92 ADVENTURES IJT RADIOISOTOPE RESEARCH 



radioactive recoil process. A portion of the active deposit is laid down 

 on the quartz disk in the form of ThA, the ThA then emits a-particles 

 with the result, according to the principle of action and reaction, that 

 a recoil of the atoms is required and thus some of the ThB atoms formed 

 from ThA are deposited under the surface of the quartz. The range of 

 such recoil atoms amounts to about ^/jq cm in air and, therefore, less 

 than 10-* cm in quartz ; this thin layer of quartz is quite sufficient 

 to protect the fraction driven inward by the recoil effect from the reaction 

 of the acid, although radiation, from which its presence can be inferred, 

 still affects the electroscope through this layer. 



The portion of the active deposit found underneath the surface 

 depends on the time and other conditions of exposure Avhich have been 

 chosen to be strictly the same in all these experiments. The portion of 

 the active deposit occurring below the quartz surface, found experiment- 

 ally to be 20 per cent, was not taken into account in compiling the tables, 

 the values in w-hich refer only to the soluble part of the active deposit. 



The determination of the effect of the stirring speed on the velocity 

 of dissolution meets with difficulties. Because of the large velocity of 

 dissolution of molecular layers the times of experiment must be limited 

 to a few minutes and the unavoidable immersion and withdrawal of the 

 quartz disk from the solution always acts as intense stirring. In our 

 experience the effect of stirring velocity on the velocity of solubility of 

 molecular layers was not considerable. 



The Relation between Velocity of Dissolution and Solubility 



As shown by the above formula, the velocity of solubility of finite 

 layers increases with the solubility of the substance ; this is true also 

 for infinitely thin layers, and thus the velocity of solubility of the lead 

 isotopes is greater than that of the bismuth isotopes (Table 1), both 

 in water and in nitric acid, corresponding to the greater solubility of 

 the lead salts involved. 



Lead peroxide dissolves more slowly than metallic lead and lead 

 monoxide, in agreement with its lower solubility in HNO3. Such peroxide 

 layers^ were produced by the anodic deposition of ThBOg on platinum. 

 Only 20 per cent of ThBOg dissolved in the same conditions in which 

 80 per cent ThB entered the solution. Since dissolution from quartz is 

 not strictly comparable with dissolution from platinum, a comparison 

 was therefore made between the velocity of solubility values of ThBOg and 

 ThB likewise deposited on platinum by a cathodic reaction ; in this case 

 also more of the latter dissolved as is proved by the figures in Table 3. 



IF. Paneth and G. Hevesy, Wien. Ber. 122, 1038 (1913). 

 2 F. Paneth and G. Heatesy, Wien. Ber. 123, 1050 (1913). 



