606 Dr. B. B. Boltwood on 



certain amount of familiarity on the part of the operator in 

 order to insure complete success, but it is comparatively 

 rapid, and there is every reason for believing in the 

 reliability of the results. 



The chemical character of the specimens of minerals 

 examined was most diverse, and the content of uranium varied 

 from 75 to 0*3 per cent. The more important characteristics 

 of the samples used are given below : — 



1. Uraninite. — This substance, which is commonly known 

 as pitchblende, is a compound of uranium dioxide (TJ0 2 ) and 

 uranium trioxide (U0 3 ), approximating to the general 

 formula U 3 8 . It contains lead and the rare earths (usually 

 including thorium). This particular sample was a very pure 

 variety and was completely soluble in dilute nitric acid. 



2. Uraninite. — This specimen contained very noticeable 

 quantities of sulphide of iron (pyrite). 



3. Gummite. — This substance is a decomposition pro- 

 duct of uraninite to which the doubtful formula (Pb,Ca) 

 U 3 SiOi 2 .6H 2 has been given. The sample used was a 

 portion of a large specimen, was orange-yellow in colour, 

 and was completely soluble in dilute hydrochloric acid. It 

 contained small quantities of the rare earths in addition to 

 the elements indicated in the formula. 



4. Uraninite. — The Joachimsthal variety, which has been 

 the source of most of the radium of commerce. This 

 specimen contained small quantities of the sulphides of iron 

 and lead. 



5. Uranophane. — A decomposition product of gummite 

 having the general composition represented by the formula 

 Ca0.2U0 3 .2Si0 2 .6H 2 0. This sample had a chalk-like 

 structure, was pale yellow in colour, and contained only 

 traces of lead and the rare earths. 



6. Uraninite. — A very impure variety, containing con- 

 siderable sulphur, iron, and tin. 



7. Uranophane. — This specimen was taken from a compact 

 piece, canary-yellow in colour, and containing appreciable 

 quantities of lead and the rare earths. Si0 2 = 13'I per cent. 



8. Tlwrogummite (?). — A mineral of doubtful composition, 

 containing a considerable proportion of thorium, lead, and 

 iron. Is chocolate-brown in colour, and occurs as a decom- 

 position-product of uraninite. Si0 2 =13'4 per cent. 



9. Carnotite. — A mineral having a composition approaching 

 that expressed by the formula K 2 0.2U 2 3 . V 2 5 .3H 2 0. 

 The sample contained a considerable quantity of sandstone 

 and a small quantity of lead. 



10. Uranothorite. — A variety of thorite (ThSi0 4 ) ; a silicate 



