THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES. ] 



Art IX. — On the Radio- Activity of Thorium Salts; by 

 Bertram B. Boltwood. 



Measurements of the a-ray activities of a number of min- 

 erals containing thorium have been described in an earlier 

 paper.* A measure of the ionization produced by known 

 weights of the finely-powdered minerals in the form of thin 

 films was obtained by introducing the films into an electro- 

 scope aud determining the rate of leak of the charge in terms 

 of the fall of the gold-leaf in scale divisions per minute. On 

 dividing the rate of leak by the weight in grams of the 

 mineral in the film a number was obtained expressing the 

 specific activity (activity per gram) of the given mineral. 

 The minerals examined contained uranium as well as thorium. 

 The activity of one gram of uranium with its equilibrium 

 amounts of disintegration products (actinium, radium, etc.) has 

 been found to be a constantf which will be called the normal 

 specific activity of the uranium-radium series. The value of 

 this constant for the particular electroscope used was deter- 

 mined by measurements of the activity of certain minerals 

 containing uranium and no thorium. Knowing the content 

 of uranium and the constant for the normal specific activity 

 of the uranium-radium series, it was possible to calculate for 

 each-, of the thorium minerals that portion of the specific 

 activity of the mineral which was due to the thorium and 

 thorium products which it contained. Dividing this by the 

 weight (in grams) of thorium contained in one gram of mineral, 

 a number representing the activity per gram of thorium was 



*This Journal, xxi, 415, 1906. 



t McCoy, Phil. Mag., ix, 176, 1906; Boltwood, loc. cit. 



An. Jour. Sci. — Fourth Series, Vol. XXIV, No. 140. — August, 1907. 



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