308 Kev. P. J. Kirkby : Chemical Effects of the 



experiments throw no light on the question whether positive 

 ions do or do not form a sensible proportion of the current 

 within the positive column : .whatever view is taken these 

 chemical effects are equally well explained by the collision 

 of the ions present in the positive column with the molecules 



of gas. 



Estimate of work required to dissociate 

 a Molecule of Oxygen. 



If one is right in attributing the chemical action within 

 the positive column to the motion of negative ions, the 

 numbers given above can be applied to estimate the work 

 done in breaking ujj a molecule of oxygen into its atoms. 

 The mean free path of a negative ion in hydrogen at 1 mm: 

 pressure is -1- cm. (Townsend, Phil. Mag. June 1902) and in 

 air is about -^5 cm - Hence we shall not be far wrong in 

 taking the mean free path of a negative ion in hydrogen and 

 oxygen to be -J- cm. At the pressure 4*5 mms. a negative ion 

 will therefore make 36 collisions in passing through a centi- 

 metre. During this passage it dissociates enough molecules 

 of the gas to produce '75 molecule of water. If we assume 

 that to be the result of dissociating -37 molecule of oxygen,, 

 it follows that one dissociating collision occurs out of nearly 

 100 collisions made by a negative ion in moving through the 

 mixed gases at pressure 4*5 mms. and under a force of 50 volts 

 per cm. Now one free path out of 100 is Aery nearly 

 4*6 times the mean free path, which, for a negative ion in the 

 case under consideration, is ^ cm. It follows that to 

 dissociate a molecule of oxygen cannot require more energy - 

 than that due to a fall of a negative ion through a potential 



difference of J^--, x4*6= j 6*4 volts. And since the charge 



on a negative ion is nearly 3 X 10 ~ 10 in electrostatic units, the 

 work required to dissociate a molecule of oxygen cannot 

 exceed 6*4 x 1.0 -12 ergs. Probably, however, this limit is 

 rather high. For even if the molecules of hydrogen do not 

 have to be dissociated as well as the molecules of oxygen 

 before water can be formed, it is highly probable that of 

 the dissociated atoms of oxygen some will recombine into 

 molecules of oxygen. Hence the above limit may be a small 

 multiple of the work which it is necessary to expend in order 

 to separate the atoms of a molecule of oxygen. 



Again, Professor Townsend has shown (Phil. Mag. Feb. 

 1901, p. 220) that the energy required to break up a molecule 

 into ions is net greater than that gained by an ion in moving 



