90 ADVENTURES IN RADIOISOTOPE RESEARCH 



gradually become deposited from the suspension in the air ; this de- 

 position process can be considerably accelerated by applying an electric 

 field. In our experiments we made use of a radiothorium preparation, 

 which provides a constant source, and the active deposit yielded by the 

 emanation was collected on a quartz surface 1.6 cm in diameter. The 

 quartz disk was covered with a mixture of ThB (lead isotope) and ThC 

 (l)ismuth isotope) because the first decay product of emanation, ThA, 

 decays very quickly with a half-life of ^1^ sec. A simple calculation yields 

 5 X 10-11 gm as the total mass of the deposit, of which about 90 per cent 

 consists of ThB and 10 per cent of ThC. In order to cover the surface 

 completely with a molecular layer of lead 2 x 10^^ gm would be necessary, 

 i.e. 50,000 times the amount actually present ; we can thus rightly 

 consider tiie surface as having an infinitely thin covering of lead and 

 bismuth. 



The velocity of dissolution was determined as follows : The quartz 

 disk was allowed to stand for several hours after cessation of the activation 

 until radioactive equilibrium had been established and the /^-activity 

 formed by the deposit, and showing the relative amounts of Pb and Bi 

 present, was then determined ; the disk was then placed in a bell- 

 shaped vessel, provided with an outlet tube and cock, and after a certain 

 time the 100 cm^ of liquid in the vessel was drained out. Care was taken 

 to attain a constant stirring speed, the disk being placed in the solution 

 only when this speed had been established. 



The /3-activity of the quartz disk treated in this way was then measured 

 again 15 min after completion of the experiment and at various later 

 time intervals, and it was thus possible to decide upon the amounts of 

 Bi and Pb present, from the change of the activity with time, and to 

 determine the percentage which had entered into solution. 



THE DETERMINATION OF THE VELOCITY OF DISSOLUTION 



The investigation was concerned with the determination of the effect 



(1) of the concentration of acid in the solution 



(2) the viscosity 



(3) of the speed of stirring 



(4) of the time 



on the velocity of dissolution of the molecular layer and finally on the 

 effect due to isotopes of the corresponding elements in the solution. 

 The percentages of Bi and Pb isotopes which dissolve in nitric acid 

 in 60 sec, under the same experimental conditions, are summarized in 

 Table 1. 



