408 Experiments on Residual Ionization. 



brass connecting wire at the top of the ionization-chamber,. 

 which junction was the only one that could possibly serve as 

 a hot junction, was heated with a small flame, the flame was- 

 removed, and the earth connexion to the quadrants of the 

 electrometer broken, but no change could be detected in 

 the normal ionization current flowing to the electrometer. 

 It seems likely though that the rise in current sometimes 

 noted at temperatures of about 100° was due to small 

 quantities of emanation being driven off from the Avails of 

 the chamber. 



It might be interesting to see if the proposed formula for N 

 agrees at all with the results of the temperature experiments. 

 The experiments on pressure show that the number of ions 

 produced per c.c. per second in carbon dioxide at 20° C. and 

 760 mm. pressure is probably less than four. In the ex- 

 pression for N we have then to assign values to u and v so 

 that the following conditions may be fulfilled: 



(1) N must be about 4 at 20° 0. and 760 mm. pressure. 



(2) N must change slowly with the temperature, at least in 



the region of 20°. 

 The only physical condition suggesting itself which will 

 fulfil the above requirement is that, for a collision to produce 

 ionization, it must be almost perfectly tangential (this will 

 make the total number of such collisions small), and that the 

 arbitrary minimum tangential velocity of each of the colliding 

 molecules must be about equal to the most probable velocity 

 for a temperature of 20° (this ensures that N shall change 

 slowly with the temperature in this region). Then for carbon 

 dioxide, if we put the minimum relative tangential velocity 

 u = 2a = 2 x 3*43 x 10 4 cm. per second, and the maximum 

 relative normal velocity v = 8*97 x 10 -10 cm. per second, we 

 find that at 20° C, N = 4, and at 100°, N = 5\L. That is, N 

 changes very slowly as the temperature is raised, which is in 

 qualitative agreement with the experimental results. A more- 

 exact application of the formula for N" does not seem worth 

 while at the present time, since, for the reasons stated above,, 

 the accuracy of the readings does not warrant it. It may, 

 however, be of interest to note that using the above values 

 for u and v, and making changes in A, p, and r) to correspond 

 to the rise in temperature, at 302° C. the value for N is 6'4. 



Summary. 



(1) It has been shown that the high residual ionization 

 in acetylene prepared from calcium carbide is due to the- 

 presence of slight traces of radium emanation. 



