Quantity of Radium in Rocks and Minerals, Sfc. 141 



the electroscope of the natural emanation derived from the 

 last, expressed as so many scale-divisions gain per hour in 

 the rate of discharge, is deducted from the like quantity 

 observed in the first experiment. The difference gives the 

 effect due to the uraninite, and enables the calibration 

 constant of the electroscope to be arrived at. 



In this manner the following values of the constant were 

 obtained : — 



070 mgrm. uraninite in 10 grins, granite (Glencullen) C=0 , 68xl0- 12 

 0-72 „ „ „ 7 „ „ „ C=0-oo „ 



0-45 „ „ „ 7 „ „ „ C=0-60 ■ 



Mean C=061 



)> 



The weighing of these small quantities of uraninite was 

 effected on a delicate balance reading to 0*01 mgrm. under 

 the conditions. 



The variations seen in these experiments are sufficiently ex- 

 plained by the results of a microscopic examination of the 

 powdered uraninite. It is then seen that the grains are very 

 evidently heterogeneous in composition. Most of them are 

 black and opaque or nearly so. These are doubtless the very 

 pure oxide, and may contain quantities of uranium up to 

 85 per cent. Other particles are colourless and translucent 

 or transparent. These are certainly impurities. Certain 

 particles of pale yellow colour may represent altered urani- 

 nite. There are quite enough of these particles, both large 

 and small, to render the chance considerable that in a very 

 small quantity of the powder there might be a variation in 

 the emanation content according to the percentage of im- 

 purities taken up. The average uranium content of this 

 powder is 64 per cent, of uranium. While this result may 

 safely be applied to such a considerable amount of it as 

 would be taken for preparation of a standard solution, it 

 might not rigidly apply to a fraction of a milligram of the 

 powder. 



By taking the mean of several such results the chemical 

 analysis must, however, be applicable. It is probable that 

 the mean of the three results is not far from the true con- 

 stant. It appears to indicate for the fusion method a yield 

 of emanation from an acid rock melt equal to that obtained 

 from pure aqueous solutions. The temperatures reached in 

 these experiments were about 1100° C. 



Assuming this as the calibration constant, the following 



