260 MR. E. M. WELLISCH ON THE MOBILITIES OF THE 



In addition to the ethyl ether obtained from Kahlbaum ether from another source 

 was fractionally distilled three times, the last time with metallic sodium, and the 

 portion boiling at 35'2 C. (barometer 769 mm.) was collected and used. The resulting 

 mobilities were in concordance with those previously obtained. 



The ionisation chambers were exhausted by means of a Topler mercury pump ; in the 

 case of a gas it was admitted to the pump and the pressure could be diminished or 

 increased with comparative ease ; in the case of a vapour the pump was first of all 

 disconnected by means of a tap and the liquid allowed to evaporate into the apparatus 

 till the requisite pressure was obtained. Often a stream of vapour was passed 

 through by means of a water pump, thus ensuring that any air which had remained 

 in the apparatus was removed. 



As is usually the case when working with vapours, the pressure at first decreases 

 owing to partial condensation ; it, however, ultimately reaches a steady state. It is 

 possible, moreover, that some chemical action between the vapour and the metal of 

 the ionisafcion chambers might interfere with the mobility values ; the best way of 

 investigating this point is to find the ionic mobilities in the vapour under conditions 

 as varied as possible. In certain instances the values obtained were not concordant ; 

 these values are given in the table of results, Imt are not used in calculating the 

 mean mobilities. In general, however, the values obtained over widely different 

 conditions were in good agreement. 



The pressures were measured by means of a mercury gauge, one limb of which had 

 been previously exhausted to a high vacuum by means of the Topler pump and then 

 sealed ; in this manner the pressure readings were made independent of the barometric 

 reading. 



7. Experimental Results. 



In estimating the ionic mobilities in a gas or vapour, it is important to secure as 

 wide a variation as possible in the experimental conditions ; such a variation was 

 obtained in the following ways : 



(i) The mobilities were estimated in different samples of the gas or vapour. 



(ii) The mobilities were often measured in each of the two ionisation chambers, the 

 remaining one in each case serving as the standard. 



(iii) The electric field was made to vary over as wide a range as was practicable. 



(iv) The mobilities were measured over a wide range of pressures. 



(v) The experiments were often repeated after a lapse of several weeks, other gases 

 or vapours having in the meantime been experimented upon. 



Under conditions so varied it was only to be expected that variations should occur 

 in the mobility values ; most of the variations were within 7 per cent, of the mean 

 value. The actual experimental conditions and the results obtained are exhibited in 

 the following tables ; the values apply to a mean temperature of about 15 C., but it 



