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 uitrite-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 the 
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, he instance here cited is representative, 
