NEW CHEMICAL REACTIONS. 85 



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 perpendicularly 

 to its axis, showed scarcely any signs of polarization. 

 Looked at obliquely the polarization was strong. 



The experimental tube being again cleansed and ex- 

 hausted, the mixed air and nitrite-of -butyl vapor was per- 

 mitted to enter it until the associated mercury column was 

 depressed one-tenth of an inch. In other words, the air 

 and vapor, united, exercised a pressure not exceeding one 

 three hundredth 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 long to permit of 

 its thorough examination. The light discharged from the 

 cloud, at right angles to its own length, was at first per- 

 fectly polarized. It could be totally quenched by the 

 Nicol. By degrees the cloud became of whitish blue, and 

 for a time the selenite colors, obtained by looking at it 

 normally, 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 color, and even for 

 some time after the blue had changed to whitish blue. 

 But, as the light continued to act, the cloud became coarser 

 and whiter, particularly at its center, where it at length 

 ceased to discharge polarized light in the direction of the 

 perpendicular, while it continued to do so at both ends. 



But the cloud which had thus ceased to polarize tiie 

 light emitted normally, showed vivid selenite colors when 

 looked at obliquely, proving that the direction 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 nitrite-of-butyl vapor in a still more atten- 

 uated condition. The instance here cited is representative. 



