360 Prof. J. J. Thomson on the Passage 



be greater than 2Fde. But if the molecule is split up by 

 the electric field, the work done by the field must equal the 

 difference between the energy of the molecule and its atoms, 

 a constant quantity if the temperature remains constant. 

 Thus Yd is constant, so that Y must vary inversely as d. Now 

 the effects produced by the collisions between molecules are 

 probably so vigorous, that the electric forces trying to split 

 up the molecules will have to commence their work afresh 

 after each collision : in this case d is the free path of a 

 molecule in the neighbourhood of the electrode, and the force 

 required to produce discharge varies inversely as the free path 

 and therefore inversely as the density. This investigation, or 

 a similar one, will hold if we suppose that the splitting up of 

 the molecules is helped by the presence of an " electric double 

 sheet " near the surface of an electrode, corresponding to a 

 definite difference of potential between the electrode and the 

 gas, or whether we suppose short " Grothaus chains '* to be 

 formed, and the splitting up to pass from one molecule in 

 the chain to another, being helped by the chemical forces 

 between the molecules. 



If we suppose the charges on the atoms to be the same as 

 those deduced from electrolytic considerations, the force be- 

 tween the atoms in the molecule is so much greater than the 

 force tending to separate them, in a field of 30,000 volts 

 per centimetre, that it seems almost necessary to assume that 

 the decomposition is helped in some such way as has just been 

 suggested. 



The increase in the strength when the density of the gas 

 falls below a certain value may be accounted for by the 

 diminution in the number of atoms available for carrying the 

 electricity, by the interference of the vessel with the free path, 

 and it may be by the increased difficulty of forming a 

 " Grothaus chain " in a rare gas. 



We ought also to notice that since Yd must be constant, 

 and since d cannot be greater than the distance between the 

 plates, the electric strength must be infinitely great when the 

 distance between the plates is indefinitely small. 



According to this view, the molecules of a gas are essen- 

 tially electrically neutral, and any electrification in the gas 

 must be due to the presence of free atoms. Now the most 

 careful experiments seem to show that air is incapable of 

 receiving a charge of electricity ; for Nahrwold's experiments 

 (Wied. Ann. xxxi. p. 448) show that apparent exceptions to 

 this rule may be explained by the presence of dust, either 

 originally present in the gas or given off from the electrodes. 



Perhaps the most conclusive evidence of the impossibility 



