THE INTERMOLECLLAR EXCHANGE OF ATOMS OF THE SAME KIND 105 



1he same molecule but with different bonding. A further range of appli- 

 cation for our method might be opened up by splitting off one of the two 

 lead atoms and making (comparative measurements of radioactivity 

 on the products. 



EXPERIMENTAL 



The radioactive lead was prepared in the following way : The active 

 deposit of a strong radiothorium preparation, which had been recovered 

 from a mesothorium, sample whose activity (gamma) corresponded to 

 5 mgm of radium, was collected on the surface of a negatively charg(>d 

 lead foil. The activated lead was then dissolved in nitric acid and the 

 resulting nitrate was converted as required into compounds such as 

 chloride, formate, acetate, etc. The salts obtained in this way wer(> 

 labelled radioactively with the lead isotope thorium- B. 



The activity was measured in the usual way with the aid of an a- 

 electroscope. The substance to be measured was spread on a metallic 

 surface and its activity compared with that of a control substance of 

 the same weight and identical surface conditions. 



1. Lead Nitrate (Active) and Lead Chloride in Pyridine 



An amount of chloride (0.76 gm) and 0.90 gm of nitrate were dissolved 

 completely in 100 gm of boiling pyridine and the solution was kept hot 

 for 14 hr. The lead chloride which crystallized on cooling was filtered at 

 the pump, washed with a little cold pyridine and with ether to remove 

 the solvent, and dried in a vacuum. The sample was quite free from nit- 

 rate. Chloride prepared from the original nitrate was used as a stan- 

 dard. 



Measurement : 0.268 gm of substance caused the following ionizations : 

 Experimental sam})le, 2.76 scale divisions per min (calculated for com- 

 plete exchange ^ X 5.64 = 2.82 scale divisions per min). Standaid 

 sample, 5.64 scale di\ isions per min. 



2. Lead Formate (Active) and Lead Acetate in Water 



A quantity of lead formate (3.00 gm) and 3.83 gm of su^ar of lead (Kahl- 

 baum) were dissolved in 25 ml hot water. After keeping warm for ^/_^ hr 

 the majority of the sparingly soluble lead formate was crystallized l)y 

 cooling the solution ; the crystals w-ere filtered at the pump, freed 

 from traces of adhering acetate by washing with alcohol and dried in 

 a vacuum. A sample of the active lead formate was used as a stan- 

 dard. Measurement with 0.604 gm of each substance* : Experimental 



