Lead and the End Product of Thorium. 827 



geological age, whatever the relative proportions of uranium 

 and thorium may be. Further, the ratio ought to vary 

 sympathetically with the geological age in the case of 

 mineral series of different antiquities. If, however, these 

 conditions are not satisfied, it may be concluded with 

 reasonable certainty that thorium lead is unstable, and that 

 it has a period which renders it ineffective in appreciably 

 altering the original age factor Pb/U. In this case, the 

 Pb/U ratios may be used as before (by Boltwood and one of 

 the writers), and with increased confidence, for the deter- 

 mination of geological time. 



§ 3. Method of Estimating Thorium in Minerals. 



(a) Preparation of Solutions, 



Except in the case of zircons, the following method was 

 used in the preparation of the solutions to be tested for 

 thorium *■ The cake obtained by fusion of the mineral with 

 excess of sodium carbonate was placed in cold water and 

 allowed to disintegrate. The use of cold water is advocated 

 because with hot water hydrolytic dissociation occurs, and 

 gelatinous silica as well as finely divided aluminium oxide 

 are separated, the precipitate being as a consequence difficult 

 to filter. The supernatant solution should be completely 

 colourless and clear. When such is not the case, a small 

 quantity of hydrogen peroxide is added to oxidise the iron 

 and manganese compounds, and the whole is gently warmed. 

 The undissolved residue eventually includes a small quantity 

 of hydrated silica. After filtering off the alkaline solution, 

 the residue is well washed, but by this means one cannot 

 completely remove the silica, owing to the formation in 

 the fusion of insoluble silica compounds. The residue is 

 therefore transferred to an evaporating dish, treated with 

 dilute hydrochloric acid, evaporated to dryness, and sub- 

 sequently taken up with water containing a little hydro- 

 chloric acid. Tho solution so obtained is next filtered, and 

 the residue of silica on the filter-paper well washed. By 

 careful working, the residual silica is completely freed from 

 mesothorium, and it may be neglected without error. The 



* By the courtesy of Professor Bamberger, the rock solutions were 

 prepared in the Chemical Laboratory of the " Technische Hockschule," 

 Vienna. We take this opportunity of expressing our thanks to him, 

 also to Dr. Weissenberger of the same Institution, who suggested the 

 method of preparing the rock solutions here described, a method which 

 has without exception given excellent results, the solutions remaining 

 perfectly clear after weeks of standing. 



3 H 2 



