356 
PROFESSOR TYNDALL ON THE ACTION OE RAYS 
II. Air and bisulphide-of-carbon vapour .... 8 inches ; then 
Air and aqueous nitric acid 8 inches. 
When the lamp was ignited the experimental tube was found optically empty ; but 
the chemical action commenced three-quarters of a minute afterwards, the convergent 
beam assuming the appearance of a fine blue spear. The action was more energetic than 
in the last case, though the battery was sensibly sinking in power. 
The light discharged normally remained perfectly polarized for two minutes after its 
first appearance. The selenite colours were rich and vivid, and the tourmaline in its two 
characteristic positions showed the same striking contrast observed in the last experi- 
ment. 
In five or six minutes the entire tube was filled with cloud, the residual blue being 
then perfectly gorgeous. 
III. Air and aqueous nitric acid .... 1 inch ; then 
Air and bisulphide-of-carbon vapour . 15 inches. 
The tube was optically empty when the lamp was ignited. The chemical action soon 
commenced, a series of layers of blue cloud stretching through the entire tube. The 
action was less energetic than in the former cases, this being due in part to the sinking of 
the battery. The light discharged normally remained perfectly polarized for ten minutes. 
Cvaxide or Ethyl (C 2 H 3 Cn) : — A transparent colourless liquid. 
Contents of experimental tube. 
I. Air and cyanide-of-ethyl vapour .... 1 inch ; then 
Air and aqueous nitric acid 15 inches. 
The tube was optically empty when the lamp was ignited. In a minute and a half 
the track of the beam became distinctly blue. The blue light was at first perfectly 
polarized. 
The beam was crossed by a series of disks, which were denser and more whitish than 
the general mass of the cloud. The extinction of these disks by the Nicol was curious 
and interesting. 
The growth of the particles in this case was so slow that the light emitted normally 
continued perfectly polarized for thirteen minutes after the first appearance of the cloud. 
A faint residual blue was afterwards developed. 
II. Air and cyanide-of-ethyl vapour . . 8 inches; then 
Air and aqueous nitric acid ... 8 inches. 
The experimental tube was optically empty for two seconds after the starting of the 
lamp ; a fine blue colour was then observed upon the upper boundary of the convergent 
beam. The light emitted normally did not remain perfectly polarized for more than half 
a minute. In two minutes the tube was filled with cloud, the anterior portion being 
