TUE I'HOBLEM OF THE ISOTOPIC ELEMENTS 



83 



In the course of a few weeks the sealed flasks, which had meanwhile 

 become coloured a (\cv]) brownish-violet, were opened, washed oul with 

 nitric acid which had been distilled through a quartz condenser, and 

 the solution was evaporated. Until completion of the electrolysis care 

 was taken to use only quartz and no glass vessels. According to the con- 

 ditions of electrolysis metallic RaD or RaD peroxide was obtained as 

 a visible coating on small platinum wires ; ])reliminary experiments 

 allowed this result to be expected since we had convinced ourselves that 

 amounts of lead smaller than 0.001 mgm, as peroxide, are still clearly 

 visible and electromotively effective, i.e. they can be used for building 

 a cell^. We have manipulated various quantities of emanation, I/2 c 

 on the avarage, but even 100 — 200 mc are sufficient for carrying out 

 an experiment . 



The activity of the wires, as checked by measuring the a- and ^-radia- 

 tion, was of the order of magnitude expected for pure lead-free RaD ; 

 moreover, our apparatus was free from lead to the extent that we were 

 able to detect an artificial contamination of 10-^ gm Pb. 



We measured the electromotive force of the following cell : 



Pt/RaD02/Ra(DN03)2, HNO3, RaDOo/KNOg/KCl, Bg^i^, H 

 10-5N 10-3N satd. IN IN satd. 



The potential of RaD Og was found to amount to —0.884 V. The PbO, 

 potential measured in the same conditions was found to be — 0.888 on 

 the average^. 



In another series of experiments lead nitrate was added gradually 

 and the following electromotive forces were found (at 20° C") (Table 1). 



Table 1 



1 Refer to J. Koenig.sbebger and W. J. Muller, Phys. Z. 6, 849 (1905) ; 

 Jbid. 12, COG (1911). 



2 The RaD nitrate conec^ntration (!Oul(l only Vjc determined to the nearest 

 order of magnitude and therefore import anee should be attached only to tho 

 agreement of the two potentials and not to their absolute values. 



e^* 



