RECENT ADVANCES IN RADIOLOGY 195 



In the present state of knowledge, estimates of tlie age of minerals 

 founded on radioactivity can not command confidence. But since the 

 time element enters in a most pronounced manner into all radiological 

 processes, this science may eventually develop methods of age determina- 

 tion which will be a boon to geologists. 



On the Earth's Eadiation 



During the past few years many determinations of the radioactivity of 

 rocks and meteorites have been made by various analysts and the activity 

 of thorium minerals has been determined. New determinations have also 

 been made of the calorific eifect of radium and of thorium. Some six 

 years ago, while I was preparing a paper for this Society on the relations 

 of radioactivity to geology, the best available average content in radium 

 of surface rock was 1.4 X 10~^- grams radium per gram of rock as deter- 

 mined by Mr. Strutt, and the heating effect of radium was believed to be 

 about 100 gram calories per gram of radium per hour, this value having 

 been found by Madame Curie. 



In 1913 Kutherford concluded that the amount of radium per gram of 

 surface rock was about 2 X 10"^-, and that it is accompanied by 1.3 X 

 10~5 grams of thorium. The heating effect of radium as determined by 

 St. Meyer and Hess and adopted by Rutherford is 133 gram calories per 

 gram per hour, while Pegram and Webb find that one gram of thorium 

 in equilibrium with its products gives 3.4 X 10"^ calories per hour. With 

 these data and taking the average density of surface rocks at 2.7, Ruther- 

 ford from the equilibrium theory" finds the heating effect of the uranium 

 present in 1 c. c. of rock 11 X 10~® calories per annum and of the thorium 

 8 X 10"^ calories per annum.'^ The sum is 19 X 10"^. The origin and 

 relations of actinium are still midetermined, and it is possible but not 

 certain that a further addition should be made for the heating effect of 

 this substance. 



Still more recent determinations by Mr. Arthur Holmes''^ indicate that 



^» A somewhat puzzling anomaly iu the behavior of certain radioactive minerals has 

 Just been explained by Messrs. S. C. Lind and C. K. Whittimore in a suggestive manner. 

 It has been known for some years that autunite and some other secondary uranium 

 minerals gave U/Ka ratios which did not accord with the theory of radioactive equilib- 

 rium. XAnd and Wliiltimore found that specimens of carnolite showed a similar abnor 

 mal behavior w Ikmi llu- samples lesU'd were small, but that when the samples rei)resfUted 

 large lots of ore the IJ/Ua ratios were normal. Tlu>y infer that radium is in some cases 

 transposed within a deposit giving rise to' local ineiiualities which are etpiali/ed by mix- 

 ing large (luiuitities of ore. It Is thus apparent that the equilibrium theory has with- 

 stood successfully a very severe lest. The possibility of local inequalities even in the 

 radium content of rocks should be borne in mind. .Jour. Amer. I'hem. Soc, vol. 30, 1914, 

 p. -JOdO. 



" Uadioactive substances and their radiation, l'J13, p. 650. 



"" Science Progress, July, li)14, p. 15. 



