804 Mobility of Positive Ions in Gases at Low Pressures. 



forms a sufficient proportion of the gas present its molecules 

 carry the positive current. At the pressures used in the 

 experiments described in the present paper, where the 

 proportion of carbon monoxide from the walls of the vessel 

 would be exceedingly small, the positive current is carried 

 by groups of the gas-molecules. 



Conclusions. 



1. The positive electricity emitted by heated aluminium 

 phosphate forms ions in the gns present identical with the 

 ions produced by X-rays, showing that the positive particle, 

 whatever it is, which is emitted from the salt hands over the 

 charge to the gas- molecules on collision. 



2. The mobility of the positive ion is inversely propor- 

 tional to the pressure of the gas down to a certain low 

 critical pressure approximately proportional inversely to the 

 density of the gas, below which pressure the mobility 

 increases very rapidly. 



3c The presence of water-vapour, which has very little 

 effect on the mobility of the positive ion in regions where 

 the inverse-pressure law holds, has a big effect when the 

 pressure is below that region, making the mobility less than 

 that required by the inverse-pressure law. 



4. The rapid increase in the mobility at very low pressures 

 may be due to the cluster of molecules forming the ion 

 breaking up into a less complex group ; or it may be due to 

 the existence of a positive unit which exists in the free state 

 for a short time at low pressures ; or both these possibilities 

 may hold good. Calculating the mass of the ion from the 

 formula k = eXjmv gives numbers indicating that the ion 

 consists at ordinary pressures usually of about six molecules, 

 and in the case of hydrogen of about twelve, round the 

 positive charge. These groups break up at low pressures, 

 giving an increased mobilit} T . 



5. In the cases of air and carbon dioxide the measure- 

 ments at the lowest possible pressures show that the positive 

 charge is carried by something less than the molecule of the 

 gas. This may be due to the existence of a universal 

 positive unit, or it may be due to hydrogen which the 

 platinum probably contained. 



In conclusion, the author expresses his sincerest thanks to 

 Professor Sir J. J. Thomson for his kind encouragement and 

 persistent interest in the experiments. 



