4 Johnstone and Boltwood — Relative Activity 



uranium-radium series with the expectation that the 

 results would throw some light on the obscure relations 

 of the earlier members of the series. 



The Radioactive Measurements. 



The determinations of the radioactivity of the differ- 

 ent solids examined were carried out in an electroscope 

 which has already been described. 9 In the present ex- 

 periments a telemicroscope was used for observing the 

 position of the gold-leaf. The natural leak of the instru- 

 ment was small and over a period of about six months 

 varied from 04 to 0-7 scale divisions per minute. 

 Before and after each series of measurements the sen- 

 sibility of the electroscope was determined by measure- 

 ments of the activity of a standard reference film of 

 pure uranoso-uranic oxide. This film was carefully pre- 

 served throughout the entire period of the measurements 

 here recorded and all the results given in this paper are 

 given in terms of this film as the standard. 



The method of preparing the radioactive materials for 

 measurement was essentially the same, with certain 

 modifications, as that described by Boltwood. The 

 material to be 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 7-5 X 9 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 15 minutes. It was placed in a desic- 

 cator 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 sur- 

 face of the aluminium with a small camePs-hair brush. 10 

 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 accu- 

 racy 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 ioniza- 



9 Boltwood, this Journal, 25, 272, 1908. 

 _ 10 The brushes used were carefully cleaned in advance by long immer- 

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

 liquid. 



