Mobility of Positive Ions in Gases at Low Pressures. 791 



£hey found that if a salt bead were introduced, the negative ions 

 were much larger than the positive. On account of the strong 

 attraction of the chlorine atoms for electrons, the negative 

 ions were atoms of chlorine, the positive ions being most 

 probably the hydrogen atoms. The exact mobilities could 

 not be determined ; but it was found that the positive ion 

 had less than one-tenth the mass of the negative ion, i. e. the 

 chlorine atom. All the experimental evidence at our disposal 

 leads one to imagine that the positive flame ion, though pri- 

 marily due to the metal salt, is not an atom of the metal, 

 but some smaller body that is always present in the flame. 



Summary. 



(1) The mobility of the positive ion due to salt vapours in 

 a Bunsen flame has been measured at intervals of temperature 

 ranging between 1150° and 1950° absolute. 



(2) Down to 1300° all salts of all conducting metals produce 

 the same ion; below this temperature the ion due to mono- 

 valent metals has a greater mobility than that due to divalent 

 metals. 



(3) Between 1950° and 1400° the mobility varies as the 

 square root of the temperature, and corresponds closely to 

 that calculated for a hydrogen atom. Below 1400° the 

 mobility drops very rapidly, as if extensive clustering took 

 place. 



In conclusion I desire to express my thanks to Professor 

 Sir J. J. Thomson for his encouragement and advice during 

 the course of these experiments. 



LXXVI. The Mobility of Positive Ions produced from Heated 

 Aluminium Phosphate in Gases at Low Pressures. By 

 George W. Todd, M.Sc, B.A., 1851 Exhibition Scholar of 

 Birmingham University ; Emmanuel College, Cambridge *. 



Introduction. 



AT the present time, when practically all phenomena 

 connected with the passage of electricity through 

 gases can be explained by the presence of ions, any experi- 

 ments which bring to light new properties concerning ions 

 are valuable. Most of what is known of the structure 

 of ions has been obtained from determinations of their 

 mobilities. The present paper is concerned with the mo- 

 bilities of the positive ions in gases at low pressures. Over 



* Communicated by Sir J. J. Thomson, F.R.S. 



