DISCOVERY AND NATURE OF RADIOACTIVITY 3 



velocity of the particles. He found the mass to be one one-thou- 

 sandth that of a hydrogen atom. 



Lenard Rays : Hertz ^^ discovered that cathode rays will pass 

 through thin aluminium foil inside of a Crookes tube. This proved 

 that aluminium is transparent to the particles constituting these rays. 

 Acting on this suggestion, Lenard ^^ succeeded in producing cathode 

 rays in a tube containing a window of aluminium. Some of the ray-s 

 passed through the window, and Lenard was thus able to experiment 

 with cathode rays outside of the tube, and to demonstrate that they 

 carry a charge of negative electricity. Thus the statement of Crooks, 

 quoted above, that we must be content to observe and experiment 

 with cathode rays " from the outside," no longer held true. Cathode 

 rays that have passed outside of a Crookes tube have been called 

 Lenard rays. 



The researches above referred to became the foundation stones 

 for the new science of radioactivity. 



Divisibility of the Atom : In 1899, Professor J. J. Thomson 

 found that carriers of negative electricity are given off from a red 

 hotwire in a vacuum, and it was for these carriers that he proposed 

 the name corpuscle. In Thomson's paper, also, we find one of the first 

 statements, based upon experimental evidence, that the atom is not 

 the limit of physical divisibility. "I regard the atom," says Pro- 

 fessor Thomson, ^^'' " as containing a larger number of smaller bodies 

 which I will call corpuscles. ... In the normal atom this assem- 

 blage of corpuscles forms a system which is electrically neutral." 



The term electron * has largely supplanted that of corpuscle. 

 The mass of an electron is always the same, no matter from what 

 gas, or from what solid produced, or by what means. It has the 

 smallest mass of any known body. It will be noted later that an 

 electron can be split off from the atom of probably every known sub- 

 stance. This fact leads to the immensely important inference that 

 unit charges of negative electricity are constituents of the atoms of 

 all matter, or, in other words, that the nature of all matter, organic 

 or inorganic, is electrical. In fact the work of Professor Thomson 

 lends much probability to the conclusion that the entire mass of the 

 corpuscle is electrical, that it is a disembodied charge of negative 

 electricity, and that matter and electricity are one and the same thing. 



Ionization : When an electron passes through a mass of a gas 



* Coined by Dr. G. Johnstone Stonej. Kelvin wrote it " electrion." 



