PLEOCHROIC HALOES 137 



atom in sinking to a lower atomic weight casts out 

 with enormous velocity an atom of helium. It 

 thus loses a definite portion of its mass and of its 

 energy. Helium which is chemically one of the 

 most inert of the elements, is, when possessed of 

 such great kinetic energy, able to penetrate and 

 ionise the atoms which it meets in its path. It 

 spends its energy in the act of ionising them, 

 coming to rest, when it moves in air, in a few 

 centimetres. Its particular initial velocity de- 

 pends upon which of the radioactive elements has 

 given rise to it. The length of its path is there- 

 fore different according to the radioactive element 

 from which it proceeds. The retardation which 

 it experiences in its path depends entirely upon 

 the atomic weight of the atoms which it traverses. 

 As it advances in its path its effectiveness in 

 ionising the atom rapidly increases and attains a 

 very marked maximum. In a gas the ions pro- 

 duced being much crowded together recombine 

 rapidly; so rapidly that the actual ionisation may 

 be quite concealed unless a sufficiently strong 

 electric force is applied to separate them. Such 

 is a brief summary of the climax of radioactive 

 discovery : the birth, life and death of the alpha 

 ray. Its advent into Science has altered funda- 

 mentally our conception of matter. It is fraught 

 with momentous bearings upon Geological 

 Science. How the work of the alpha ray is some- 

 times recorded visibly in the rocks and what we 

 may learn from that record I propose now to 

 bring before you. 



