OF HIGH BEFBAN GIBILIT Y TJPOJST GASEOUS MATTES. 
357 
white, and the posterior portion bluish. The posterior portion could be utterly extin- 
guished by the Nicol long after the anterior portion had began to show a residual 
blue. Passing with the Nicol from the densest to the least dense portion of the cloud, 
the residual colour changed from a bright blue through a gorgeous Alpine skyblue to 
absolute extinction. 
Looked at obliquely in a vertical plane, the two semicylinders into which the cloud was 
longitudinally divided were found in opposite states of polarization. 
This was a truly splendid action. The chemical effect was exceedingly vigorous, and 
the cloud-form fine. 
III. Air and aqueous nitric acid 1 inch ; then 
Air and cyanide-of-ethyl vapour .... 15 inches. 
On starting the light the experimental tube was found optically empty. In a quarter 
of a minute, however, the track of the beam, which previously had been invisible, was 
coloured blue. The chemical action appeared to exert itself with almost the same inten- 
sity throughout the entire length of the experimental tube. 
For a brief interval the whole of the light emitted normally was polarized. Then for 
a time about three-fourths of the length of the cloud could be quenched by the Nicol, 
the remainder showing a fine residual blue. This sank from a brilliant azure at the 
densest portion of the cloud through deep rich blue to entire extinction. 
The selenite bands were exceedingly vivid long after this cloud had ceased to be blue. 
An immense quantity of polarized light was discharged normally, even after the cloud 
had become white. Placed between the cloud and the eye, a plate of tourmaline with 
its axis parallel to the beam was practically black, while when placed across the beam 
a bright green light was copiously transmitted. 
In one position of the Nicol this cloud was yellow, in the rectangular position it was 
blue. Here also the chemical action was very vigorous, and the cloud-form very fine. 
Benzol (C g H 6 ) : — A transparent colourless liquid. 
Contents of experimental tube. 
I. Air and benzol vapour 1 inch ; then 
Air and aqueous nitric acid ... 15 inches. 
Nitric acid is known to form with benzol nitro-benzol, a liquid possessing a high 
boiling-point. But though the mixed vapours were allowed to remain together for ten 
minutes before starting the lamp, when the beam passed through the experimental tube 
it was optically empty. 
Chemical action commenced a quarter of a minute after the ignition of the lamp ; a 
very delicate blue light was then discharged from the beam, the centre of which was 
particularly bright and transparent. The light emitted normally remained perfectly 
polarized for one minute. 
3 B 
MDCCCLXX. 
