MOLECULAR PRESSURE, AND THE TRAJECTORY OP MOLECULES. 
157 
the other. If the molecules are projected from the negative pole with different 
velocities we might expect that under a constant magnetic deflection the higher 
velocities would show the flatter trajectories. With the apparatus shown at figs. 15 
and 16, these variations in the trajectory of the molecules were not obtained in a 
decided manner, although indications of an alteration of curve by intensifying the spark 
were apparent. 
556. Another apparatus was accordingly made in order to test this point, and also 
to obtain a more definite relation between the dimensions of the dark space round the 
negative pole, the commencement of the green phosphorescence, and the magnetic 
deflection under varying conditions of pressure in different gases. 
I have spoken of shadows being deflected by the magnet as a convenient way of 
describing the phenomena observed ; but it will be understood that what is really 
deflected is the path of the molecules driven from the negative pole and whose 
impact on the phosphorescent surface causes light. The shadows are the effect of a 
material obstacle in the way of the molecules. 
In the apparatus now about to be described, a ray of light was used instead of 
a shadow. Fig. 19 shows the arrangement. 
Pig. 19. 
The poles are at a and b. The negative pole a is a flat aluminium disk with 
a notch cut in it. The pole b is a ring of aluminium; c is a mica screen with a 
small hole in the middle about 1 millim. in diameter ; cl is a flat plate of German 
glass with a millimetre scale engraved on it vertically; e is a mica scale of millimetres. 
The scale e is to measure the thickness of the dark space as the exhaustion proceeds ; 
the hole in c is to enable a spot of light to be thrown on the scale d from the 
pole a ; the notch in the pole is to enable me to see if the spot of light projected 
on d is an image of the pole a, or of the hole in c ; the scale on d is to enable 
me to measure the deflection of the ray proceeding from a, through c, to d, when 
bent by the magnet; ff is a vertical screen of mica in the plane of the movement 
of the ray, covered with a phosphorescent powder. On this the path of the ray 
traces itself in a straight line when the magnet is absent, and curved when the 
magnet is present. 
