1886.] electric discharge in a uniform electric field. 397 



Suppose we have two vortex rings AB and CD of equal 

 strength, whose planes are parallel and whose cores are nearly 

 coincident, they will rotate round each other, the cores remaining 

 at an approximately constant distance apart. Let us suppose 

 that these rings are moving in a fluid which is in motion but in 

 which the distribution of velocity is not uniform ; then we know 

 (see a Treatise on the ' Motion of Vortex Rings ' by J. J. Thomson, 

 p. 65) that the radii of the rings will alter, and that the alteration 

 will not be affected by reversing the direction of motion of the 

 rings. 



Now let us suppose that the radius of AB in consequence 

 of the distribution of velocity in the surrounding fluid increases 

 faster than that of CD, then since the velocity of a ring diminishes 

 as its radius increases the diminution in the velocity of AB will 

 be greater than in that of CD, so that CD will now move faster 

 than AB, the distance between the rings will therefore increase, 

 and if the difference between the velocities is great enough they 

 will ultimately separate. Next let us suppose that the rings are 

 turned round so as to be moving in the opposite direction, as in 

 fig. 8. Then, since the alteration in the radius of either rin^ is 

 the same after the direction of motion has been reversed ; under 

 the same circumstances as before, the radius of the ring AB, which 

 is now in front, will still increase faster than that of the ring CD, 

 which is now in the rear ; that is, the diminution in the velocity 

 of the ring in front will be greater than that of the one in the 

 rear, that is, the front ring will move more slowly than the one 

 behind, so that the distance between the rings will diminish and 

 the connection between the atoms in the molecule be made firmer, 

 while in the other case the molecules tended to separate. The 

 only difference between the cases, however, is the direction in 

 which the molecules are moving, so that a molecule of this kind 

 may tend to be decomposed when it is moving in one direction 

 and not when it is moving in the opposite one. 



It would, I have no doubt, be possible to give an illustration 

 of this property by taking an ordinary mechanical system and 

 supposing it to be acted on by a proper distribution of forces : the 

 above illustration, however, is sufficient for my purpose, which is 

 to shew that the properties of molecules may be such that they 

 are decomposed when moving in one direction in an electric field 

 but not when moving in the opposite. 



Let us trace some of the consequences of supposing that the 

 molecules are decomposed when moving in the direction of the 

 lines of force and not Avhen moving in the opposite direction. 

 If we consider the electric field near the electrodes, this means 

 that at the negative electrode those molecules which are moving 

 towards it are the only ones which have any tendency to be 



