October 2"], 1923] 



NA TURE 



619 



differ much according to the conditions of excita- 

 tion. This also, as is well known, is the case with the 

 zirconium spectrum. 



The spectra were photographed with the same 

 instrument as before, and the arc spectra were pro- 

 duccfi in tlie wav previonslv described ; the spark 



spectra were omaincd iarL;c iinuutifin « 



between carbon electroi' ted with the liafnin 



saltsolnli'-- '•• "■•■ ' •• ■ "■■ '■■'••• 



lines in i; 



iron nor!i..i. .. -. , , ..i; . . , 



intensity I, both in Ihc ar( ;umI m the ^ii.niv -jicctnun 



(scale \ 1() ''i_ lines WMi.i-i ^'•\.\\\ i 'iinit'i.! ■ '> denotes 



2 faint, I weak, \ extremely v/eak, d diffuse). The 

 wave-lengths are given to 0-05 A. U., the accuracy 

 which is generally reached ; at the longer wave- 

 lengths an error of about o-i A. U. may be possible. 

 Where (Ti), (Mn), (Nb), or (Zr) is added 'it means that 

 the line in question is nearly coincident with a line 

 belonging to the spectrum of one of these 

 elements, traces of which we have men- 

 tioned were present, but that the line is so 

 much stronger relative to the other lines of 

 that element present in the spectrum that 

 it is almost certain that the line really 

 belongs to the hafnium spectrum. 



A similar examination of the hafnium 

 lines in the remaining part of the spectrum 

 which is obtainable photographically, will 

 appear shortly. We publish this table first, 

 because it comprises the region in which 

 high accuracy is easily reached by smaller 

 spectrographs, and will therefore mainly 

 be used for identification, as the hafnium 

 spectrum shows few characteristic lines in 

 the visil>le part of the spectrum. 



H. M. Hansen. 

 S. \Vi:kxkr. 

 Universitetets Institut for 



teorr* • •'- '-"■•■•]:, 

 Copenhat^i nhcr jo. 



The Isotopes of Lead. 



It was suggested by the writtr in luij 

 that the end-products of the uranium and 

 thorium disintegration series should differ 

 in atomic weight from that of common 

 lead, which chemically they close I \ re- 

 semble. As the atomic weiglits ot these 

 products, determined experimentalK- later 

 by Richards and others as approximately 

 206 and 208 respectively, " bracketed " the 

 atomic weight of common lead, it was not 

 unreasonable to suppose tlial cummon lead 

 is a mixture of isotopes of whieh the mass- 

 numbers _'oo and JoS are chief. This, 

 althoui^h ])rol)ahle, is still uiH)ro\ed. The 

 fnrt her deduci ion that conceivably lead in 

 .Nature has been niaiuly produced by the 

 (lisjiue-ration of uranium and thorium has 

 re(ei\e.l soiiic, l)Ut iiol a i^reat measure of 

 supiiiu-t. [f it could l)e shown experiment- 

 alK" that common lead has some isotoiies 



wliich are not 

 disintegralii m, 



more dittu nit t 

 were comiWete 

 (lisjiro\ cd. 



M\ ,iual\'sis 



likelv to be 

 this 'hxpotlu 

 I mainl.uu : if 1 1 



the hx-jxithesi-- 



of the CO' 

 I some aci 



pr 



nluced by 

 would be 

 (lillerence 

 would be 



oO, 

 •n6 



■uei a I 

 W Weivi 



' > lie derived fi- 

 ipes of cleuiii: 

 hich, like'- ■ 

 iiiass-nur 



, .,; .iderat 11 Ml . ..,.-... 



ma\- lia\'e an isotope ol 



olliv W i).ll t he dil'felCli 



nuts, air : 

 e.se numl - 



jMiMM. ned results 101 On 



; \v\ 50, t he all luuc nuuil)ers 



I- ' "• ;'"•'■ 1. in ad<lition, 



i lucible from t his 



! 1 c in 1 ii 1 1 !i 1 1 1 bi a" 



m\ element , 



25 



V(J|, 



IIJJ 



