248 Prof. J. J. Thomson. [June 20, 



of the tube ; indeed I never succeeded in getting rid. of th.e hydrogen 

 lines at the very lowest pressures. By heating the tube and allowing 

 dry air to run through it for a long time, however, I got the tube so 

 dry that it did not show the hydrogen lines at a pressure quite low 

 enough to allow the discharge to pass freely through it. When the 

 tube was in this state and hydrogen was let into the middle of the 

 tube, the hydrogen spectrum appeared at the negative electrode, but 

 not at the positive. 



The appearance of hydrogen at the negative electrode when mixed 

 in a discharge tube with other gases has been described by Mr. Baly 

 in a very interesting paper in the ' Philosophical Magazine,' vol. 35, 

 p. 200. 



The preceding experiments suggest, I think, that this separation of 

 two gases, A and B, by the discharge is due to the decomposition by 

 the discharge of a chemical compound formed of A and B, in which 

 the A atoms have a charge of electricity of one sign, the B atoms a 

 charge of electricity of the opposite sign ; these charged atoms under 

 the influence of the electromotive force in the tube travel in opposite 

 directions. Further, it follows from the experiment with the bromine 

 vapour in an atmosphere of chlorine that the sign of the electrical 

 charge on an atom of the same substance is not invariable, but 

 depends on the substance with which this atom is in combination. 

 We shall find numerous other instances of this change in the sign of 

 the charge on an atom in experiments described in a later part of 

 this paper. 



Polarisation of the Electrodes. This in the electrolysis of liquids is 

 due to the accumulation at the electrodes of ions which have ceased 

 to act as carriers of electricity. We have, I think, distinct evidence 

 of a similar accumulation in the electrolysis of gases. For, as has 

 been already described, after the discharge has been running for some 

 time in one direction, giving the spectrum of some gas at one of the 

 terminals, the spectrum of the gas at that terminal is momentarily 

 brightened to a very great extent by suddenly reversing the direc- 

 tion of the discharge. After the current has been flowing for some 

 time in one direction through, say, Cl in an atmosphere of H, the 

 spectrum of the chlorine, though still visible at the positive elec- 

 trode, gets faint, the chlorine apparently to a great extent ceasing to 

 carry the discharge ; when, however, the current is reversed, the 

 atoms of chlorine can move freely, as they are not obstructed by the 

 electrode, so that immediately after the reversal of the current there 

 is probably more of the discharge carried by the chlorine than at any 

 other time, and the chlorine spectrum is consequently brightest. 



Discharge through a Compound Gas. The separation of the ions by 

 the discharge can be readily observed in a tube of the kind shown in 

 fig. 2. 



