﻿in the Earth and the Atmosphere. 199 



lead cut off the penetrating rays entirely, 2*5 cms. of 

 iron would reduce the penetrating radiation to about half 

 value. 



It is, however, possible that I underestimated the radiation 

 from the sides of the taDk and the penetrating radiation 

 passing through them. On the whole, the tank experiments 

 seem to prove that the emanation in the atmosphere is an 

 important ionizing agent in virtue of its a radiation; and 

 the excited activity actually collected in the tank bears out 

 the results of direct ionization experiments. 



It is certain that emanation exists in the atmosphere and 

 that it produces ionization, but more experimental work must 

 be done before any exact value can be assigned to the number 

 of ions thus produced. My work and calculations point to 

 the conclusion that the y rays from the emanation in the 

 atmosphere may be disregarded as a negligible quantity. 

 The rays may probably be ignored in the same manner. 

 On the other hand, the a rays from the emanation do produce 

 several ions per c,c. per second, whilst the penetrating rays 

 from the active matter in the earth probably contribute the 

 balance of those observed near the earth's surface. 



Summary. 



1. The ionization produced within closed vessels at a given 

 distance from a given quantity of radium, and due to the 

 7 rays alone, is dependent on the substance and thickness of 

 the metal employed, owing to the secondary radiation (see 

 Table I.). 



2. From the results obtained within a vessel of aluminium 

 an approximate value of K was obtained equal to 3*1 x 10 9 , 

 where K is the number of ions produced per c.-c. per second 

 by the 7 rays from 1 gram of pure radium bromide at 1 cm. 

 from the source, supposing the rays are all absorbed in air. 



3. Hence the total number of ions produced by the 7 rays 

 from 1 gram of pure radium bromide completely surrounded 

 by air would be about i> x 10 9 per c.c. per second. 



4. Concluding from a former paper that the equivalent 

 radium bromide required to supply the observed quantity of 

 emanation per cubic kilometre of the atmosphere near the 

 earth's surface is between *14 and "5 grain ; it follows 

 that the number of ions per c.c. per second due to the 7 rays 

 from the active matter in the air is between about *06 and 

 *22 near the earth's surface. 



5. The a rays from the emanation in the atmosphere pro- 

 duce much more ionization per c.c. than do the 7 rays. The 



