ARTIFICIAL SKY 121 



been permitted to bubble through fresh commercial hy- 

 drochloric acid. On sending the beam through this mixt- 

 ure, 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 perpen- 

 dicularly to its axis, showed scarcely any signs of polar- 

 ization. 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 is of an inch. In other words, the air and 

 vapor, united, exercised a pressure not exceeding ish of 

 an atmosphere. Air, passed through a solution of hydro- 

 chloric acid, was then added, till the mercury column was 

 depressed three inches. The condensed beam of the elec- 

 tric 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 examina- 

 tion. The light discharged from the cloud, at right angles 

 to its own length, was at first perfectly 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 exceed- 

 ingly 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 main- 



SOIKNOE— —6 



