the Electric Discharge in Gases. 429 



lines of force than at right angles to them; and thus those 

 molecules which are moving along the lines of force will be 

 split up into atoms sooner than those moving at right angles 

 to them. Thus the ratio of the paired to the free time will be 

 less for those molecules which are moving along the lines of 

 force than for those moving at right angles to them; and 

 therefore the pressure will be less along the lines of force than 

 at right angles to them. Maxwell, in his ' Treatise on Elec- 

 tricity and Magnetism/ has shown that a distribution of stress 

 of this character will account for the mechanical actions between 

 electrified bodies. 



To show that it is conceivable that this cause should pro- 

 duce effects sufficiently large to account for electrostatic 

 attractions and repulsions, it may be useful to point out that 

 the electric tension along the lines of force is very small when 

 compared with the atmospheric pressure; for air at the atmo- 

 spheric pressure the maximum electric tension is only about 

 20V0 °f the atmospheric pressure. The theory that electro- 

 static attractions and repulsions are due to stresses in the 

 gaseous dielectric admits of an experimental test; for it is 

 evident that, according to this theory, the tension along the 

 lines of force cannot exceed the pressure of the gas. Thus, if 

 we have a gas sufficiently rare to support an electric field 

 so intense that the pressure of the gas does not greatly exceed 

 the electric tension along the lines of force when calcu- 

 lated by the ordinary expression, viz. KH 2 jSir (where K is 

 the specific inductive capacity of the gas, and H the electro- 

 motive force), then, if the theory of stress in the gas be correct, 

 the tension along the lines of force will soon reach a maximum 

 value, and will not increase with an increase in the electro- 

 motive force. Thus the attraction between the two electrodes 

 in this case would reach a maximum, and would not after- 

 wards increase with an increase in the difference of potential 

 between them. I may point out that, for this to happen, the 

 density of the gas would have to be much less than the density 

 for which the electric strength is a minimum, which the re- 

 searches of Dr. De La Rue and others have shown to be at a 

 pressure about *6 millimetre. For down to this pressure the 

 electric force necessary to produce discharge is, speaking very 

 roughly, proportional to the pressure, but the electric tension 

 is proportional to the square of the electromotive force. Thus, 

 down to the pressure of minimum strength, the ratio of the 

 greatest electric tension to the pressure of the gas diminishes 

 with the pressure; and it would be no use seeking for any 

 effect such as is described above, except at pressures very much 

 less than this. Taking the formula given by Dr. Macfarlane 

 in the Philosophical Magazine for December 1880, for calcu- 



