SHORT PAPER 



PROFESSOR R. A. MILLIKAN, of the University of Chicago, presented a paper to 

 this Section on " The Relation between the Radioactivity and the Uranium Con- 

 tent of Certain Minerals," of which the following is an abstract: 



In March, 1904, the author, assisted by Mr. H. A. Nichols, Assistant Curator of 

 Geology at the Field Columbian Museum (Chicago) , began an investigation of the 

 relation between the radioactivity and the uranium content of uranium-bearing 

 minerals with a view to ascertaining whether the radioactive substances found 

 in pitchblende are not all decomposition products of uranium. If such be the 

 case the ratio between the uranium content and the radioactivity of uranium 

 ought obviously to be constant, in case the assumption may be made that the 

 active products of the decomposition are not washed out of the mineral by per- 

 colating water or other agencies. 



Since the beginning of this investigation some preliminary results have been 

 published in Nature by Boltwood which indicate a constancy in this ratio in the 

 case of a few American ores which he has examined. McCoy (cf. Ber. d. Chem. Ges. 

 36, 3043) has also found a similar indication of constancy in the case of the six 

 different kinds of uranium minerals which he has studied. 



The present investigation is not yet complete, but so far as it has gone it fur- 

 nishes additional evidence in support of the view that uranium is the parent of 

 radium, for it extends somewhat the number of minerals for which the ratio 

 between the activity and the uranium content is approximately constant. The 

 following table gives the results thus far obtained. 



It will be seen that the departures from the mean value of the ratio amount in 

 some cases to as much as 13 %, but this was found to be no more than the dif- 

 ferences which might be obtained by "resurfacing" the same specimen of a given 

 substance. 



The measurements on activity were all made as follows: three hundred mg. of 

 the very carefully powdered mineral were spread as uniformly as possible over 

 three square inches of a metal sheet. This sheet was then placed upon the lower 

 plate of an air-condenser which was connected with one pair of quadrants of an 

 electrometer, the other pair being earthed. The condenser-plates were ten cm. on 

 a side and 3 cm. apart. A potential of one hundred and thirty volts was applied to 

 the upper condenser-plate, and the rate of charging of the electrometer noted. 

 The potential to which the needle of the electrometer was charged was one hundred 

 and twenty-five volts. The chemical analyses were all made by Mr. Nichols. 



