142 CONCEPTION OF THE CHEMICAL ELEMENT 



one time it seemed that neon had been so resolved, 

 but this has not yet been confirmed. 1 It would be 

 interesting- also if the rotation of the salts of some 

 optically active acid with different varieties of lead, 

 separated from uranium and from thorium minerals, 

 were examined. A difference is to be expected, 

 although it is likely to be small, and possibly may be 

 too minute to be detectable. Recent experiments at 

 Harvard have shown that the refractive index of 

 a crystal of lead nitrate is independent of the atomic 

 weight of the contained lead, but the solubility, as is 

 to be expected, is different, the molar solubility of 

 different varieties being the same. 



Isotopes need not, however, have different atomic 

 weights. One of the clearest cases is in the two end- 

 products of thorium, but, if the scheme is correct as 

 regards the branching point of the actinium series, 

 ionium and uranium- K, actinium-^4 and radium- C', 

 actinium- C and radium-Zf, actinium-/? and radium-/?, 

 and the actinium and uranium isotopes of lead, are 

 other cases. These result by branchings of the 

 series, and, since in the respective branches the 

 amount of energy evolved in the successive changes 

 is different, the internal energy of the various pairs 

 must be different, although for them atomic weight 

 as well as spectroscopic and chemical character are 

 all identical. I recently suggested in the case of the 

 two end-products of thorium that possibly only one 

 of these survives in geological time, namely, that 

 produced in the smaller quantity, and that the other 

 continues to break up in changes as yet undetected 

 (Royal Institution Lecture, i8th May 1917; Nature, 

 iQi 7> 99, 414 and 433). This would account for the 

 relative poverty of thorium minerals in lead, which 

 was the original basis for the conclusion that lead 



1 Mr Aston tells me this work is still being actively prosecuted at 

 the Cavendish Laboratory. 



