of Electricity through Gases. 321 



electric effects of A ; as one of these effects is to bold the 

 atom B in combination, the affinity between the atoms A and 

 B will be almost annulled by the presence of the sphere. 

 Molecules condensed on the surface of a sphere will thus be 

 practically dissociated. The same effect would be pro- 

 duced if the molecules were surrounded by a substance 

 possessing a very large specific inductive capacity. Since 

 water is such a substance it follows, if we accept the view 

 that -the forces between the atoms are electrical in their 

 origin, that when the molecules of a substance are in 

 aqueous solution the forces between them are very much 

 less than they are when the molecule is free and in the 

 gaseous state. 



Since w T ater- vapour produces so great an effect on chemical 

 combination, the question suggests itself whether its presence 

 has any considerable influence on the passage of electricity 

 through gases, as there is strong evidence that this pheno- 

 menon is closely connected with chemical changes taking- 

 place in the gas through which the discharge passes. A very 

 large number of experiments have been made on the effect of 

 the presence of aqueous vapour on the potential-difference 

 required to produce a spark of given length in air. The 

 results of these experiments are very discordant; it is not 

 necessary, however, for our purpose to enter into any detailed 

 discussion of the greater part of them, as they for the most 

 part consisted of determinations of the effect of increasing 

 the amount of aqueous vapour in air already damp, whereas 

 the most striking result produced by aqueous vapour on 

 chemical combination is not so much the difference between 

 the behaviour of gases which contain a moderate amount of 

 aqueous vapour and those which contain still more, as the 

 difference between gases which are damp and those from 

 which as much moisture has been removed as is possible with 

 the means at our control. 



The only investigation with which I am acquainted where 

 the gases were comparable in dryness with those used in 

 the chemical experiments previously referred to is one by 

 Warburg* on the cathode fall, t. e. the potential-difference 

 between the cathode and the luminous boundary of the dark 

 space in nitrogen and hydrogen, (1) when the gases are very 

 dry, (2) when the gases are damp. Warburg found that the 

 cathode fall in dry nitrogen is greater than that in damp, 

 while in hydrogen the reverse is the case, the cathode fall in 

 the damp gas being greater than that in the dry. The dif- 



* AVied. Ann. xxxi. p. 545 (1887). 



