* PLEOCHROIC HALOKS.! 
By J. Jouy, F. R. 8. 
[With 3 plates.] 
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 2X 10° centimeters 
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 recognize 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 far dismem- 
bered as to yield charged electrons or ions, the atoms remaining 
’ charged with an equal and opposite charge. Such a medium of free 
electric charges becomes a conductor of electricity by convection 
when an electromotive force is applied. The gas also acquires other 
properties in virtue of its ionization. Under certain conditions it may 
acquire chemical activity and new combinations may be formed or 
existing ones broken up. When its initial velocity is expended the 
helium atom gives up its properties as an alpha ray and thenceforth 
remains possessed of the ordinary varying velocity of thermal agita- 
tion. Bragg and Kleeman and others have investigated the career 
of the alpha ray when its path or range lies in a gas at ordinary or 
obtainable conditions of pressure and temperature. We will review 
some of the facts ascertained. 
1 Being the Huxley lecture, delivered at the University of Birmingham on Oct. 30, 1912. Reprinted 
by permission. Published in Bedrock, London, January, 1913. 
313 
