129 



PLEOCHROIC HALOES.* 



By Prof. John foly, F.R.S. 



It is now well established that a helium atom 

 is expelled from certain of the radioactive 

 elements at the moment of transformation. The 

 helium atom or alpha ray leaves the transforming 

 atom with a velocity which varies in the different 

 radioactive elements, but which is always very 

 great, attaining as much as 2 x io 9 cms. per 

 second; a velocity which, if unchecked, would 

 carry the atom round the earth in less than two 

 seconds. The alpha ray carries a positive charge 

 of double the ionic amount. 



When an alpha ray is discharged from the 

 transforming element into a gaseous medium its 

 velocity is rapidly checked and its energy 

 absorbed. A certain amount of energy is thus 

 transferred from the transforming atom to the 

 gas. We recognise this energy in the gas by the 

 altered properties of the latter ; chiefly by the fact 

 that it becomes a conductor of electricity. The 

 mechanism by which this change is effected is in 

 part known. The atoms of the gas, which appear 

 to be freely penetrated by the alpha ray, are so 



* Being the Huxley Lecture, delivered at the University 

 of Birmingham on October 3Oth, 1912. 



