Quantity of Radium in Rocks and Minerals, 8fc. 149 



experiment, the melt is at once cleanly removable, and a fresh 

 charge may be immediately placed in the furnace. Thus a 

 succession of experiments may be carried out, the results 

 being available in a little more lhan an hour after starting, 

 if the maximum reading reached by the electroscope, about 

 15 or 20 minutes after admission of the emanation, is 

 availed of. 



The use of the fusion method in the convenient form de- 

 scribed involves the assumption that there is no serious loss 

 of emanation attending the pulverization of the material dealt 

 with. Under the conditions this assumption seems fullv 

 justified. It is not necessary to grind the rock very fine. In 

 the experiments cited above the rock powder was generally 

 sifted through a sieve GO mesh to the .inch. Boltwood (Phil. 

 Mag. April 1905) found that "very finely pulverized" radio- 

 active minerals lost from 0*2 to 26 per cent, of emanation, 

 and in a later paper (Am. Journ. Sc. xxx. April 1908) that the 

 emanation loss in powdered uraninite varied from 1*1 to 14*1 

 per cent, Geiger and Rutherford (Phil. Mag. Oct. 1910) 

 found that a sample of Joachimsthal pitchblende when finely 

 powdered lost 6'2 per cent, of its emanation. These refer to 

 substances brought to a very fine state of subdivision. The 

 experiments which have been made upon the escape of helium 

 from pulverized minerals are, very probably, quite applicable 

 to escape of emanation. Now Moss determined that quan- 

 tities up to one per cent, of the helium contained in a mineral 

 could be liberated by grinding in vacuo. Strutt found that a 

 quantity of monazite, powdered and passed through a wire- 

 gauze sieve of 120 mesh to the inch, from immediately after 

 grinding to 33 days later, showed a helium loss probably less 

 than the 500th part of the whole. Lastly Gray (Proc. Roy. 

 Soc. March 10, 1909) has made a careful quantitative exami- 

 nation of the helium loss attending different states of sub- 

 division. Thus thorianite, of known helium capacity, reduced 

 to very fine particles, ranging in size from 0*001 mm. to 

 0*030 mm., showed a loss of 5 per cent, of its helium. Gray 

 concludes that when the size of the particles is greater than 

 0*01 mm. (1/250 inch) scarcely any helium is liberated. The 

 liberation of the helium probably begins in the neighbourhood 

 of 0-005 mm. 



In order to test directly whether the anticipations founded 

 upon the foregoing investigations were justified in the ease 

 of the rock powders, a quantity of the Deccan trap of Ghats, 

 the radium content of which I had found to be 2"2 x 10~ 12 grin. 

 per grin., was enclosed in a large U-tube. The powder had 

 been put through the same sieve — 60 mesh to the inch — used 



