i 4 SCIENCE PROGRESS 



which a brief account of the mode of precedure is appended. 

 The results obtained were very surprising in the light of 

 Rutherford's calculation. The average of twenty-eight igneous 

 rocks gave 16 x io -12 grams of radium per gram of rock, about 

 thirty-five times as much as Rutherford had demanded for 

 thermal equilibrium of the earth. 



Strutt's figures have been confirmed by several other 

 observers, and rocks from every continent have now been 

 examined. Prof. Joly, in his later measurements, employed a 

 method of extracting the radium emanation which avoids the 

 labour of preparing clear solutions of rock material. A mixture 

 of finely powdered rock and fusion mixture is heated in an 

 electric furnace, and the emanation is driven off with the expelled 

 gases. Carbon dioxide is absorbed in a soda-lime tube, and the 

 remaining gases, containing the emanation, are finally passed 

 into the electroscope, after which the rate of fall of the leaf is 

 measured just as in the solution method. 1 Joly's fusion method 

 gives consistently higher results than those obtained by the 

 solution method, but the reason for the discrepancy is not 

 altogether clear. There is no doubt that suspended particles 

 in the solution may bring about a partial occlusion of the 

 emanation, which would prevent a complete expulsion of the 

 latter by boiling, and would therefore lead to a low deter- 

 mination. 2 



With careful chemical treatment, however, this source of 

 error may be avoided, and Mache has recently developed a 

 method of extracting emanation by aspirating air through the 

 solution, which renews our confidence in the accuracy of the 

 method devised by Strutt. 3 Mache has standardised his 

 apparatus directly with the Honigschmid radium standard, 

 and he finds that his results conform with surprising closeness 

 to those obtained from the same material when treated accord- 

 ing to Strutt's method of boiling out the emanation. 



All the results obtained for igneous rocks up to the present 

 are summarised and analysed in the adjoining table. 



1 Joly, Phil. Mag. vol. xix. p. 695, 1912. 



3 Eve and Mcintosh, Phil. Mag. vol. xiv. p. 237, 1907 ; Trans. Roy. Soc. 

 Canada, series iii. p. 67, 1910 ; Joly, Phil. Mag. vol. xix. p. 695, 1912. 

 3 Mache (in the press, 1914). 



