the Origin of Radium. 611 



minute*. It is evident that this affords a far more delicate and 

 exact measure than can be obtained by analytical methods of 

 the pereentge ot' the uranium present, which, on the assumption 

 of the value 22$ for the constant of the instrument, indicates 

 the following percentages of uranium in the monazites 

 examined : — North Carolina 03<S per cent. ; Norwegian 0*37 ; 

 Brazilian 033 : Connecticut 0'2$. That the uranium is a 

 constituent of the monazite itself and is not due to the 

 ail mixture of other substances f is indicated by the fairly 

 constant proportion in which it occurs in all of the samples 

 examined. Xos. 18 and 20 were sands containing minerals 

 other than monazite, but No. 19 was a crystalline variety, 

 and Xo. 21a massive variety of a high degree of purity. 



The presence of these notable quantities of uranium and 

 radium in monazite J affords a plausible explanation of the 

 occurrence of helium in this mineral, without recourse to the 

 unsubstantiated hypothesis of the formation of helium from 

 thorium. The age of monazite from a geological standpoint 

 is evidently extreme, since it occurs as a primary constituent 

 of some of the oldest igneous rocks of the globe. It is, 

 moreover, extremely compact in its structure, and even in 

 the form of fine sand loses at ordinary temperatures less than 

 one thousandth of its radium emanation. It may therefore 

 be safely assumed that it originally contained considerably 

 more uranium than at present, and that the accumulated 

 helium represents the disintegration product of radium for 

 countless ages. 



The question of the bearing of the emanating power of the 

 different minerals on the proportion of the various disintegration 

 products contained in them is not without interest. It will be 

 observed from an examination of the numbers in Column IV. 

 of the table on p. 609, that the emanating power of the cold 

 minerals varies from practically zero to 26 per cent, of the 

 total emanation. These numbers are of course considerably 

 influenced by the state of fine division in which the minerals 

 existed when tested, but the more strongly emanating 

 mineral- have a very measurable emanating power even when 

 in good-sized fragments. Rutherford has shown § that in all 



ted for the normal leak of the instrument, which was 012 

 division per minute. 



f Zerbian (loc. fit.) attributes the uranium in the North Carolina 

 monazite Band t<» the presence of samarskite and other uranium minerals. 



| Tlie presence of radium has been previously observed by Haitinger 

 and Peters [Sitzungsberichte d. kaisert. Akademie d. Wissenschaft. Wien t 

 May L90J . 



$ Phil. I Soc. London, cciv. p. 169 ( 1! 04): and Phil, Magi 



Nov. 1904. 



