76 Scientific Proceedings^ Royal Dublin Society. 



solids by such missiles than at the remarkable potency of the solid in 

 absorbing the kinetic energy. One-tenth of a millimetre of glass would stop 

 them all. The absorbing body does not, however, oppose the bombardment 

 with impunity. Violent work is done upon the opposing atoms, many of 

 which are, in a sense, partially dismembered. 



The influence of these fast-moving atoms on tlie matter through which 

 they pass is revealed in their power of conferring on the air in the 

 electroscope the properties of a conductor of electricity. The air in the 

 electroscope, if we could quite shelter it from all external influence, would, 

 possibly, be a perfect insulator. When the rays from a radioactive body or 

 from the X ray tube enter it, it becomes, for the time being, a conductor. 

 Now we know that if, say, a negative charge resides in the gold leaf, this 

 must get an equal positive amount of electricitj' in order to be neutralized or 

 discharged. The view that best explains the action of radiant matter and 

 other rays, and harmonizes with many observed facts, is that wliich assumes 

 the creation by their means of free - and + charges in the gas. To account 

 for the former, it is supposed that an electron is torn from the disturbed 

 atom of the gas. This electron carries the unit negative charge. Its removal 

 from the atom leaves the latter charged witli an equal positive cliarge. The 

 atom in this state is said to be "ionized." A single a ray will give rise to 

 about 100,000 ions in the coui'se of its flight in air. 



Under normal conditions such ions and electrons created in a gas 

 speedily reunite by mutual attraction. But this reunion is hindered if a 

 sufficient electric force exists. Thus in the electroscope, if the leaf carries 

 a negative charge, the electric force between it and the walls of tlie 

 electroscope draws the ions to the negative leaf, and drives the electrons to 

 the walls. So long as the radiation continues, however, fresh ions and 

 electrons are being made in the gas. Thus there is a continuous flux 

 of positively electrified atoms to the gold-leaf, and hence its gradual 

 neutralization and discharge. 



Ionization, or the creation of a free charge upon the atom, is well known 

 to attend many chemical actions. We may say that the a ray does chemical 

 work upon the air in the electroscope. The effect of these rays upon a 

 photographic plate is similar to that of light. Ionization also takes place 

 here, we may be sure ; and as the medium is in the solid state, the ions do 

 not recombine as they would in a gas, and the effects of the rays accumulate. 

 Similarly many substances — such as glass— become coloured when acted 

 upon by radiant matter. We must regard the a ray as an influence of 

 transcendent subtlety ; remembering that, in virtue of its own atomic 

 dimensions, it has power to deal directly with the atom it encounters, 



