222 W. Crookes—Lines of Molecular Pressure. 
: The rays exciting green phosphorescence will not turn a 
corner in the slightest degree, but radiate from the negative pole 
in straight lines, casting strong and sharply defined shadows 
from objects which happen to be in their path. On the other 
hand, the ordinary luminescence of vacuum-tubes will travel 
hither and thither along any number of curves and angles. 
Projection of Molecular Shadows. —The author next examines 
the phenomena of shadows east by the green light. The best 
and sharpest shadows are cast by flat disks and not by narrow- 
pointed poles; no green light whatever is seen in the shadow 
itself, no matter how thin, or whatever may be the substance 
from which it is thrown. 
rom these and other experiments, fully described in the 
paper, he ventures to advance the theory that the induction- 
spark actually illuminates the lines of molecular pressure 
caused by the electrical excitement of the negative pole. 
thickness of the dark space is the measure of the mean length 
of the path between successive collisions of the molecules. 
e extra velocity with which the molecules rebound from the 
excited negative pole keep back the more slowly moving mole- 
cules which are advancing towards that pole. The conflict 
glass. The shadows are not optical, but are molecular shadows 
revealed only by an ordinary illuminating effect ; this is proved 
a the sharpness of the shadows when projected from a wide 
ole 
Phosphorescence of Thin Films.—An experiment is next de- 
scribed in which a film of uranium glass, sufficiently thin te 
show colors of thin plates, is placed in front of a thick plate of 
the same glass, the whole being closed in a tube with terminals 
and exhausted to a few millionths of an atmosphere. Of this 
the following observations are recorded :— 
. The uranium film, being next to the negative pole, casts 
a strong shadow on the plate. 
eS Se en 
