OF HIGH EEEEANGIBILITY UPON GASEOUS MATTES. 
355 
The cloud for a time was divided from beginning to end into two longitudinal lobes, 
separated from each other by an apparently empty space about a quarter of an inch 
wide. When the cloud was looked at obliquely in a vertical plane, one of these lobes 
was found to polarize the light positively, the other negatively. In passing from the 
one to the other the selenite tints were reversed. 
The quantity of light scattered by this cloud was very considerable ; it brightly illu- 
minated the walls and ceiling of the laboratory. As the cloud became denser, the cen- 
tral empty space, which at first divided it into two lobes, gradually disappeared. 
Looked at normally the polarization of the one half of this cloud was positive, and 
that of the other negative. Between the two a neutral point existed. The oblique pola- 
rization of the dense cloud was strong. 
III. Air and aqueous nitric acid . . 1 inch ; then 
Air and toluol vapour . . . . 15 inches. 
The action here was not so prompt as in the last case, nor was the cloud generated so 
dense. The cloud-particles, moreover, were coarser, and showed iridescent colours. 
Still the chemical action of the light was distinct and copious. 
Looked at normally, a portion of this light was salmon-coloured. The selenite bands 
appeared to be of this colour, and its complementary greenish tint. 
Bisulphide op Caebon (C $ 2 ) : — A transparent colourless liquid. 
Contents of experimental tube. 
I. Air and bisulphide-of-carbon vapour . linch; then 
Air and aqueous nitric acid . . . . 15 inches. 
On starting the experimental tube was optically empty ; but in a minute afterwards 
the track of the beam became blue, which was particularly deep and rich in the middle 
portion of the beam. 
The blue light discharged normally was perfectly polarized, but the least deviation of 
the line of vision from the normal caused a portion of the light to pass through the Nicol. 
The growth of this cloud and the gradual brightening and subsequent whitening of 
the blue were very instructive. 
The light discharged normally remained perfectly polarized for seven minutes after 
the first appearance of the blue colour. A faint but rich residual blue was seen for 
some time afterwards. 
The selenite colours were exceedingly vivid with this cloud. When, moreover, a plate 
of tourmaline was placed with the crystallographic axes parallel to the beam it was 
black; placed at right angles to the beam, a large portion of the light of the cloud was 
transmitted. 
After ten minutes’ exposure the cloud itself still showed a distinct trace of blue. The 
residual blue was then particularly rich and pure. After fifteen minutes the selenite 
colours were still vivid, though the cloud had then become greyish white. 
