the Natural Ionization in a Closed Vessel. 225 



i£ it is one of the known ones, must be that of radium, since 

 the emanations of thorium and actinium have much too short 

 a life to exist during the time required to take a reading. 



The conditions of the experiment are not sufficiently well 

 known to be sure of the explanation of the slight curvature 

 of the lower portion of the curve, but it can very likely be 

 ascribed to radiations shot out obliquely from the sides of 

 the vessel, which even at atmospheric pressure have not 

 completed their full path in air before striking another side. 



The amount of emanation required to produce the sloping 

 part of the curve can be calculated from these results and is 

 found to be exceedingly small. 



If we assume that the final portion of the curve is due 

 exclusively to the radiation from the emanation, we see from 

 the slope of the line that the ionization due to it is '00104 

 division per minute at atmospheric pressure. Now, "017 

 division per minute on the scale corresponds to an ionization 

 of 13 ions per cc. per second. Consequently, the ionization 

 due to the emanation present is only *80 ion per c.c. per second. 



Also, Rutherford has shown that one a particle from radium 

 produces about 86,000 ions in its path through air. Taking 

 into account the difference in the ranges, an a particle from 

 the emanation must produce about 100.000 ions before it 

 conies to rest. 



Now, 800 ions are produced per c.c. per second by the 

 emanations in the whole of the vessel which has a capacity 

 of 1000 c.c. This corresponds to the passage of one a particle 

 in every 125 seconds on the average. 



And the radioactive constant of the radium emanation is 



±(\i ooo' *kat * s ' one a ^ om m evel T 463,000 disintegrates 



per second. 



Therefore, since we have only one atom breaking up in 

 every 125 seconds, the number of emanation atoms present 

 must be 3700 or 3'7 per c.c. 



This is an excessively small amount, and we can hardly 

 expect to get rid of it, no matter how clean the vessel in 

 which the air is stored. The effect of this small quantity is 

 of course practically immeasurable at atmospheric pressure, 

 with the electroscope used, and only becomes at all marked 

 when the pressure is raised considerably. 



This research was suggested by Prof. Rutherford, and 

 I wish to acknowledge his ever ready advice and en- 



couragement. 



Physical Laboratory, 

 The University, Manchester. 



Phil. Mag. S. 6. Vol. 17. No. 97. Jan. 1909, Q 



