1895.] On the Electrolysis of Gases. 249 



FIG. 2. 



A 



I 



It differs from an ordinary discharge tube merely in having a flat 

 metal plate, AB, fastened, across the tube. When the discharge 

 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 a lever 

 so as to bring one side or other of the plate opposite the slit of the 

 spectroscope ; 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 electrodes 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 it has the advan- 

 tage of not requiring a plate across the tube ; all that is necessary is 

 to use for one of the terminals a disc whose plane is parallel to the 

 slit of the spectroscope. 



If the plate AB is thin, it is necessary to fuse it into the glass tube 

 all the way round, otherwise when the pressure is low, the discharge, 

 instead of crossing the plate, goes through any little crevices there 

 may be between the plate and the tube. The easiest way of making 

 the tube is to use a plate about 0'5 cm. thick, cut from an aluminium 

 cylinder which tightly fits the tube ; with a plate of this thickness the 

 narrow spaces between the tube and the plate are .so long that the 

 discharge goes through the plate rather than through the crevices. 



The tube was filled with the gas to be observed and the spectra at 

 the two sides of the plate compared. These spectra were in many 

 cases found to differ in a very remarkable way ; it was, however, only 

 in exceptional cases that a line which was bright at one side of the 

 plate was absolutely invisible on the other. The method used was to 

 take two sets of lines, say A and B, as close together in the spectrum 

 as possible, and compare the brightness of these sets of lines on the 

 two sides of the plate ; if it was found that the A lines were brighter 

 on the positive side of the plate than on the negative, while on the 

 other hand the B lines weie brighter on the negative side of the plate 

 than on the positive, then it was inferred that electrolytic separation 

 had occurred, and that the substance giving the A lines was in excess 

 on the positive side of the plate, that giving the B lines on the nega- 



