Certain Minnesota Soils. 393 



after passing through amber insulation and guard ring in the 

 top of the ionization vessel, served as a support for the gold 

 leaf in the electroscope. The latter had double walls, to pre- 

 vent thermal air currents, and a magnetic charging device, 

 which was very reliable and convenient, and, perhaps, merits 

 a short description. A A', in the figure, is a flat permanent 

 magnet, pivoted at O, and with a counterpoise W which makes 

 its equilibrium unstable in a horizontal position. Under the 

 end A' is an electromagnet. A small loop of fine wire is 

 attached to the end A by a bead of sealing wax. In one posi- 

 tion of the magnet, this loop dips into two cavities in the 

 piece of amber B ; in the other position it is withdrawn. The 

 cavities contain mercury, and one is permanently connected to 

 the central electrode and gold leaf ; the other to the high 

 potential battery and the guardring. One lead of the electro- 

 magnet is grounded ; the other passes out of the case through 

 a glass tube. A similar tube serves for the high potential 

 lead. A single battery cell and the momentary throw of a 

 reversing switch send the magnet AA' from one position to 

 the other, alternately charging and insulating the leaf and 

 electrode. The handling of dry, tine soils is very dusty work, 

 and the arrangement just described is particularly desirable, 

 in that it involves no openings in the electroscope through 

 which dust might enter. A galvanized iron cylindrical vessel, 

 60 centimeters in height and 14*5 centimeters in diameter, was 

 provided, at the top, with a narrow channel of rectangular cross- 

 section, and a depth of about one centimeter, which was filled 

 with paraffine and into wmich the cover fitted. Brass connect- 

 ing tubes were soldered at the center of the base and the 

 center of the cover. 



Method of Procedure. 



A piece of cotton was placed over the small opening in the 

 bottom of the cylinder, which was then filled completely with 

 the soil to be tested, w T ell packed and shaken down. The 

 cover was sealed in place and the lower end of the cylinder 

 was connected with the filter pump. The soil was rinsed 

 thoroughly, with frequent exhaustions, to get rid of all pos- 

 sible free radium emanation. During a known period of three 

 or more days the radium emanation was allowed to build up 

 in the soil, the lower end of the cylinder being sealed, and 

 the tube in the cover left open, so that any rinsing action, due 

 to changes in atmospheric pressure, might affect only the 

 upper layers of soil. The air from these layers did not reach 

 the ionization vessel. 



After this period of recuperation, the lower end of the 

 cylinder was connected to the ionization vessel through a short 



