454 Mr. S. Ratner on the 



in purer hydrogen and nitrogen, as well as in carefully dried 

 air, are now in progress. 



4. Experiments in heavy gases. — Experiments were also 

 made in order to test the mobility of the negative ion in 

 heavy gases, such as the vapours of chloroForm, carbon 

 tetrachloride, and methyl iodide *. The mobility in these 

 gases was found to be constant for the negative as well as 

 for the positive ion, the mobility curves being straight lines 

 parallel to the X axis even at low pressures (down to 5 mm.) 

 and with large electric forces. 



A striking effect illustrating the influence of impurities on 

 the mobility of the negative ion was observed during these 

 experiments. Even after the chloroform was removed and 

 the apparatus, as well as the bell-jar and the pump, was 

 thoroughly cleaned and freed from traces of this gas, no 

 further experiments in other gases were possible for many 

 days. It was found that chloroform vapour present in air 

 to an amount as small as 1 part in 1,000,000 changes com- 

 pletely the aspect of the mobility curves. 



Control Experiments and Sources of Error. 



1. It is important for the accuracy of the experiments, 

 that the total wind-pressure produced by the ions should be 

 given by the gauge, and this is secured if all the ions reach 

 the plate B within the area of the grating. In order to 

 ascertain this the grating was insulated from the plate by a 

 narrow air-gap, and the galvanometer connected by means 

 of a suitable key, either with the plate or with the grating. 

 It was found that even at the largest distance between the 

 plates, and under conditions in which the lateral diffusion of 

 ions is abnormally great, the total current is received by the 

 grating, the charge picked up by the plate being imper- 

 ceptible. 



2. The temperature of the gas close to the heated strip 

 must be very high, and therefore the mobility of the ion at 

 the moment when it leaves the strip must be large. In order 

 to ascertain the magnitude of this source of error, curves 

 representing the mobility of an ion as a function of the 

 electric force were drawn for distances between the plates 

 varying from 5 to 45 mm. No difference between them 

 could be observed, which shows that the path traversed by 



* In these experiments the silvered surface of the mirror was attacked 

 by the halogens. I wish to thank Mr. E. Everett for suggesting to me 

 the idea of platinizing the mirror and for setting up the cathode-ray 

 apparatus necessary for this operation. 



