W. Crookes—Lines of Molecular Pressure. 219 
the surface of the disk, forming an oblate spheroid around it. . 
6. The diameter of this dark space varies with the exhaus- 
tion, with the ki 
a velvety violet light forms on the metallic side of the vanes, 
the mica side remaining dark throughout these experiments. 
As the pressure diminishes a dark space is seen to separate the 
violet halo from the metal. Ata pressure of half a millim. 
this dark space extends to the glass, and positive rotation com- 
mences, 
On continuing the exhaustion, the dark space further widens 
out and appears to flatten itself against the glass, and the rota- 
tion becomes very rapid. 
