106 THE EVOLUTION OF MATTER 



in thorium has been found to be higher than that 

 of common lead, whereas the atomic weight of lead 

 derived from minerals rich in uranium is lower. 

 The values in fact vary from 206-0 to 207-7. 



The densities of the varieties of the lead, the 

 writer recently found, differ in exactly the same way 

 as the atomic weights, showing that the volume 

 of the atom is the same though the weights are 

 different, as was to be expected from general 

 theoretical considerations. The difference is only 

 small. "Thorium " lead is about i per cent, heavier 

 than common lead. Prof. Richards, of Harvard, 

 has since found "uranium" lead to be J per cent 

 lighter than common lead. But if such a difference 

 occurred with gold, a bank-teller would be liable 

 to be out by one sovereign, or two, in every 400, 

 if he weighed the coins instead of counting them. 



Gold was the goal of alchemy, and it is interest- 

 ing to ask whether the new discoveries have thrown 

 any light on the alchemical problem of how to 

 make gold from lead or mercury. The answer may 

 be given at once. Gold is followed in the Periodic 

 Table by mercury, thallium, lead, and bismuth, 

 occupying successive places without gaps, as the 

 figure shows. To get gold from mercury, expel 

 from the atom of mercury one /3-particle, whicty will 

 make thallium, then one a-particle, which will turn 

 the thallium into gold. Or, to get gold from 

 lead, expel from the atom of lead one a-particle, 

 which will turn it into mercury, and proceed as 

 before. 



It is interesting to note that, in the case of both 

 the thorium and uranium disintegration series, at 

 a certain stage, the expulsion of an a-particle 

 instead of a /3-particle would have resulted in gold 

 being produced, for in each case the place occupied 

 by thallium is entered in the course of the changes. 



