The mobility of ions in air. Part III.—Air containing organic vapours

The data on mobility of gaseous ions available at present have not led to a satisfactory quantitative theory of ionic structure because of the large number of variables necessarily entering into the formulæ deduced. The writers feel that the most fruitful line of experimental attack is to carry out a series of different experiments under conditions chosen so that one or more of these variables may be regarded, at any rate approximately, as unchanged throughout the series. The work herein described is an attempt in that direction and deals with the mobility of ions in air containing the vapours of a series of closely related organic substances. It is well known that the presence of water vapour in air causes a reduction in the mobility of the negative ions. The effect has recently been studied in detail from small traces up to saturation by one of the writers (A. M. T.) and Mr. G. C. Grindley. Other vapours are also known to be similarly effective. Thus Rutherford showed that alcohol and ether vapour lower the mobility of negative ions in air. Przibram found that various saturated organic vapours affect both positive and negative mobilities, though quantitatively his method is not free from objection. Wellisch found that 10 mm. of alcohol or acetone introduced into air at atmospheric pressure caused a reduction in both mobilities, but that 6 mm. of ethyl bromide or iodide had no effect.