1869.] 



and on the Polarization of Light. 



225 



other gas or vapour, be caused to exhibit under light vigorous, if not violent 

 action. The case is similar to that of carbonic acid gas, which diffused in 

 the atmosphere resists the decomposing action of solar light, but when 

 placed in contiguity with the chlorophyl in the leaves of plants, has its mo- 

 lecules 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 mix- 

 ture after fifteen minutes' exposure was slight. The tube was afterwards 

 filled with half an atmosphere of the mixed air and vapour, and another 

 half atmosphere of air which had been permitted to bubble through fresh 

 commercial hydrochloric acid. On sending the beam through this mixture, 

 the action paused barely sufficiently long to show that at the moment of 

 commencement the tube was optically empty. But the pause amounted 

 only to a small fraction of a second, a dense cloud being immediately pre- 

 cipitated upon the beam which traversed the mixture. 



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 perpendicularly to its axis, showed scarcely any signs of 

 polarization. Looked at obliquely the polarization 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 j 1 ^ 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 condensed beam of the 

 electric light passed for some time in darkness through this mixture. 

 There was absolutely nothing 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 long to permit of its thorough 

 examination. The light discharged from the cloud at right angles to its 

 own length was 'perfectly polarized. By degrees the cloud became of whi- 

 tish blue, and for a time the selenite colours obtained by looking at it nor- 

 mally were exceedingly brilliant. The direction of maximum polarization 

 was distinctly at right angles to the illuminating beam. This continued to 

 be the case as long as the cloud maintained a decided blue colour, and 

 even for some time after the pure blue had changed to whitish blue. But 

 as the light continued to act the cloud became coarser and whiter, particu- 

 larly at its centre, where it at length ceased to discharge polarized light in 

 the direction of the perpendicular, while it continued to so at both its ends. 



But the cloud which had thus ceased to polarize the light emitted nor- 

 mally, showed vivid selenite colours when looked at obliquely. The direc- 

 tion of maximum polarization changed with the texture of the cloud. This 

 point shall receive further illustration subsequently. 



A blue, equally rich and more durable, was obtained by employing the 



