382 RADIO-ACTIVITY OF THE ATMOSPHERE, ETC. [CH. XI 
radio-activity observed, and the distribution of radio-active matter 
throughout the constituents of the earth, render it difficult to be 
certain that any substance, however carefully prepared, is freed 
from possible radio-active impurities. A careful comparison of 
the rates of decay of the activity of the emanations obtained from 
ordinary matter, and of the excited activity, with the corresponding 
rates of decay of the activity of the products of the known radio- 
active substances, may throw some light on the question. 

