727 



closure.^) When in one of the basins of diameter IV 1^ cm., 50 

 grammes of the dried substance were strewn eqnablj', it was possible 

 in a very simple way to exchange the basins without allowing the 

 air to enter the ionisation space. As to the electric measurement, 

 for this the ionisation lime was only determined for 50 scale divisions 

 over the same part of the scale. This was subsequently done with 

 all the samples, the results of which are given in the table below; 

 only the ionisation time is mentioned in it as, because of the above- 

 mentioned reasons, a proper calculation of activity will not be 

 possible. Y^i the various ionisation times will approximately denote 

 the different values of activity. For a closer meaning of the numbers 

 cf. Fig. 1. 



In order to be able to form a somewhat, more accurate notion of 

 the order of radio-activity with which such ionisations agree, I have, 

 while supposing among others that the absorptions of the various samples 

 are the same and, besides, that they only contain radium, traced 

 the calculation of activity for sample I (cf. table 2) namely by com- 

 paring the ionisation time of I with that of the Fango of Battaglia. 

 It should be taken into consideration here, that the natural leak will 

 also give its share to attain these effects, so that in reality the ratio 

 of the activity of the Rockanje mud to that of the Fango is still less 

 than one might think one could derive directly from these figures. 

 Thus I found N°. 1 to contain l.l >< lO^^^ grammes of radium pro 

 gramme; i.e. 1.8 times a larger value for this sample than follows 

 for this sample from the emanation method (cf. Table 1). That these 

 values do not agree, might partly be caused by the fact that the 

 sample in question contains at the same time thorinm. Many minerals 

 are even exempt from radium, whereas they contain a large amount 

 of thorium. 



So also this second comprehensive series of experiments had not 

 disclosed an appreciable radio-activity for any of the samples. This 

 notwithstanding, we did not yet consider our experiments as having 

 come to an end and that in connection with the assertion uttered 

 on various sides that radiograms had been obtained by means of 

 Rockanje mud ; this had been the matter with the already mentioned 

 sample 1913 and with two samples forwarded by Dr. Keys from 

 the Hague. These samples were investigated, both electrically and pho- 

 tographically ; it was already mentioned that according to the first 



') Cf. also foi' the drawing: These Proceedings, p. 684. The closure principally 

 consists in this, that instead of the circular bottora plate a ring is fastened to the 

 ionisation cylinder carrying two ways over which 2 metal basins can slide, which 

 alternately form the bottom of the ionisation space. 



52* 



