384 D. MAWSON AND T. H. LABY. 



M. GL Bardet 1 arranges a number of the more common 

 uranium and thorium minerals in a comparative table, 

 according to the activity exhibited photographically. 

 R. W. Brocke 2 has published a comparative table of activi- 

 ties measured by a simple type of electroscope. 



Experimental Methods. 

 A preliminary examination of a number of the minerals 

 was made photographically, and the active ones were 

 readily detected. No disagreement was found between the 

 photographic and the electrical methods. Since the P and 

 7 rays are the photographically active ones, it is an unsafe 

 method, as Rutherford has pointed out, to rely on alone. 

 Our results as stated in the table, were obtained by the 

 use of a C. T. R. Wilson's 3 electroscope, consisting of a 

 rectangular brass box, with a narrow gold leaf, insulated 

 throughout with sulphur, and made by one of us in the En- 

 gineering Laboratory. The plate potential used was about 

 209 volts (as measured by a static voltmeter of the Physics 

 Department), which gave the maximum sensitiveness. 

 The case was preserved throughout at a constant sensitive 

 angle of tilt. The movement of the gold leaf was read by 

 means of a microscope moved by a screw thread. The leaf 

 was connected to the upper of two insulated brass plates 

 11 cm. in diameter separated by a 5 cm. air gap, and sur- 

 rounded by a metal case which was earthed. The lower of 

 the plates was kept at a positive potential of 300 volts, by 

 means of a battery of test tube accumulators, which was 

 found sufficient to produce a saturation current. On this 

 plate the mineral was placed in a circular lead dish of 5 cm. 

 diameter, 3 mm. in depth — a fresh one being used for each 

 mineral. 



1 Bull. Soc. Franc, de Mineralogie, part 1. 27, p. 63, 1904. 

 3 Engineering and Mining Journal, June 2, 1904. 

 9 Proc. Camb. Phil. Soc, 3 2, part ii., 1903. 



