Boltwood — Thorium, Minerals and Salts. 419 



chamber 16 cm long, 9'5 cm wide and 7 cm high, with a charged 

 plate 13 cm long and 5 cm wide suspended 3'5 cm from the bottom, 

 and a larger electroscope with an ionization chamber 15 to 19 cm 

 in diameter and 14 cm in height, having a circular plate 7*5 cm in 

 diameter at a distance of 9 - 5 cm 'from the bottom. The electro- 

 scopes are described in greater detail in an accompanying 

 paper (p. 411). 



Calculation of the Thorium Activity. 



From the weight of mineral in a given film and the corre- 

 sponding activity as measured in the electroscope, it is a simple 

 matter to calculate the activity of one gram of the mineral in 

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

 It has been stated by McCoy* that for uranium minerals con- 

 taining no thorium, the total activity is directly proportional to 

 the amount of uranium present, and experiments made by the 

 writer on a series of uranium minerals have led to a similar 

 conclusion, provided that corrections are introduced for the 

 amount of radium emanation which escapes from the minerals 

 when in a finely powdered condition. There are therefore two 

 methods available for calculating the activity due to thorium 

 in a series of minerals containing both thorium and uranium. 

 One of these is to determine, from the measurement of a min- 

 eral containing uranium only, the activity corresponding to one 

 gram of uranium with its active disintegration products. 

 Knowing this value, it. is a simple matter to deduct from the 

 activity of a uranium-thorium mineral of known composition 

 that part of the activity due to uranium and products. If no 

 other radio-active bodies than thorium are present, the remain- 

 ing activity will be due to thorium and its products only, and 

 on dividing this by the weight in grams of thorium present 

 the activity of thorium per gram will be given. The other 

 method of calculating the activity of the thorium is to solve 

 by algebraic methods the equation obtained through the meas- 

 urement of two or more unlike thorium-uranium minerals, the 

 equations having the form 



aV + bTh = C, 



where a is the weight of uranium in one gram of the mineral, 

 b the weight of thorium in the same quantity, and C is the 

 total activity of one gram of the mineral. This second method 

 is only applicable if the activity of the thorium is a constant. 

 The results as given in this paper were calculated by the first 

 method. The activity corresponding to one gram of uranium 

 and products in a mineral retaining all of its radium emanation 



* Phil. Mag. (6), xi, 176, 1906. 



