290 Rev. P. J. Kirkby: Chemical Effects of the 



end of this paper. These seemed to show that when the 

 electrodes are widely separated the chemical combination 

 within the positive column may become o£ equal importance 

 with that within the region of the cathode fall of potential. 



To examine completely the part played by the gaseous 

 ions in causing chemical action, one ought to determine the 

 chemical effect of their passing through a centimetre of 

 hydrogen and oxygen at various pressures and under various 

 fields of uniform force. But unfortunately the number of 

 molecules of water formed by the motion of a pair of ions is 

 so small"* that no results can be obtained by using the small 

 steady currents that can be generated by the help of ultra- 

 violet light in a uniform field of force, even though the force 

 is increased — the current increases with the force owing 

 to the multiplication of ions by collision — to its sparking 

 value. 



Recourse must therefore be had to the large steady 

 currents between parallel plate electrodes that are attended 

 by a glow. And though with such currents f the electric 

 force varies greatly in different parts of the discharge, yet, 

 if the distance between the electrodes is great enough, it 

 nevertheless becomes practically uniform towards the anode, 

 the region of uniformity being known as " the positive 

 column." 



It was therefore intended by these experiments to deter- 

 mine the amount of chemical action produced by the passage 

 of a coulomb in a centimetre of the positive column when an 

 electric discharge was pnssed through a mixture of hydrogen 

 and oxygen in equivalent proportions, and also to determine 

 the electric force in the positive column. These determina- 

 tions would disclose the chemical effect of ions moving 

 through a centimetre in hydrogen and oxygen at a known 

 pressure and under a known uniform force. 



Description of the Apparatus. 



The apparatus designed for this purpose is illustrated 

 diagram matically in fig. 1. ABGD is a long vertical piece 

 of glass tubing about 115 cms. in length and 2'2 cms. 

 in diameter dipping into a jar of mercury J. The end A 



* When a luminous discharge passed between electrodes whose dis- 

 tance apart did not exceed 1*5 cm. this number never exceeded 10. 



t In the case of very small currents attended by a glow Professor 

 Townsend has shown that the force is practically uniform. These, how- 

 ever, would be too small for our purpose and far too unsteady. For the 

 complete theoretical solution of the distribution of force between two 

 parallel plate electrodes when a steady self- maintained discharge is 

 passing, see Townsend, Phil. Mag. March 1905. 



