January 31, 1895] 



NATURE 



331 



A second experiment which shows the same thing is the 

 following. Two long electrodes are fused inio a bulb, so that 

 the tip of an electrode is a considerable distance from the place 

 where it passes through the glass. We will now send an alter- 

 nating discharge through the tube, and you will see, I think, 

 that the discharge, instead of going straight across the short 

 distance between the ends of the electrodes, goes from the tip 

 of one electrode to the place where the other passes through 

 the glass, thus staying as long as possible in the gas before 

 passing into the metal. The appearance of the discharge 

 shows that the positive electroHe is at the end of the wire, the 

 negative at the junction of the wire with the glass. 



.\nother interesting example of the difficulty the discharge 

 experiences in passing from gas to metal is the discovery made 

 by Profs. Liveing and Dewar, that when the discharge passes 

 through a gas containing a large quantity of metallic dust, the 

 light from the discharge, when examined in the spectroscope, 

 does not show any of the lines of the metal. 



The difficulty which the positive electricity finds in passing 

 from the gas to the electrode depends a great deal upon the 

 nature of the gas, as well as upon that of the electrode ; it is 

 influenced by the position of the gas and the electrode relatively 

 to one another in the electrochemical series. 



I have lately made a series of experiments on this point in 

 the following way. An alternating discharge from a high 

 tension transformer was made to pass between two electrodes 

 fused into a bu|l>, which could be filled with the gases under 

 examination, .\nother electrode connected to an electrometer, 

 passed into the bulb, and was arranged so that it could be 

 moved about from one part of it to the other. When the 

 electrodes were metal and the bulb was filled with the electro- 

 negative gas oxygen, the electrode received a positive charge in 

 whatever part of the bulb it was situated ; if now the bulb was 

 filled with hydrogen at atmospheric pressure, then in the regions 

 remote from the arc the electrode received a positive charge, but 

 in the immediate neighbourhood of the arc itself it received a 

 negative charge. When the pressure was reduced the region 

 in which the charge was negative contracted, and finally at 

 pressures about one-third of an atmosphere, seemed to disappear, 

 and the electrode got a slight positive charge in whatever 

 position it was placed. If now, instead of using metallic 

 electrodes we use well-oxidised copper ones, and repeat the 

 experiment in hydrogen, working at a pressure when there was 

 only positive electricity, when the electrodes were bright and 

 polished, we find that with the oxidised electrodes every particle 

 of positive electricity is taken out of the tube, and a negative 

 charge is left. This negative charge remains until the copper 

 oxide is completely reduced; when this occurs the negative 

 charge disappears, and is replaced by positive. Thus, under 

 the same conditions as to the nature of tbe gas and the pressure, 

 the bright copper electrodes leave a positive charge in the gas, 

 while the oxidised ones leave a negative charge. 



The most probable explanation of these results seems to me 

 to be the view that the communication of electricity from gas to 

 the electrode, or from the electrode to gas, is facilitated by the 

 temporary formation of something of the nature of a chemical 

 compound between the gas and the metal. In all such com- 

 pounds the metal is the electro-positive element, and has the 

 positive charge, the gas being the electro-negative and carrying 

 the negative charge. Now consider the case when the negative 

 charge is on the gas, and the positive charge on the metal ; then 

 the gas and metal have got the charges proper to them in any 

 compound they may form, and are thus in a fit state to combine, or 

 according to this view, allow the negative electricity to pass from 

 the gas to the copper. Hut, now, suppose the gas w as positively 

 electrified, the g.is and the metal have now opposite charges to 

 those proper to them in a compound, and before the union of 

 gas and metal in this slate could result in anything but a most 

 unstable compound, an additional process must be gone through 

 — i.e. the charges on the gas and metal must be interchanged. 

 Thus the conditions for the combination of the gas and metal 

 arc more complex when the gas is positively electrified than 

 when it is negatively electrified, and thus, on the view that the 

 communication of electricity between the gas and the metal 

 involves a sort of chemical combination, we see '.hat the negative 

 electricity will escape more easily from the gas to the metal 

 than the positive. Now consider the case when the gas was 

 hydrogen, the electrodes oxidised copper ; the hydrogen 

 combines now not with the metal, but with the oxygen, forming 

 water, in which hydrogen is the electro-positive element ; thus, 



NO. I 3 18, VOL 5 l] 



in this case, it is the positively charged hydrogen which is in 

 the state best fitted for pairing. The consequence is, the 

 positive charge would be most readily removed from the gas 

 and the negative left — exactly the opposite to that which 

 occurred when the electrodes were bright. This reversal, as 1 

 stated before, is verified by experiment. 



I have hitherto only spoken of the phenomena which accom- 

 pany the passage of electricity from the electrode to the gas, or 

 from the gas to the electrode. 



I shall now pass on to consider the properties of the discharge 

 when it is entirely confined to the gas. 



We may produce a discharge which, during the whole of its 

 course, shall be confined to the gas in the way represented in 

 the diagram. 



The two poles of a Wimshurst machine are connected to the 

 insides of two jars A and B, while the outsides of these jars are 

 connected together by a metal wire wound so as to form a coil. 

 The electricity from the Wimshurst machine charges up the 

 jars, the difTerence of potential between the poles increases 

 until a spark passes. The passage of the spark puts the insides 

 of the two jars in connection, and the jars are discharged. The 

 discharge of the jar, as was proved from the theory ol electro- 

 magnet action by Lord Kelvin more than forty years ago, and 

 shortly afterwards confirmed by the experiments of Feddersen, 

 is an oscillatory one, producing currents surging backwards and 

 forwards through the wires with extraordinary rapidity. The 



subject of these oscillatory currents is one which is tinged with 

 melancholy. In the beginning of 1894 we lost Hertz, whose 

 splendid work on these electrical oscillations is known to you 

 all. The Managers of this Institution have marked their sense 

 of the importance of this work by devoting a special lecture to 

 this work alone, and they have entrusted that lecture to a most 

 distinguished worker in the same field as Hertz. It would 

 therefore be presumptuous on my part to refer in any detail to 

 Hertz's work ; but no physicist, and least of all one who is a 

 member of Maxwell's University, could pass over in silence the 

 death of Hertz. 



When Hertz began his magnificent experiments on electric 

 oscillations, there were many theories of electrical action. 

 When he had finished them there was only one, Clerk 

 Maxwell's. 



Hertz's work was done with very much quicker vibrations 

 than those produced by the apparatus no* on the screen ; this, 

 however, gives rise to currents through the coil changing 

 their direction some million times a second. If we place in 

 the coil an exhausted bulb, the bull) in reality will be the 

 secondary of an induction coil, and will be exposed to electro- 

 motive forces tending to produce circular currents parallel to 

 the plane of the coil. 



I will now place a bulb inside this coil, and you see that a 

 circular ring discharge passes through il, and this discharge 

 passes entirely in the gas. 



The gas in the bulb now in the coil is the vapour of silicon 

 tetrachloride ; it happens to be the bulb which gives a brighter 

 ring than any others 1 possess. 



