MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 
149 
shadow is therefore seen on it. When the lower pole, h, is made negative, so as to 
project the shadow of the mica star cl, no shadow is seen on the mica plate, neither 
is any seen on the uranium plate above the mica. The thin film of mica entirely 
prevents the uranium glass from becoming fluorescent under the influence of the 
negative pole. Other experiments have, however, shown that the mica star gives j ust 
as sharp and intense a shadow as the aluminium star, provided a suitable screen is 
used to receive it on. 
529. If the aluminium star is made the positive pole, any one of the others being 
the negative pole, it casts an enlarged and somewhat distorted image of itself all over 
the upper part of the tube. This image is not sharply defined. 
The sharpness of the shadows cast by the negative pole is slightly affected by the 
intensity of the current; when the spark is very strong, the shadow widens out a little. 
530. I have already advanced the theory that the thickness of the dark space sur¬ 
rounding the negative pole is the measure of the mean length of the path of the gaseous 
molecules between successive collisions (507). The electrified molecules are projected 
from the negative pole with enormous velocity, varying, however, with the degree of 
exhaustion and intensity of the induction current (498). In the dark space they are 
few in number in comparison to what they are at the luminous boundary. When the 
exhaustion is so high that the mean path of the molecules stretches right across the 
tube, their velocity is suddenly checked by the glass walls, and the production of light 
is the consequence of this sudden arrest of velocity. The light actually proceeds from 
the glass, and is caused by fluorescence or phosphorescence in or on its surface, and 
not by an evolution of light by the molecules themselves, crowding together and 
striking each other on the surface of the glass. Had this been the case—had the 
molecules themselves been the lamps—they would shine equally well whatever were 
the arresting surface, and their light would have shown the spectral characteristics of 
the gas whose residue they constituted. But no light is caused by a mica or quartz 
screen, however near it may be brought to the negative pole ; and generally speaking 
the more fluorescent the material of the screen, the better the luminosity. 
531. The theory best supported by experiment, and the one which although new is 
not at all improbable in the present state of our knowledge respecting molecules, is 
that the greenish-yellow phosphorescence of the glass is caused by the direct impact of 
the molecules on the surface of the glass. The shadows are not optical but molecular 
shad ows, only they are revealed by an ordinary illuminating effect. The sharpness of 
the shadow, when projected from a wide pole, proves them to be molecular. Had the 
projection from the negative pole radiated in all directions, after the manner of light 
radiating from a luminous disk, the shadows would not be perfectly sharp, but would 
be surrounded by a penumbra. Being, however, projected material molecules in the 
same electrical state, they do not cross each other, but travel on in slightly divergent 
paths, giving perfectly sharp shadows with no penumbrse. 
532. It was suggested by Professor Stokes, as an alternative to the above theory, 
