17-J -/. C.Sanderson — ProbahL Infiuena <>f th< 



sealing wax insulation and guard ring, into the upper vessel, 

 and served to support the gold leaf. The guard ring was 



kept charged to — 300 volts. The walls of both vessels were 

 grounded, while the gold leaf and central electrode were 

 charged negatively. The source <>f potential was a battery 

 of small storage cells. The position of the gold Leaf was read 

 by means of a telemicroscope through mica windows in the 



- of the upper vessel. A small tube near the bottom of 

 the ionization vessel admitted the current of air. which was 

 drawn out through a similar tube near the top. 



To get the underground air. several bricks were removed 

 from the cellar floor and a deep hole dug in the sandy soil 

 beneath. An iron cylinder, 13 centimeters in diameter and 

 30 centimeters high, with many holes drilled in its top, bottom 

 and walls, was fitted with a brass tube, 130 centimeters long 

 and 0*4 centimeter in diameter, and. after being wrapped in a 

 covering of cheese cloth, was buried deep in the hole, so that, 

 when the earth had been rilled in. only two or three centi- 

 meters of the tube protruded vertically above the surface. 

 Connection was made with the inlet of the ionization vessel 

 by a rubber tube and a glass bulb containing glass wool, to act 

 as a dust and iou filter. The outlet of the ionization vessel 

 was connected with a filter pump through rubber and lead 

 tubing. Suitable pincheocks were placed at the outlet and 

 inlet of the ionization vessel. 



Radioactivity of tin: Underground Air. 



After the normal air leak of the electroscope had been 

 determined, the current of underground air was started and 

 readings were taken every teu or fifteen minutes, over thirty 

 divisions of the scale, for an hour and forty-five minutes. 

 As will be seeu later, these readings, when plotted, fitted 

 almost perfectly the curve showing the increase of the amount 

 of ionization due to a-particles from radium A and C. The 

 current of air was maintained for about forty-four hours, when 

 the active deposit of thorium was fifteen-sixteenths of its 

 equilibrium value and practically the maximum activity was 

 attained. At this point, the current of air was stopped 

 exactly on the minute, and readings taken every minute for 

 the next four minutes. This was repeated a number of times. 

 As will be shown later, these readings approximated closely to 

 the decay curve of thorium emanation. Finally the under- 

 ground air connection was removed and the emanations swept 

 out by drawing the air from the room through the electro- 

 scope. Eeadings were taken, at first every few minutes and 

 then at longer intervals, which, when plotted, showed the dis- 



