731 



(cf page 725) it is certainlj- remarkable to have (o gather from 

 these curves that either these substances contain only the active 

 radium or that also the other active substances in it must show the 

 same course as that element. 



Besides, this conformity in the results of the two methods may 

 be looked upon as a valuable check of the one upon the other. 



But to what cause do the different values for sample 12 point? One 

 could notice already that this sample is much more active than any of 

 the others in series I, but again looking up the notes taken when I 

 investigated these various samples electrically, I found that it was 

 just for this sample that a very strange course was mentioned of 

 its behaviour with regard to the radio-activity during the day of 

 its measurement. Though, however, N°. 12 only contains 2.16 X lO-^^ 

 gr. of radium pro gramme yet the above mentioned fact considered 

 by itself will be the motive that this sample will be subject to 

 further investigation. 



If then, having come to the end of all the mentioned experiments, 

 we recapitulate results, we will see that neither the electric research, 

 according to various methods, nor the photographic research have 

 disclosed any appreciable values for the radio-activity of the Rockanje 

 mud. The only exception to the large quantity of samples is sample 

 1913, but, as it did not come to us straight from Rockanje, but 

 was kept in the Utrecht-laboratory for 4 years, we need not take it 

 into serious account. 



Our final conclusions from these researches can only be 

 that in our opinion the Lake of Rockanje possesses no radio- 

 activity of any importance. 



§ 3. Conclusions. 



After the detailed researches described above by Miss Folmer 

 there is no occasion left to me to seek geological explanation for 

 an especially strong, local radio-activity, for not one of the samples 

 shows an action which would be ± 100 times stronger than those 

 of igneous rocks (Cf. BücHiNER 19J 3, EscHER 1915) and all the samples 

 have an activity as has generally been found in soils of the kind. 



But though the occasion for further inquiry into an hypothesis 

 for the explanation of a strong local activity ma}' be disregarded by 

 me, I yet want to point out that it is now that this question becomes 

 generally of importance: where do the radio-active parts come 

 from which are found in the alluvium (or in a more genei'al sense 

 in the ground)? In this I will not xenture too far on a territory 

 with which others are so much more familiar and better entrusted 



