82S Messrs. A. Holmes and R. W. Lawson on 



acid solution, obtained by the method just described, contains 

 practically all the mesothorium. The alkaline solution, 

 obtained after the first filtration, may usually be neglected, 

 as it only contains quantities of thorium which are within 

 the limits of experimental error in the measurements. It 

 should be mentioned, however, that whilst in the case of 

 minerals containing a small quantity of thorium, the alkaline 

 solutions always contained negligible quantities of: thorium, 

 it often happened that the alkaline solutions from minerals 

 rich in thorium (e. g. thorite) contained several per cent, 

 of the total thorium. In nearly all cases both solutions were 

 tested. 



(b) Treatment of Zircons, 



The zircons were treated by the following method. After 

 allowing the fused mineral cake to disintegrate in cold water, 

 the supernatant liquid was decolorised with hydrogen per- 

 oxide as before, but neither solution nor residue was heated. 

 The whole operation must be performed in the cold, because 

 the alkali zirconates contained in the melt are unstable, and 

 readily break up into zirconium oxide, which is insoluble in 

 acids. Having filtered off the supernatant liquid from the 

 decolorised solution, the residue is allowed to stand for a 

 further period in excess of distilled water. The residue 

 remaining after filtering off the overlying fluid is then 

 covered with a quantity of cold concentrated hydrochloric 

 acid, as the zirconates are not readily soluble in dilute acid. 

 After this has stood for several hours, a quantity of water is 

 added to the acid solution and residue, and the solution 

 filtered off through the same filter as that used previously. 

 The residue in the experimental dish is repeatedly so treated 

 with concentrated hydrochloric acid, allowed to stand, and 

 filtered, until the remaining residue has either completely 

 disappeared, or is totally white. The acid solution for 

 measurement is that composed of the separate flltrations 

 after the acid treatments above described. In the zircons 

 examined it was always found that the alkali solution could 

 have been neglected without appreciable error. 



(e) Determination of Thorium *. 



The measurements of the thorium content of the minerals 

 used was performed by the method of aspirating and shaking 



* For the methods employed in the determination of lead and 

 uranium see Holmes, Proc. lloy. Soc. A. lxxxv. p. 252 (1911). 



