724 



From this table it is clear amongst other things that the investi- 

 gated samples of the boring, as well as those of the Betjenskekier 

 or its neighbourhood only show a radio-activity in the order of 

 IQ~i2 grammes of radium pro gramme; i.e. no larger quantity than 

 is normally found in most rock ^). The activity of mould is of the 

 same order, whereas only the sample of comparison : F'ango exhibits 

 an effect considerably higher. Notwitiistanding this small radio-activity, 

 however, it shows in relation to the depth and the nature of the 

 layers, a certain regular course as may appear more distinctly yet 

 from the geological interpretation of the results by Dr. Blaauw as 

 mentioned below '). The two values for N". 23 which agree within 

 the limits of errors of observation, relate lo the values of activity 

 of two solutions, made and investigated at quite different periods. 

 Having arrived at the end of these experiments we could not but 

 conclude that our results did indeed contrast greatly with the values 

 for samples of mud from the Lake as indicated by Dr. Büchner in 

 1913; he even found a value of 462 X 10 ^^ grammes of radium 

 pro gramme in the blue mud from the Betjenskelder. 



At the very moment that these researches had proceeded thus 

 far, a circumstance intervened, which especially has been the occasion 

 of our resolving not yet to consider the experiments as being finished ; 

 on the contrary : to continue them in a yet more extensive w^ay. 

 From Utrecht Prof. Zwaardemaker sent us two samples of Rockanje 

 mud; from one of these, which Prof. Zwaardemaker had received 

 in 1913 from Mr. Trouw at Rockanje, the radio-activity had been 

 clearly shown in the Utrecht Laboratory, because they had succeeded 

 there in obtaining radiograms through this sample. After all that 

 preceded it was certainly remarkable that, according to the "direct 

 method" of investigation (Cf. Part 11) I could determine that this 

 sample certainly possessed radio-activity IV, times that of Fango of 

 Battaglia. In order to find out whether we might have been mistaken 

 with other samples, Dr. Blaauw investigated this sample with regard 



1) In the table the values for activity only apply to those of acid solution ; in 

 the alkaline solutions, even in the Fango I could find no activity. An experiment 

 with only chemical substances was made to check this with a solution which 

 contained the quantity of chemical substances only necessary for each experiment ; 

 also this had a negative result. 



2) It is of importance to remark here that very little information has been 

 given up to now on the relation of the quantity of radium to the depth under 

 the surface of the earth. However, experiments showed no relation. Cf A. F. Eve 

 and D. Mc. Intosch, Trans. Roy. Soc. Canada 1910; E. H. Büchner, Jahrbuch 

 Rad. u. El. 10, 1H13; H. E. Watson and G. Pal, Phil. Mag. 28, 1914. 



