1878.] the lllamination of Lines of Molecular Pressure. 109 



tive pole. The molecules are projected from the negative, but the posi- 

 tion 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 producing phosphorescence, or mechanical effects, or in their 

 magnetic deflection. The magnet gives their line of path a spiral 

 twist, greater or less according to its power, but diminishing as the 

 molecules get further off. 



Numerous experiments were tried in this apparatus with different 

 gases, and with the magnet in and out of position. 



Working with exhausted air it was found that the spot of 

 green phosphorescence on the screen is visible at an exhaustion of 

 102*6 M, when the free path of the molecules, measured by the 

 thickness of the dark space round the negative pole, is only 

 12 millims. Hence, it follows, that a number of molecules sufficient 

 to excite green phosphorescence on the screen are projected the whole 

 distance from the pole to the screen, or 102 millims., without being 

 stopped by collisions. 



Alteration of Molecular Velocity. 



If we suppose the horseshoe magnet to be permanently in position, 

 and thus to exert a uniform downward pressure on the molecules, 

 we perceive that their trajectory is much curved at low exhaustions, 

 and gets flatter as the exhaustion increases. A flatter trajectory corre- 

 sponds to a higher velocity. This may arise from one of two condi- 

 tions ; either the initial impulse given by the negative pole is stronger, 

 or the molecules experience less resistance. The latter is probably the 

 true one. The molecules which produce the green phosphorescence 

 must be looked upon as in a state differing from those arrested by 

 frequent collisions. Any action which impedes the velocity of the 

 free molecules allows longer time for the magnetism to act on 

 them; and, although the deflecting force of magnetism might be 

 expected to decrease with the velocity of the molecules, Professor 

 Stokes has pointed out that it would have to decrease as the square 

 of the velocity, in order that the deflection should be no greater at 

 low than at high velocities. 



Comparing the free molecules to cannon balls, the magnetic deflection 

 to the earth's gravitation, and the electrical excitation of the negative 

 pole to the explosion of the powder in the gun, the trajectory will be 

 flat when no gravitation acts, and curved when 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, as already shown, 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 



