Dust and Smoke on the Ionization of Air. 671 



With a high potential the smoke increased the current 

 fourfold, and at a low potential decreased it to one-fifth value. 

 The critical potential was about 900 volts, and then the 

 increase of current due to the larger number of feebly mobile 

 smoke ions bain need the loss of the fewer but mobile small 

 ions, which the former had replaced. 



In a similar manner the critical potential of B was deter- 

 mined to be about 760 volts. 



Volts on centre. No Smoke. Smoke. 



Div./min. Div./min. 



1000 2-9 5-0 



900 2-0 33 



800 1-7 2-0 



700 1-5 1-0 



600 1-5 0-4 



The readings in a column are not taken over the same part 

 of the scale, but the corresponding readings in the right two 

 columns are so taken. 



The air passes the electrode B with twice the velocity it 

 passes through A, hence the critical potentials are more 

 nearly equal than would be otherwise expected. 



Ammonium Cliloride Fumes. 



An air current was next used into which the fumes from 

 ammonium chloride were introduced. Brogiie * has shown 

 that such fumes do not contain charged centres, but these he 

 fimU are readily formed by contact with glass or guttapercha 

 in their passage through tubes, on the same principle perhaps 

 as dust acquires a charge in Lichtenberg's figures. In my 

 experiment air was driven, without bubbling, through a flask 

 containing HC1 and along a metal tube, at the very end of 

 which was cotton wool moistened with ammonia. The fumes 

 were found to contain charged centres when no radium was 

 near, but possibly they received the charge in their passage 

 along the metal tubes to the testing vessel. However that 

 may be, the results obtained with these fumes were analogous 

 to those found with smoke. The results in divisions a minute, 

 with radium present, were : — 



Elect. A (low pot.). Elect. B (high pot.). 



Air current alone Ill 3*6 



with HC1 83 9-5 



withNH 3 62 5-0 



„ with NH 4 01 fumes 38 750 



Le Radium. Jul)'. L909. 



