ON CHEMICAL COMBINATION AND ELECTRIC DISCHAKGE. 483 



electricity througli gases is influenced by various circumstances, such as 

 the presence of very small quantities of water vapour, which exert so 

 strange an effect upon chemical combination. 



As for the opportunities for the study of chemical problems given by 

 the phenomena of gaseous discharge, they seem to excel even those 

 afforded by electrolysis, for the substances are in the gaseous state, the 

 state in which the properties are the simplest and have been the most 

 closely studied, while the visibility of the discharge facilitates the study 

 of the electric phenomena, as it allows us to see, to some extent at least, 

 what is going on. 



The first point by which I shall illustrate the connection between 

 chemical action and the discharge through gases is the influence exerted 

 by water vapour on the potential difference required to initiate a spark 

 through gas. The researches of Dixon, of Pringsheim, and of Baker, have 

 established that water vapour exerts a remarkable influence on chemical 

 combination. In several typical cases the presence of water seems indis- 

 pensable for chemical action ; two perfectly dry gases, even when they 

 have as strong an affinity for each other as hydrogen and chlorine, seem 

 utterly unable to combine with each other. The addition of a little water, 

 liowever, is all that is necessary to cause combination to take place. As 

 an analogue to this I shall now show some experiments which prove that 

 it is extremely difficult to start a spark through a perfectly dried gas. 

 When all traces of moisture are abstracted from the gas, the gas is able 

 to withstand without sparking a potential difference many times more 

 than that which would be sufficient to spai'k through it if it were slightly 

 moist. The experiments suggest that, if it were possible to get a perfectly 

 dry gas, then to spark through it would require so large a potential dif- 

 ference as to be far beyond any means of production at present at our 

 command. 



The experiments are of the following kind. The ends of a coil of wire 

 are attached to a and b (fig. 1), the outside coatings of two Leyden jars, 

 the insides of which ai'e connected to the terminals of a Wimshurst elec- 

 trical machine, or of an induction coil. When sparks pass between the 

 terminals of the machine electric oscillations are started, and we have 

 electric currents of very high frequency passing to and fro through the 

 wires connecting the coatings of the jars. By using jars of suitable size 

 it is easy to send through the coil electrical currents which reverse their 

 dii'ection several million times per second. This coil, with the alternating 

 currents passing through it, may be compared to the primary of an induc- 

 tion coil, and it will induce currents in any neighbouring conductor. If 

 we place inside the coil a bulb containing some gas at a very low pressure, 

 the gas will be subject to electro-motive forces due to the electro-magnetic 

 induction of the alternating currents in the coil ; if these forces are very 

 intense, they may be sufficient to cause a luminous discharge to pass through 

 the gas. This discharge will take a ring shape, since the electro-motive 

 force due to the alternating currents in the coil will act round rings pa- 

 rallel to the plane of tlie turns of the coil. I now place in the coil this bulb, 

 which contains gas wliich has not been specially dried, and you see that 

 the ring discharge flashes through the bulb whenever sparks pass between 

 the terminals of the Wimshurst machine. I will now use this arrangement 

 to show the effect of drying the gas, remarking in passing that the effect 

 of moisture is not confined to any particular method of producing the elec- 

 tro-motive force, and is just as marked when the E.M.F. is steady as when 



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