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XXIV. On the Radium Content of Secondary Rocks. By 

 Arnold L. Fletcher, B.A., B.A.I., Research Assistant 

 to the Professor of Geology in the University of Dublin*. 



IT was suggested to me by Professor Joly that an 

 investigation of the radium content of typical sedi- 

 mentary rocks was desirable in view of the discrepancies 

 between some of the published results, — his own more 

 especially being in the mean higher than those of other 

 observers generally, — and also on account of the small number 

 of observations actually available. 



An additional reason for now making such an investigation 

 is to be found in the facilities for rapid determination which 

 the method of fusion offers {. This method being also freer 

 from the uncertainties and risks of error besetting other 

 methods, might be trusted to afford correspondingly more 

 reliable data. The geological importance of a sufficiently 

 comprehensive knowledge of the radioactivity of common 

 sedimentary rocks has already been pointed out §. 



The experiments were carried out in the manner described 

 by Professor Joly (loc. cit.) with the exception of slight 

 unimportant variations in the relative quantities of chemical 

 fluxes employed to meet the varying degrees of basicity met 

 with between the different rock types. 



Attention has been drawn to the fact that the complete 

 disengagement of the imprisoned emanation depends of 

 necessity to a large extent upon two conditions : an active 

 chemical change and a correspondingly free evolution — 

 with effervescence if possible — of carbon dioxide, involving 

 therefore a mobile rock-melt. 



Professor Joly has recorded how the acidification of his 

 more basic and viscous rock-melts by the addition of a 

 small quantity of boracic acid resulted in a reduction of the 

 constant of one of his electroscopes by over 20 per cent., and 

 this increased yield of emanation consequent upon acidi- 

 fication has been observed in dealing with sedimentary 

 materials. For example, a Zechstein dolomite from Hartz 



* Communicated by the Author. 



I desire here to pay my grateful acknowledgments to the Royal 

 Society, under whose grant the following experiments were performed as 

 a prelimiuary to the further investigation of Plutonic and Volcanic 

 rocks. 



% Joly, " On a method of estimating the quantity of Radium in Rocks, 

 Minerals, etc." Phil. Mag., July 1911. 



§ Joly, ' Radioactivity and Geolcgy,' p. 45. 



