. 



This cloud began blue, but the advance to whiteness 

 was so rapid as almost to justify the application of the 

 term instantaneous. The dense cloud, looked at per- 

 pendicularly to its axis, showed scarcely any signs of 

 polarisation. Looked at obliquely the polarisation was 

 strong. 



The experimental tube being again cleansed and 

 exhausted, the mixed air and nitrite-of-butyl vapour 

 was permitted to enter it until the associated mercury 

 column was depressed -^ of an inch. In other words, 

 the air and vapour, united, exercised a pressure not 

 exceeding -$^ of an atmosphere. Air, passed through 

 a solution of hydrochloric acid, was then added, till the 

 mercury column was depressed three inches. The con- 

 densed beam of the electric light was passed for some 

 time through this mixture without revealing anything 

 within the tube competent to scatter the light. Soon, 

 however, a superbly blue cloud was formed along the 

 track of the beam, and it continued blue sufficiently 



