274 Boltwood — Radio-activity of Uranium Minerals. 



gold-leaf over a certain definite portion (8*0 divisions) of the 

 scale in the eye-piece. The natural air-leak of the electroscope 

 was low and averaged about O04 division per minute. The 

 maximum and minimum noted in the course of over two years 

 were 0*058 and 0*029 div., respectively. The air-leak was 

 always higher in winter, when the windows of the laboratory 

 were closed, than in summer, when the windows were open. 

 With proper precautions the natural leak could be kept con- 

 stant within a few per cent for days at a time. In every series 

 of measurements the sensitiveness of the electroscope was 

 determined by measurements of the leak produced by a cer- 

 tain standard film of uranium oxide. This film was attached 

 to a plate of aluminium in a manner which will be described 

 later. This standard film was carefully preserved during the 

 entire course of the investigation. When the electroscope 

 and reading microscope were undisturbed the sensitiveness of 

 the electroscope remained quite constant for long periods, but 

 during the time occupied by the experiments it was necessary 

 on one occasion to completely dismount and re-assemble the 

 entire apparatus. On several other occasions a slight readjust- 

 ment of the microscope was made. These alterations produced 

 slight changes in the constants of the instrument, which could 

 be readily determined by the measurements of the standard 

 film mentioned above. By means of these readings obtained 

 with the standard film the different measurements can all be 

 calculated in terms of a single standard. The results of earlier 

 measurements as given in this paper have thus been reduced 

 to the terms of the present sensitiveness of the instrument. 



Preparation of the Films. — The method followed in pre- 

 paring the minerals and solid preparations for the radio-active 

 measurements has been described in an earlier paper.* The 

 material to be tested was ground to an impalpable powder in 

 an agate mortar with freshly distilled chloroform. At the end 

 of the grinding operation the mixture of solid and chloroform 

 consisted of a thin paste. A small amount of this paste was 

 removed on the point of a small camel's-hair brush, f a little fresh 

 chloroform was dropped on the brush, and the material was 

 quickly and quite evenly spread over the surface of a sheet of 

 aluminium 7*5 0m wide, 9 om long and 0*l mm in thickness. The 

 aluminium sheets weighed about 2 grams, and the weight of 

 the film of solid remaining after the chloroform had evaporated 

 could be determined with considerable accuracy. The mate- 

 rial adhered quite firmly to the surface of the plate and showed 



* The Radio-activity of Thorium Minerals and Salts, this Journal, xxi, 

 418, 1906. 



f The brushes before use were carefully cleaned with chloroform in order 

 to remove any soluble substances contained in them. 



