Originally communiratod in Ndtnre, 128, 1038 (1931). 



3. LEAD CONTENT OF ROCKS 



G. Hevesv and R. Hobbie 

 From the Institute of Physical Chemistry of the University of Freiburg 



In recent years various geochemical problems have arisen which make 

 it important that our scanty knowledge of the lead content of rocks 

 should be amplified and made more precise. To this end we have deter- 

 mined the lead in a series of samples, representing in all about 220 rocks, 

 some of which we owe to the kindness of Prof. Arthur Holmes of 

 the University of Durham. 



The sample to be analysed was brought into solution ; silver sulphate 

 was added and the silver and lead present in the solution were simultane- 

 ously precipitated as sulphide. The precipitate was them brought into 

 solution and the lead deposited electrolytically as peroxide. That the 

 deposit was actually lead peroxide was confirmed by a colorimetric 

 test, tetramethyl-diamino-diphenylmethane being added to the solution 

 of the deposit. To ascertain that the total lead content was actually 

 recovered, we used the method of radioactive indicators. We added to 

 the rock sample a known amount of the lead isotope radium D, prepared 

 from radium emanation, and checked the yield obtained by measuring 

 the activity of the lead peroxide deposit. As the purest chemicals com- 

 mercially obtainable were found to contain quantities of lead that 

 would have influenced our results, all the chemicals used were first 

 purified from lead. Moreover, every precaution was taken to avoid 

 contamination of the samples by dust which might have contained 

 traces of lead. The results obtained are listed in Table 1. 



The average value found is 16xl0~^ gm lead per gm rock, a some- 

 what larger value than that given by Clarke and Steiger^ who found 

 7.5xl0~6 gm per gm rock. As shown in the communication lliat 

 follows, the amount of lead accumulated in the rocks since the solidifica- 

 tion of the earth's crust (as a result of the decay of uranium and thorium) 

 is very much smaller. Thus, as between the atomic weights of rock- 

 lead and ore-lead we have in most cases to expect differences only in the 



^Clarke and Steiger, ./. Wash. Acad. Set. 4, -IS (1014). 



