372 Prof. Joly and Mr. Poole : Attempt to determine 



intervals. " The fractions " — he says—" can be obtained by 

 oilier pendulums," with my italics. Thus, other and other 

 pendulums are to be declared as fresh standards (for instance, 

 for t = ^ we should require a smaller pendulum, such that its 

 three oscillations just fill out the interval between two clicks, 

 for t = ^ yet another, and for what Siemens's chronograph 

 yields, a pendulum of ultra-molecular dimensions) ; thus the 

 postulate (1) would have to be extended and enriched without 

 any end. (Moreover, the 'fractional' pendula could only be 

 found by endless trials, for Dr. Campbell's set (1), (2), (3) 

 does not yield a method of constructing the required sub- 

 divisions. Nor is, of course, such a scheme adaptable to any 

 somewhat refined chronometry.) Is this satisfactory ? Is 

 such a set as Dr. Campbell's (1), (2), (3) satisfactory as the 

 basis, logical or physical, of a theory of time-measurement? 

 I think not. 



Moreover, Dr. Campbell believes (1),. (2), (3) to be good 

 enough for a theory of the measurement, not only of time, 

 but also of any other " magnitude." He quotes lime only 

 as a little example. Now, temperature is certainly an 

 example of u magnitude," and better still, length or distance 

 is another, and it would be extremely interesting to see 

 Dr. Campbell setting up an intrinsic scale in both of these 

 cases, most especially in the latter one. 



The psychological clue to all fallacies of Dr. Campbell is 

 contained in his concluding sentence : "Of course, this is all 

 as elementary as AB C." If this were so, gigantic mentalities, 

 such as was Cayley and many of his successors here and 

 abroad, would never have devoted so much time to what is 

 known as the Theory of Distance. 



November 4, 1919. 



XXXIV. An attempt to determine if Common Lead could be 

 separated into Isotopes by Centrifuging in the Liquid State. 

 By J. Joly, F.R.S., and J. H. J. Poole, B.A.I* 



SINCE it has been discovered that both the Uranium and 

 Thorium radioactive families yield elements which 

 are isotopic with ordinary lead but differ from it slightly in 

 atomic weight and density, it has often been suggested that 

 common lead itself is not a homogeneous element, but consists 

 of a mixture of isotopic uranium and thorium lead. This view 

 of the constitution of common lead is based on the fact that 



* Communicated by the Authors. 



