294 Wellisch — Experiments on Active Deposit of Radium. 



produced per cc. per second is approximately 10 8 . If we 

 assume that the value of the coefficient of recombination 

 (a) is unaltered by the presence of the emanation and take for 

 a the value assigned by Townsend, viz., 3400 X 0, then 

 the number of ions of each sign present per cc. in the vessel is 



/ 



q = 7-95 X 10 6 . 



When the density of ionization in the gas falls below this 

 amount there seems to be a rapid diminution in the amount of 

 charged activity in the gas. When the emanation decays still 

 further the active deposit particles in the gas appear to be 

 uncharged and in amount to be a constant fraction of the total 

 amount of deposit in equilibrium with the emanation. It 

 appears from the results that aggregation of the deposit 

 particles ceases fairly abruptly at the critical stage, but there is 

 also evidence that a considerable fraction of the deposit 

 particles in the gas can still be charged even after they have 

 ceased to aggregate. Aggregation implies a slow rate of dif- 

 fusion, which is made manifest by the very small amount of 

 deposit collected on a central rod in the absence of an electric 

 field. It is easy to tell, by measuring the proportionate activ- 

 ity collected on such a rod, the stage when aggregation prac- 

 tically ceases. As stated before, aggregation ceased fairly 

 abruptly when the saturation current was 5*0 X 10 -9 ampere, 

 but evidence at any rate of negatively charged deposit particles 

 in the gas was found even with the current as low as 10" 9 

 ampere. 



The general effect seems to be that the active deposit atoms 

 situated in the gas charge up, both positively and negatively, 

 in appreciable numbers when the density of ionization exceeds 

 a certain amount ; the atoms thus charged act then as nuclei of 

 condensation and build up aggregates of active particles when 

 these are present in sufficient quantity. 



There is, of course, a possibility that both the aggregation 

 and the charging-up may be conditioned by the presence in the 

 gas of foreign nuclei, such as traces of dust particles, etc. 

 Against this view, however, is the fact that practically identi- 

 cal results were obtained in different series of experiments 

 when in every instance great care was taken to exclude foreign 

 matter. Such matter might, however, have been introduced 

 on the insertion of the electrode. It should, in this connec- 

 tion, be mentioned that when the emanation had decayed so 

 that the saturation current was in value between 5'0xl0" 9 and 

 10" 9 ampere, the number of charged particles for any given 

 amount of emanation, although small, was subject to irregular 

 variations. 



