130 HUXLEY MEMORIAL LECTURES 



far dismembered 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 ionisation. Under certain con- 

 ditions 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 agitation. 

 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. 



The range or distance traversed in a gas at 

 ordinary pressures is a few centimetres. The 

 following table, compiled by Geiger, gives the 

 range in air at the temperature of 15 C. : 



cms. 



Uranium i 2.50 



Uranium 2 ... 2.90 



Ionium ... ... ... ... ... 3- 



Radium 3-3 



Ra Emanation 4-*6 



Radium A 4-75 



Radium C 6.94 



Radium F 3-77 



