1] RADIO-ACTIVE SUBSTANCES 27 
with a rate of decay different from those already known would 
afford strong evidence that a new radio-active body was present. 
The presence of either thorium or radium in matter can very 
readily be detected by observing the rate of decay of the emana- 
tions given out by them. When once the presence of a new 
radio-element has been inferred by an examination of its radio- 
active properties, chemical methods of separation can be devised, 
the radiating or emanating property being used as a guide in 
qualitative and quantitative analysis. 
