634 PROCEEDINGS OP THE AMERICAN ACADEMY. 



value of this product we can have motion along the line of the electric 

 intensity, motion at right angles to the lines of magnetic force, or 

 motion along these latter lines. In a vacuum, where the only force act- 

 ing is that of the magnetic field, there is a strong tendency in the ion to 

 follow the lines of magnetic force. This tendency was early shown by 

 the experiments of Plucker ; it is commented upon at length in Professor 

 J. J. Thomson's work upon electric discharges in gases, and the impor- 

 tant paper by Lehmann gives many experimental proofs of it. 



One feels strongly in the presence of the phenomena that the mathe- 

 matical theory is very inadequate ; and the conviction has arisen in my 

 mind that no theory based merely upon the impact and collision of 

 particles moving with different speeds can explain the characteristic 

 phenomena which I shall describe. A mathematical theory which shall 

 embrace the entire range of such phenomena must draw upon the known 

 laws of electrodynamics, those of the kinetic theory of gases, and above 

 all those of hydrodynamics. 



I shall first describe the experience with a tube of rarefied air 36 cm. 

 long, 4 cm. in diameter. The electro-magnets were excited by a current 

 of twenty-five amperes. The current through the discharge tube varied 

 from 10 milliamperes to 25 milliamperes. The difference of potential 

 between the terminals of the tube varied from 3000 volts to 6000 volts. 



1. The lines of magnetic force being along the line of discharge, the 

 cathode forming the magnetic pole, the striations are drawn into or toward 

 the cathode ; the Faraday space can be completely annihilated, new 

 striations being formed, and the increase being proportional to the 

 current and the field. 



2. In this experiment the tube was 15 cm. long, 4 cm. wide. The 

 cathode was the core of the electro-magnet. This core was pointed 

 at the end. The anode was a pointed aluminum rod. The distance 

 between the terminals was 12 cm. At a high pressure, about 1 cm., 

 when the only light in the tube consisted of a slight glow at the ter- 

 minals, the excitation of the magnetic field drew a luminous discharge 

 from the anode up to a Faraday space, forming at the end of the dis- 

 charge a very sharp line of demarcation, with one or two striae. This 

 luminous discharge came up slowly, and retreated very slowly, after a 

 minute or two, into the anode. The pressure was far below the ordinary 

 stratification stage without the influence of magnetism. 



3. At a lower pressure, 2 mm., a series of striae emerged from the 

 anode with a bulge at their centre (Figure 2). 



At still lower pressures ring-formed striae issued from the anode and 



