410 Penetrating Radiation from the Eartli^s Surface. 



voltages, the curve thus obtained being practically linear 

 between 100 and 340 volts, these being the extreme limits 

 of potential for the observations. For different gold-leaf 

 systems this calibration gave between 2*5 and 3 volts per 

 division. 



To determine the capacity of the gold leaves the electro- 

 scope was set on an insulating block of paraffin, and the 

 outside cylinder connected to the quadrants of a very 

 delicate Dolezalek electrometer, which was connected in 

 parallel with a standard capacity of "002 microfarad. The 

 leaves of the electroscope were charged, and the reading 

 observed. The leaves were then discharged, and the charge 

 which had been attracted to the case of the instrument 

 thus released and allowed to charge up the electrometer and 

 standard capacity. The deflexion of the electrometer was 

 then read, and was a measure of the charge which had been 

 on the leaves of the electroscope. The observations were 

 taken five successive times, using 200 and 300 volts, and 

 showed an agreement of better than four parts in a hundred. 

 The sensitiveness of the electrometer was 543 divisions per 

 volt. From the data thus obtained the capacity of the system 

 was calculated, and came to "85 and "91 cms., for 200 and 

 300 volts respectively. As the observations generally ranged 

 between 325 and 175 volts the average capacity of the system 

 was taken as '88 cm. 



The volume of the air inclosed in the electroscope was 

 1100 c.c. 



With a gold-leaf system giving 2*6 divisions deflexion for 

 a volt, the rates of collapse of the leaves for the electroscope 

 exposed in the laboratory, and then surrounded with a very 

 thick lead screen, were 2*4 and 1'7 divisions per hour. This 

 gives as the number of ions 7*5 and 5 respectively. The 

 number given by Wilson, 14, is for conditions similar to 

 those under which the value 7*5, above, was obtained. The 

 lower number here obtained may be accounted for by the 

 fact that Wilson used silvered glass in the construction of 

 his electroscope, whereas an instrument of brass 2 mms. thick 

 was used in these experiments. 



To sum up, the results of this investigation are : — 



(1) The proof of the existence of a very penetrating radia- 

 tion, present everywhere under ordinary conditions. This 

 radiation is similar in properties to the radiation from radium, 

 and is comparable to it in penetrating power. This radiation 

 is accountable for between 30 and 33 per cent, of the natural 

 ionization observed in ordinary testing-vessels, 33 per cent, 

 beiug the greatest reduction obtained by the use of massive 



