62 Mr. "W. Crookes on the Illumination of 



Magnetic Deflection of Lines of Molecular Force. — With this ap- 

 paratus another phenomenon was investigated. . It is found that 

 the stream of molecules whose impact on the glass occasions evo- 

 lution of light is very sensitive to magnetic influence ; and by bring- 

 ing one pole of an electromagnet, or even .of a small permanent 

 magnet, near, the shadow can be twisted to the right or to the left. 



When the little indicator was placed entirely within the mole- 

 cular shadow no movement was produced. As soon, however, as 

 an adjacent electromagnet was excited, the shadow was twisted half 

 off the indicator, which immediately rotated with great speed. 



The Trajectory of Molecules. — The amount of deflection of the 

 stream of molecules forming a shadow is in proportion to the mag- 

 netic power employed. 



The trajectory of the molecules forming the shadow is curved 

 when under the magnetic influence : the action of the magnet is to 

 twist the trajectory of the molecules round in a direction at an 

 angle to their free path, and to a greater extent as they are nearer 

 the magnet, the direction of twist being that of the electric current 

 passing round the electromagnet. 



Laws of Magnetic Deflection. — An apparatus was constructed so 

 that the deflection of a spot of Light was used instead of that of a 

 shadow, a horseshoe magnet being placed underneath the negative 

 pole to deflect the trajectory. The action of the north pole being 

 to give the line of molecules a spiral twist one way, and that of 

 the south pole being to twist it the other way, the two poles side 

 by side compel the line to move in a straight line up or down, 

 along a plane at right angles to the plane of the magnet and a line 

 joining its poles. 



The ray of molecules does not appear to obey Ampere's law, as 

 it would were it a perfectly flexible conductor, joining the negative 

 and the positive pole. The molecules are projected from the nega- 

 tive ; but the position of the positive pole — whether in front, at the 

 side, or even behiad 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 millim., when the mean 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 milhms., with- 

 out being stopped by collisions. 



Alteration of Molecular Velocity. — If we suppose the magnet to 

 be permanently in position, and thus to exert a uniform downward 

 pull on the molecules, we perceive that their trajectory is much 



