574 The Evolution of Matter 
emanation decayed. By isolating a very narrow pencil of a-rays, and 
watching through a microscope their impact on a fluorescent screen, 
Rutherford has lately counted the individual a-projectiles, and con- 
firmed his original conclusion that their mass corresponded to that of 
helium atoms and their charge to double that on a univalent atom’. 
Still more recently, he has collected the a-particles shot through an 
extremely thin wall of glass, and demonstrated by direct spectroscopic 
evidence the presence of helium”. 
But the most thorough investigation of a radio-active pedigree is 
found in Rutherford’s classical researches on the successive disinte- 
gration products of radium. In order to follow the evidence on 
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Fig. 1. 
which his results are founded, we must describe more fully the 
process of decay of the activity of a simple radio-active substance. 
The decay of activity of the body known as uranium-X is shown in 
the falling curve of Fig. 1. It will be seen that, in each successive 
22 days, the activity falls to half the value it possessed at the 
beginning. 
This change in a geometrical progression is characteristic of simple 
radio-active processes, and can be expressed mathematically by a 
simple exponential formula. 
As we have said above, solid bodies exposed to the emanations of 
1 Proc. Roy. Soc. A, p. 141, 1908. 
2 Phil. Mag. Feb. 1909. 
