Matter present in the Atmosphere. Ill 



Thus we obtain 4*1 from the natural air in the iron tank, 

 and 5*2 from the air mixed with the radium emanation in the 

 smaller vessel. . These figures are of the same order, and the 

 difference between them is probably caused by the difficulty 

 of drawing all the excited activity to a wire charged even to 

 — 10,000 volts from a tank whose sides measure 150 cms. 

 These results show that the ionization in the large tank was 

 very largely due to the presence of radium emanation, and 

 cannot be ascribed to radiations from the walls of that vessel. 

 But the results obtained were not altered after air had been 

 freely driven through the tank, and we thus come to the con- 

 clusion that the rate of production of ions in the air of the 

 tank was probably the same as from the air outside the 

 building ; and if such is the case, we have seen that the 

 ionization can mainly be attributed to the presence of radio- 

 active matter in the air itself. Since the activity is due to 

 the emanation which decays to half value in four days, its 

 amount cannot have sensibly decayed in passing from the 

 outside air to the tank. 



The results strongly indicate that the radioactive matter in 

 the atmosphere near the earth will fully account for the rate 

 of production of ions observed in it. 



V. Summary. 



The general conclusions derived from these experiments 

 are as follows : — 



(1) An estimate of the amount of radium required to main- 

 tain a steady supply of the emanation in one cubic kilometre 

 of air near the earth's surface, lies between *14 and *49 

 gramme. 



(2) This amount of emanation and its successive products 

 cause a production of ions at the rate of about 9'6 per cubic 

 centimetre per second. 



(3) The radium emanation in the air is probably sufficient 

 to account wholly for the natural ionization observed in large 

 closed vessels consisting of non-radioactive materials, and for 

 the rate of production of the ions in the atmosphere near the 

 earth. 



(4) The collecting distance of a wire charged to —10,000 

 volts is about 40 to 80 cms. The active matter derived from 

 the carriers is not drawn in appreciable quantities from still 

 air at a greater distance. 



