at Low Pressures through, the Passage of Electricity. 183 



agreement with ir. Hence, probably, p may be at least as 

 much as 40 mm., although this cannot be said with certainty. 



The lower limit for the mean pressures was about 1 mm. 

 At pressures less than this the results were nugatory. For 

 the current G rapidly decreased, and the sparking was no 

 longer confined between the electrodes, but appeared any- 

 where within the bell-jar. 



It should be added, however, that after about three-quarters 

 of the mixture has been made to combine into water, the 

 union of the rest does not appear to conform to the same 

 laws. This seems to be the case with silver electrodes. With 

 zinc electrodes this irregularity appears to occur sooner, viz., 

 when about two-thirds of the mixture has gone. 



Returning now to the equation 



Ap pD 3X 

 AQ~ 10 + spo* 



let us denote by W the number of molecules of water-vapour 

 whose formation is connected with the passage to the elec- 

 trodes of each pair of ions. Then, since it has been shown 

 above that 



it follows that 



W-4 A/ ' 



w ./ pD 3X\ 

 W=4 (l0 + 800> 



an equation which is independent of the capacity of the 

 apparatus. 



Now either — or '—molecules of a mixture of hydrogen 



and oxygen must he dissociated into atoms before W mole- 

 cules of water can be formed ; the first if it is not necessary 

 to dissociate the hydrogen, the second if it is necessary. 



This necessary dissociation may be assumed to be brought 

 about by the ions colliding with the molecules of the gas as 

 they move to the electrodes. The first term of the right side 

 of the equation 



w / P D 3X\ 



w= <Io + «oo) 



then becomes intelligible. For pT> is proportional to the 

 quantity of gas between the electrodes, and therefore to the 

 nnmber of collisions made, on the average, by each negative 

 ion. if each is supposed practically to cross the whole distance 

 between the electrodes. But this is probably what happens. 

 For the ions which constitute the current, as they move to 



