802 Mr. G. W. Todd on the Mobility of Positive Ions 



the product o£ pressure and mobility 1 



las a va 



lue of about 



360, and the same thing occurs in the case of carbon dioxide 

 at approximately 240. If these bends in the curve are real, 



Gas. 



Mass of 

 molecule 



xio 2i . 



A(0° & 760) 



xios. 



!/(0°j. 



971 X 10 2i . 



Ordinary press. 



Lowest press. 



H 2 



CH 4 ... 



Ah- 



C0 2 ... 

 S0 2 



2 

 16 

 29 

 44 

 64 



1780 

 800 

 960 

 650 

 470 



169800 

 60000 

 44700 

 36100 

 29800 



25 (12 mols.) 



96 ( 6 „ ) 



216 ( 7 „ ) 



243 ( 5 „ ) 



361 ( 6" V ) 



5 ( 2 mols.) 



29 ( 2 „ ) 



24(<^1 „ ) 



25 (<1 „ ) 



107 ( 2 „ ) 



it means that the ion at that stage of its dissociation becomes 

 very stable, and resists disintegration until the pressure is 

 lowered further. This stage in both air and carbon dioxide 

 corresponds to an ion consisting of about two molecules. 



In the case of hydrogen the product of pressure and 

 mobility remains fairly constant between 4'0 mm. and 0'5 mm. 

 pressure, and it could not be ascertained whether the mobility 

 increased again at lower pressures because the limit of the 

 apparatus had been reached. This stage in hydrogen also 

 corresponds to about two molecules to the ion. 



Note on the Carriers of the Positive Charge 

 from Hot Salts. 



Although several experiments have been made to deter- 

 mine what actually is the carrier of the positive charge, the 

 question is still uncertain. Sir J. J. Thomson *, who first 

 measured the ratio ejm for the positive particles from an 

 iron wire and a carbon filament, found that m was about 

 26 times the hydrogen atom, and he suggested that the 

 carrier might be the molecule of carbon monoxide, because 

 the spectrum of a discharge in the tube gave the carbon- 

 monoxide lines. Richardson t has shown that the mass of 

 the positive carrier from heated metals is independent of the 

 metal, and is the same as that found by Sir J. J. Thomson. 

 He suggested that the carrier was a common impurity in all 



* ' Conduction in Gases,' 1906, p. 148. 

 f Phil. Mag. xvi. p. 740 (1908). 



