48 THE FOUNDATIONS OF SCIENCE 



and Soddy the theory now accepted, that radio- 

 activity consists in the explosive disintegration of 

 isolated chemical atoms, atomic and sub-atomic 

 projectiles being shot off, till a simple atomic residue 

 is left behind. 



All the types of radiation possess the power of 

 making a gas through which they pass a conductor 

 of electricity. The rays, as they impinge on the 

 molecules in their path, produce gaseous ions. It is 

 by this effect that they are chiefly investigated. Our 

 methods for the detection of electric effects are 

 so sensitive that Rutherford and C. T. R. Wilson 

 have been able to trace the effects of single a particles 

 as one by one they are shot off by disintegrating 

 radio-active atoms. In this marvellous way, by 

 watching the motions of the needle of an electro- 

 meter, or the formation of lines of cloud in the air 

 in a glass vessel, has the effect of the atomic structure 

 of matter been made visible to the human eye. 



If, from one point of view, the cathode ray cor- 

 puscles and the /? particles of radio-active changes 

 can be regarded as sub-atomic particles of matter, 

 from another they are ultimate units of negative 

 electricity. An atom with one corpuscle too much 

 is an atom negatively electrified, while a defect of a 

 corpuscle from the normal number means positive 

 electrification. 



But here we come in contact with another line of 



