THE ALUMNI JOURNAL. 



299 



hydrogen lines at the very lowest pres- 

 sure. 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 nega- 

 tive electrode, but not at the positive. 



The appearance of hydrogen at the 

 negative electrode when mixed in a dis- 

 charge tube with other gases has been 

 described by Mr. Baly in a very interest- 

 ing paper in the Philosophical Magazine, 

 vol. xxxv. 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 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 vapor in an atmosphere of chlor- 

 ine 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. 



Polarization 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 direc- 

 tion, 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 direction of the 

 discharge. After the current has been 

 flowing for sometime in one direction, 

 through, say, CI in an atmosphere of H, 

 the spectrum of the chlorine, though still 

 visible at the positive electrode, 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 spec- 

 trum is consequently brightest. 



Discharge through a Compound Gas. — 

 The separation of the ions by the dis- 

 charge can be readily observed in a tube 

 which differs from an ordinary discharge 

 tube merely in having a flat metal plate 

 fastened across the tube. When the dis- 

 charge passes through the tube, one side 

 of the plate acts as a positive, the other 

 as a negative, electrode. The tube is 

 mounted on a stand, which the observer 

 at the spectroscope can move by means of 

 lever, so as to bring one side or other of 

 the plate opposite the slit of the spectro- 

 scope; a very slight movement of the 

 lever is sufficient to do this, so that the 

 spectra at the two sides of the plate can 

 readily be compared. I found that the 

 results were more satisfactory when the 

 current was kept flowing through the 

 tube in one direction and the tube moved 

 so as to bring the spectra at the two elec- 

 trodes into the field of view than when 

 the tube was kept fixed in one position 

 and the current reversed. The latter 

 method, however, suffices to show the 

 separation of the ions in many cases, and 



