in Air by the Action of a Radio-active Substance, &c. 123 



Table I. Positive Ions in 

 Dry Air. 



Table II. Negative Ions in 

 Dry Air. 



Table III. Positive Ions in 

 Moist Air. 



Table IV. Negative Ions in 

 Moist Air. 



The values of K are expressed in ^ '- . 



second 



The values of P are expressed in millimetres of mercury. 



each case. It will be noticed that the experiments at 772 mm. were 

 made when the temperature of the air was higher than the tempera- 

 tures during the other experiments. 



The negative ions which are produced when ultra-violet light falls on 

 a zinc plate diffuse into air at nearly the same rate as the negative 

 ions produced by a radio-active substance. The values of the co- 

 efficients of diffusion for dry and moist air are 0*0435 and 0'0375 

 respectively, the pressure being 760 mm. and temperature 17 C. in 

 each case. 



The rates of diffusion of ions produced by a point discharge were 

 found to vary considerably. The discharges were usually produced 

 from a steel needle or platinum wire pointing along the axis of a metal 

 tube. It was found that when the point was at the open end of the 

 tube the ions which were produced diffused more rapidly than those 

 given off when the needle, or wire, was drawn back into the tube, so 

 that the point should be a few centimetres from the open end. The 

 differences obtained in this way were greater when the air was dry 



