738 Prof. Townsend and Mr. Hurst on the 
The other values are given in the following table. 
Distance from end of Path. Radius of Curvature. 
Ieee PU EL a Tee ae eke ae 
72 oh 
1:3 a1 
1:4 a 
5 oe 
db 6) a) 
1'8 ey 
2°0 40) 
The radius of curvature should be approximately pro- 
portional to the square root of the distance from the end of 
the path: and the agreement is quite as close as we could 
expect. 
This argument applies of course to the outer edge of the 
trace only. Butit may be argued that this edge should have 
more character than the other because we know that the rays 
end with precision. The inner edge of the trace is made up 
of the paths of rays which gradually decrease in intensity as 
we proceed from right to left in the figure. It is probable 
also that the photographic action, like the ionizing action, is 
more intense at the end of the path. The figures drawn 
greatly exaggerate the actual deflexions; for in Becquerel’s 
trace the length of AP is only 1 cm., and the deflexion at P 
only ‘05 cm. | 
The University of Adelaide. 
Adelaide, Sept. 8, 1904. 
LXXV. The Genesis of ons by the Motion of Positive Ions, and 
a Theory of the Sparking Potential. By JOHNS. TOWNSEND, 
M.A. F.RS., Wykeham Professor of Physics, Fellow of 
New College, Oxford; and H. E. Hurst, B.A., Hertford 
College, Oxford *. 
| Es a paper on the “ Genesis of Ions in a Gas” (Phil. Mag. 
Nov. 1903) the theory of ionization by collision in a 
uniform field of force was explained, and a number of experi- 
ments with air and hydrogen were given in illustration of 
the phenomena. The apparatus which was described in that 
paper was used for a further set of experiments with the 
same two gases in order to test the theories over a wider 
range of pressures. | 
* Communicated by the Authors. 
