Relative Adit-it,// of Radium and Uranium. 53 



examined was ground as finely as possible in the form of a 

 thin paste with pure ethyl alcohol in a small agate mortar. 

 A sheet of aluminium 7*5x9 cm. and 0*01 cm. thick was 

 first carefully cleaned with liquid soap and distilled water, 

 and was then placed in a drying oven at 65° C. for fifteen 

 minutes. It was placed in a desiccator over sulphuric acid 

 for half an hour, and then weighed on a. sensitive chemical 

 balance. The paste of material and alcohol was then thinly 

 spread on the surface of the aluminium with a small camel's- 

 hair brush*. The coated plate was placed in the oven, 

 cooled in the desiccator and weighed as before. The weight 

 of the films could be determined in this manner with an 

 accuracy of one per cent. The solid material adhered quite 

 strongly to the plate and showed no tendency to fall off' even 

 when the plate was inverted. 



The measurements of radium emanation were made with 

 a gold-leaf electroscope having an air-tight ionization cham- 

 ber with a capacity of about three litres. The separation 

 and collection of the radium emanation, its transfer to the 

 electroscope, and the measurement of its radioactivity were 

 carried out according to methods which have already been 

 described f. 



Ratio of the Activity of a Uranium Mineral to the Activity of 

 the Contained Uranium. 



The relation of the activity of the parent element, uranium, 

 associated with equilibrium amounts of all of its disintegra- 

 tion products, to the activity of the parent element alone, is 

 a fundamental quantity of great importance to any considera- 

 tion of the relations existing between the individual products 

 themselves. The actual progenitor of the series is uranium I., 

 but this cannot be isolated from its invariable associate and 

 isotopic product uranium II. The combined effect of these 

 two elements when mixed in equilibrium proportions can be 

 determined, however, and this can be compared with the 

 activity of a similar mixture containing all the other disin- 

 tegration products in equilibrium proportions. 



Such a mixture is furnished by a pure, primary, unaltered 

 uranite. A mineral containing a low proportion of thorium 

 is preferable since a correction must he made for the activity 



* The brushes used were carefully cleaned in advance by long- immer- 

 sion in alcohol and subsequent washing in fresh quantities of the same 

 liquid. 



t Boltwood, Am. Journ. Sci. xviii. p. 378 (1904) ; Phil. Map:, ix.p.599 

 (1905). 



