Matter present in the Atmosphere, 109 



vessel, (3) penetrating radiation through the sides of the 

 vessel, due to radioactive matter in the surrounding hodies. 



In the present experiments, the sides of the tank consisted 

 of iron one inch thick, and were sufficient to cut off all but 

 7 rays coming from without. The deposit on the sides of the 

 tank could not have been appreciably radioactive, or the low 

 values of q (9*6) would not have been obtained. For in no 

 apparatus, however carefully cleaned, has a lower value of 

 the volume ionization been noted. The chief factor, probably 

 the only factor, in producing the discharge of the wire is the 

 emanation in the air, and the successive products of rapid 

 decay. And of this there is undoubted evidence, inasmuch 

 as the excited activity was actually collected on a negatively- 

 charged wire in the tank. 



The experiment shows that the radioactive matter is in the 

 tank, and the only question is whether it is sufficient to account 

 for the effect obtained. 



In order to test this point, Professor Rutherford proposed 

 to me a new method of directly determining this important 

 point. I venture to describe it at some length, because it is 

 desirable that similar experiments under various conditions 

 should be carried out in other places. Such a radioactive 

 survey of the atmosphere would be of interest at the present 

 time. 



If the radioactive matter in the air is the sole cause of the 

 ionization observed, there should be a direct and proportional 

 relation between the excited activity and the ionization ; for 

 both these effects must originate from the same cause, namely, 

 the emanation present at any given moment in the mass of 

 air under observation. 



The method employed was as follows : — The natural ioni- 

 zation of the air in the tank was observed, and the active 

 deposit in the tank was then collected ; the first with a 

 cylinder of wire connected with the electroscope (as in fig. 3), 

 the second on a wire suspended in the tank, and charged to 

 — 10,000 volts. The wire after removal was wound on the 

 reel, and measured by the same electroscope, as in figure 2. 



Thus, if C x be the capacity of the cylinder of wires in the 

 large tank, V, be the potential fall per minute in the electro- 

 scope whose gold-leaf system is connected with the central 

 wires, then (.j^Vx is a measure of the natural ionization. 



Again, if C 2 be the capacity of the electroscope, fitted with 

 the upper cylinder and reel as i:i figure 2, and if V 2 be the 

 fall of potential per minute due to excited activity on a 

 negatively-charged wire, as observed ten minutes from the 



