300 



THE ALUMNI JOURNAL. 



it has the advantage 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 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 crevice 

 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 aluminium cylinder which 

 tightly fits the tube; with a plate of this 

 thickness the narrow spaces between the 

 plate and the tube 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 were 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 negative. It is not safe to 

 draw any conclusions from the variations 

 in intensity of one line or one group of 

 lines on the two sides of the plate, as the 

 total quantity of light coming from the 

 neighborhood of the cathode often differs 



considerably from that coming from the 

 anode. When, however, we get an in- 

 crease in the brilliancy of one set of lines 

 accompanied by a diminution in the 

 brightness of another set, when we move 

 across the plate we eliminate this source 

 of error. The differences in the spectra 

 at the two sides of the plate are most 

 easily observed at pressures where there 

 is not any very great difference between 

 the luminosity of the cathode and the 

 anode. As was mentioned at the begin- 

 ning of the paper, there is a range of 

 pressure within which the effects are ir- 

 regular, and no decided differences are 

 observed between >the spectra at the two 

 sides of the plate. It is desirable in these 

 experiments to keep the tube on to the 

 pipe as long as the experiment lasts, for 

 the discharge always decomposes the 

 compound gas, and unless the products of 

 decomposition are continually pumped 

 off and replaced by fresh supplies of the 

 compound gas, the spectra of the dis- 

 charge keep changing. With organic 

 compounds this is especially necessary, as 

 the character of the spectrum often 

 changes entirely very shortly after the 

 commencement of the discharge unless 

 fresh gas is continually introduced. 



In the following experiments the cur- 

 rent was produced by a large induction 

 coil with a mercury slow break. 



When the tube was filled with hydro- 

 chloric acid gas at a low pressure, the 

 separation of the hydrogen and chlorine 

 was seen very distinctly, the hydrogen 

 line being very much brighter on the 

 side of the plate which acted as the cath- 

 ode (which we shall call the negative 

 side of the plate) than on the positive 

 side, while the chlorine, on the other 

 hand, was brighter on the positive than 

 on the negative side of the plate. 



When the tube was filled with ammonia 

 gas, the hydrogen lines were bright on 



