296 



THE ALUMNI JOURNAL. 



ON THE ELECTROLYSIS OF GASES.* 



In the experiments described in this 

 paper I have used the spectroscope to 

 detect the decomposition of gases by the 

 electric discharge and the movement of 

 the ions in the opposite direction along 

 the discharge-tube. 



The method consists in sending the 

 electric discharge through a tube so ar- 

 ranged that the spectra close to the posi- 

 tive and negative electrodes can easily 

 be compared, the presence or absence of 

 certain ions at these electrodes can thus 

 be ascertained. This method is capable 

 of much wider application than the one 

 I previously used in my experiments on 

 the "Electrolysis of Steam," (Proc. Roy. 

 Soc, vol. Hi. p. 90), the use of which is 

 attended with very great difficulty for 

 any substance other than steam. The 

 earlier method has, however, the ad- 

 vantage of being a quantitative method 

 — the present one is only qualitative. 



In my former experiment with steam, 

 when I worked at atmospheric pressure, 

 and varied the length of the spark, I 

 found that when the spark-length exceed- 

 ed a certain length, d lf there was an ex- 

 cess of hydrogen at the negative electrode 

 and of oxygen at the positive, equal in 

 amount to the quantities of hydrogen 

 and oxygen liberated from a water volta- 

 meter placed in series with the steam- 

 tube. When the sparks were shorter 

 than a certain length, d 2 , the hydrogen 

 appeared at the positive, the oxygen at 

 the negative electrode, but the quantity 

 of these gases was again equal to the 

 quantities liberated in a water voltameter 

 placed in series with the steam-tube. 



When the spark-length was between 

 d x and d 2 the effects were irregular, and 

 there seemed to be no connection between 



♦Paper read at the Royal Society, by J. J. Thomson, M. 

 A., F. R. S., Cavendish Professor of Experimental Phys- 

 ics, Cambridge. Reprinted from Nature. 



the amount of gases liberated in the 

 steam-tubes and those liberated in the 

 voltameter. 



In the following experiments in which 

 the sparks were of constant length and 

 the pressure was altered, corresponding 

 effects were observed. Within certain 

 limits of pressure definite and perfectly 

 regular evidence of the separation of the 

 ions of the gas sparked through was ob- 

 tained; and the electrode at which a 

 given ion appeared could be reversed by 

 altering the pressure; there was, how- 

 ever, a range of pressures in which the 

 separation of the ions was either not well 

 marked or was irregular in character. 



I shall begin by describing a very sim- 

 ple method of showing the separation of 

 the ions produced by the discharge of 

 electricity through a compound gas such 

 as hydrochloric acid gas, which is appli- 

 cable when the discharges through the 

 constituent gases of the compound are of 

 distinct and different colors; this is emi- 

 nently the case with the hydrochloric acid 

 gas, as the discharge through hydrogen 

 in a capillary tube is red, through chlo- 

 rine green. 



Take a capillary tube of very fine bore, 

 the finer the better (the tube I used was 

 thermometer tubing of the finest bore I 

 could procure), and insert platinum wires 

 for electrodes in two small bulbs blown 

 on the ends of the tubes; then fill the tube 

 with HC1 gas, allowing it to run through 

 the tube for a considerable time so as to 

 get rid of any extraneous gas, and ex- 

 haust the tube so that the gas in it is at a 

 very low pressure. Then when a dis- 

 charge from a large induction coil passes 

 through the tube the following phenom- 

 ena are observed : When first the dis- 

 charge passes through the tube the color 

 is uniform throughout and of a greenish- 

 grey ;after the discharge has been passing 

 for a little time, the end of the tube next 



