MOLECULAR PRESSURE, AND THE TRAJECTORY OE MOLECULES. 
161 
initial impulse given by the negative pole is stronger, or the collisions are less 
frequent. I consider the latter to be the true cause. The molecules which produce 
the green phosphorescence must be looked upon as in a different state from those which 
are arrested by frequent collisions. These impede the velocity of the free molecules 
and allow longer time for the magnetism to act on them; and although the deflecting 
force of magnetism might be expected to increase with the velocity of the molecules, 
Professor Stokes has pointed out that it would have to increase as the square of the 
velocity in order that the deflection should be as great at high as at low velocities. 
Comparing the free molecules to cannon balls, the magnetic pull to the earth’s gravi¬ 
tation, and the electrical excitation of the negative pole to the explosion of the powder 
in the gun, the trajectory will be quite flat when no gravitation acts, and gets curved 
under the influence of gravitation ; it is also much curved when the ball passes through 
a dense resisting medium ; it is less curved when the resisting medium gets rarer, 
and, from par. 562, it is seen that intensifying the induction spark, equivalent to 
increasing the charge of powder, gives greater initial velocity, and therefore flattens the 
trajectory. The parallelism is still closer if we compare the evolution of light seen 
when the shot strikes the target, with the phosphorescence produced in the glass 
screen by molecular impacts. 
578. In carbonic acid the mean free path of the molecules is shorter for the same 
degree of exhaustion than in air, and the velocity of the molecules measured by the 
flatness of the trajectory under magnetic influence is also lower than what it is in air 
at similar pressures. 
579. Although in many cases, especially at a moderate exhaustion and using a feeble 
spark, the image on the screen was a perfectly well-defined circle, I could detect no 
image of the notch cut in the negative pole. The hole in the mica was quite small 
enough to have given a good image of the negative pole inverted on the screen had it 
been shining by ordinary light, but the rays being corpuscular and the particles not 
crossing, no image of the pole is formed, but only the image of the hole in the mica.* 
580. Attempts to obtain continuous rotation of the ray of molecular light by means 
of a magnet have hitherto failed. The stream of molecules does not obey Ampeee’s 
Law as it would were it a perfectly flexible conductor joining the negative and positive 
pole. The molecules are projected from the negative pole, but the position of the 
positive pole, whether in front, at the side, or even behind the negative pole, has no 
influence on their subsequent behaviour, either in causing phosphorescence, producing 
mechanical effects, or in their magnetic deflection (519, 526, 527, 549). The magnet 
seems to give them a spiral twist, greater or less, according to its power, but diminishing 
as the molecules get further off, and independent of their direction. 
* With, another apparatus, in which the pole and screen are nearer together and the mica, with the 
hole in it capable of being moved to and fro, I have succeeded in seeing an ill-defined image of the 
negative pole. This only occurs at low exhaustions and soon disappears, giving way to a sharp image of 
the hole. 
MDCCCLXNIX. 
Y 
