Physics. — " Uesearches into the Radio- Activity of the Lake of 

 Rockanje" . By Miss H. J. Folmek and Dr. A. H. Hlaauw. 

 (Communicated by Prof. Haga). 



(Communicated in the meeting of September 29, 1917). 



§ 1. Tntroduction. 



In a treatise on the Lake of Rockanje ') ') 1 have communicated 

 that an inquiry into the radio-activity of tlie samples collected by 

 me was being made by an expert. Now that this inquiry, carried 

 out by Miss H. J. Foi.mek in the Physical Laboratory at Groningen, 

 has led to fixed results, we w4sh to bring them before the public. 

 I shall begin with a short explanation from what considerations this 

 inquiry has been planned and elaborated so amply. 



When in July 1915 I began my researches into the tloi-a and 

 history of the origin of the Lake of Rockanje 1 got acquainted 

 with Dr. Buchner's researches, which showed what a surprisingly 

 strong radio-activity the latter had found in the Rockanje mud 

 according to his publication in the Chemical Weekly. ') Subsequently 

 Dr. B. G. EscHKR communicated to the Geological Mining Society 

 a report ^) made in April 1914. Esciter discerned in the Rockanje 

 mud organic and mineral parts and came to the hypothesis that the 

 mineral parts and together with these the radio-active substance, 

 had been blown into it from the dunes. In the said meeting and in 

 the above mentioned treatise I have already pointed out that the 

 large mass of mineral substance is nothing but clay, which has 

 settled on the bottom of salt and brackish water in the form of 

 fine slime and mixed with many organic remains. 



If, therefore, this mud were so radio-active there would be a far 

 greater chance that radio-active parts had been carried there as 

 slime by the rivers from the mountains of Central-Europe, than that 



1) The Lake of Rockanje in the Isle of Voorne, which forms part of the province 

 of "Zuid-Holland". It is situated near Hook of Holland, separated from ii by 

 the Isle of Rozenburg. 



2) A. H. Blaauw (1917) Treatise. Royal Acad, of Sciences Amst. 2nd Section XIX 

 NO. 3. 



») E. H. BüCHNEE (1913) Chem. Weekly. Part 10 N". 35. 



4) B- G. EscHER (1915) Report Geol Mining Soc. Gen. Geol. Section Part 2. 



