134 Prof. J. Joly on a Method of Investigating the 



uniform over the wave-front, as may be supposed in dis- 

 cussing the effect of the toothed wheel ; but it exists 

 nevertheless, and it involves for its expression the intro- 

 duction of more than one frequency, from which circumstance 

 the group-velocity takes its origin. 



A development of the present method would probably 

 permit the solution of the problem of a series of equidistant 

 moving apertures, or a single moving aperture. Doubtless 

 in all cases the aberration angle would assume the value 

 w/U, 



Terling Place, Witham, 

 May 19, 1911. 



IX. On a Method of Investigating the Quantity of Radium 

 in Rocks and Minerals, Sfc. By J. Joly, F.R.S.* 



THE method commonly in use for investigating the 

 radium content of rocks and minerals leaves much to 

 be desired. The rock is first treated chemically so as to 

 bring it into solution. Generally this is effected by fusion 

 with alkaline carbonates. Leaching, filtration, and acidifi- 

 cation follow ; the rock substance being finally obtained in 

 two bulky solutions: an alkaline, resulting from the leaching 

 of the melt, and an acid, resulting from the solution in acid 

 of the insoluble residue. It is difficult to obtain quite limpid 

 acid solutions ; several nitrations and successive fusions with 

 carbonates have often to be resorted to ; and it not infre- 

 quently happens that even then a solution of perfect limpidity 

 cannot be obtained. A trace of precipitate may involve the 

 imperfect solution of the radium ; and the complete liberation 

 of the emanation by ebullition may be, subsequently, im- 

 possible. This may, in some cases, be demonstrated by 

 adding to the acid solution a trace of sulphuric acid, when it 

 will be found that if any precipitate is formed a fractional 

 part only of the emanation can be subsequently boiled off. 

 By treating a solution of a tinguaite in this manner, Eve 

 and Mcintosh (Phil. Mag. Aug. 1907) found that only one- 

 fifth of the amount of emanation originally obtained could 

 be removed by boiling. I have found that a trace of barium 

 added to the rock solution along with sulphuric acid, will 

 bring about this result. So also Levin (Rhys. Zeit. xi. 

 1910, p. 323) found that by adding some BaCl 2 and H 2 S0 4 to 

 mineral waters less than one per cent, of the radium remained 

 unprecipitated as judged by the emanation. 



* Communicated by the Author. 



