of charged Ions through a Gas. 379 



G. Mr Wellisch has proved to a high degree of accuracy that 

 in these mixed gases there is only one mobility. Let us take 

 the case of O2 and SO2 investigated by Mr Wellisch and cal- 

 culate the velocity (1) of an oxygen ion consisting of a charged 

 molecule of oxygen, and (2) that of an ion of SO2 consisting 

 of a charged molecule of SO2 ; let us suppose that the partial 

 pressures due to these gases are the same so that Vi = v^. If rn^ 

 is the mass of an oxygen molecule, m^ that of an SO2 molecule, 

 nis = 2m2, and /i2 — 1 = '0003, /A3 — 1 = -00066 ; substituting these 

 numbers we find that the velocity of a charged oxygen molecule 

 through the mixture would be about 15 per cent, greater than 

 that of a charged molecule of sulphur dioxide, a difference which 

 could easily have been detected in Mr Wellisch's experiments. If 

 the ions from oxygen and sulphur dioxide had been much more 

 complex than the single molecule of these gases their mobilities 

 through the mixed gases would have been much more nearly 

 equal than for the simple ions, and the difference in this case 

 might have escaped detection. 



We shall consider in the light of the expression given above 

 for the mobility the various views that have been taken of the 

 nature of the ion. There are two main points to be considered 

 with regard to the ion, (1) is the ion more complex in structure 

 than a molecule, i.e. does it consist of an aggregation of molecules, 

 and (2) can the electric charge on the ion leave the ion and find 

 another home, thus producing a new ion, or is the charge bound 

 by an indissoluble bond to the molecules forming the ion. It 

 would seem clear from the great increase in the mobility of the 

 negative ion which takes place in flames that the negative charge 

 must be able to leave one ion, exist for a time as a corpuscle and 

 then form a fresh ion: the question then is confined to the 

 positive ion, and we have to consider whether or not a positive 

 charge, carried by something much more massive than a corpuscle, 

 can leave one positive ion, unite with a molecule of the gas, and 

 form a new positive ion. If there were no transference of charge 

 from the positive ion, then if the positive ion was a single 

 molecule or even two or three molecules, there would in mixtures 

 of gases be two sets of positive ions moving with speeds sufficiently 

 different to have been detected in experiments like those made by 

 Mr Wellisch. Wellisch made some other experiments where a 

 small quantity of methyl iodide was mixed with a large quantity 

 of hydrogen, and it was found that the velocity of the positive 

 ions (which had originated from the methyl iodide) through the 

 mixture was the same as that of the positive ion through pure 

 hydrogen when the ion had originated from the hydrogen. If the 

 positive ion in hydrogen had been a single molecule of hydrogen 

 the velocity of the positive ion in pure hydrogen would be \/2 times 



