186 Prof. E. Rutherford on Radioactivity 



value at atmospheric pressure. Still further diminution of 

 pressure does not have much effect. 



A special experiment on the distribution of the radio- 

 activity at low pressures throws some light on the phenomena. 

 If we expose a rod charged — at a low pressure to the action 

 of thorium, it will be found that the rod is only slightly 

 radio-active, while the top of the paper over the thorium 

 oxide and the sides of the vessel are strongly radio-active. At 

 atmospheric pressure, other conditions remaining the same, it 

 will be found that most of the radioactivity is confined to 

 the rod, and only a slight amount is produced on the paper 

 and sides of the vessel. It appears as if the radio-active 

 particles are unable to be all carried to the negative electrode 

 at low pressures. This may be due to the increased rate of 

 diffusion of the active particles at low pressures, or more 

 probably to the small number of ions produced by the " emana- 

 tion " at low pressures. 



It is found that the current through the gas due to the 

 " emanation " falls off nearly proportionally to the pressure, 

 so that the number of ions present between cylinders at low 

 pressures is a very small fraction of those at atmospheric 

 pressure. 



The following table gives results of the variation of the 

 current, due to the emanation, with pressure of air in the 

 apparatus of fi< 



ng. J 1 a. 





Pressure of Gas. 



Current due 



millini. 



to Emanation 



760 



1 



587 



•819 



402 



•582 



214 



•297 



145 



•203 



93 



•133 



25 



•046 



Fig. 12 shows the results graphically. The curve is nearly 

 a straight line. If the conveyance of the radio-active 

 particles to the electrode is due to the movements of the ions 

 between cylinders, at low pressures the number of ions may 

 be too small to be effective in that respect. 



Effect of Gases. 



The apparatus shown in fig. 11 was used. The amount of 

 radioactivity produced on the central rod was not very 

 different whether the gas was hydrogeu, air, or carbonic acid. 



