292 Wellisch — Experiments on Active Deposit of Radium. 



small (12). A large applied negative potential brings over 

 similarly the negatively charged aggregates. 



An essential condition for the presence in the gas of a large 

 amount of active deposit is that sufficient time should be 

 allowed for the particles to accumulate. Experiments 10 and 

 12 show well the difference which results from varying the 

 time allowed for accumulation. From a knowledge of the 

 activity resulting from a long exposure (cf. 11) it is easy to 

 calculate by the aid of standard transformation formulae the 

 activity which should result from an exposure of one minute in 

 the absence of any accumulation. The theory gives that the 

 former activity is 40 times the latter, which shows that the 

 activity in experiment 12 is due practically entirely to the 

 formation during one minute ; experiment 10 shows in con- 

 sequence the preponderating influence of the deposit which has 

 been allowed to accumulate in the gas. 



There is abundant evidence that the aggregates owe their 

 charge to the ions present in the gas. Experiments 13, 14 and 

 16 bring out this point most clearly ; in experiment 13, where 

 the duration of exposure is long, the ions are removed from the 

 gas by the applied field and the activity measured is due prac- 

 tically entirely to the diffusion of the neutral deposit particles ; 

 in experiment 14 there has not been sufficient time for 

 accumulation of deposit in the gas and the consequent forma- 

 tion of aggregates ; in experiment 16 the central electrode 

 receives the negatively charged aggregates. Moreover for long 

 exposures there is more deposit on the central rod with an 

 applied potential of —400 volts than with —990 (cf. 1 and 5). 

 With the former potential it was easy to show by readings of 

 the ionization current that there was considerable recombina- 

 tion of the ions ; this implies the presence of ions in the gas 

 and hence an increased number of charged aggregates of active 

 deposit.* Experiments 16 and 17 or 20 and 21 show that the 

 amount of positively charged gas activity is slightly greater 

 than the amount which is negatively charged ; the ratio is 

 roughly the same as that of the mobilities of the negative and 

 positive gas ions ; in other words, the amounts of positive and 

 negative activity are roughly in the same proportion as the 

 numbers of positive and negative ions present in the gas. 



More direct evidence that the aggregates acquire their charge 

 from the ions present in the gas is furnished by investigating 

 the effect produced by causing Kontgen rays to pass through 

 the gas. A beam of fairly intense Rontgen rays passed 

 through the aluminium bottom of the vessel for about 15 

 minutes before the introduction of the electrode ;- the rays con- 



*Of course the charged aggregates also suffer recombination, but they 

 acquire a fresh charge from the ions present in the gas. 



