ELECTRONS 93 



as there were with weights of 37. On the average such 

 a mixture would then have the weight indicated by the 

 chemical method. 



Two kinds of atoms, differing in the total number of 

 protons, were thus found to possess the characteristic 

 behaviour of chlorine. They are called "isotopes" of 

 each other because they occupy in the chemical tables 

 the same position. The term was introduced by Pro- 

 fessor Soddy who has met a similar phenomenon in his 

 study of the chemistry of the radioactive elements. 

 The case of uranium which was cited earlier will illus- 

 trate it. Uranium loses from its nucleus successively 

 an alpha particle and two electrons. Thereby it has 

 reduced its nuclear content by four protons and four 

 electrons. The atomic number of the new element, 

 uranium II, is the same as uranium; the chemical 

 behaviour is the same although, of course, the radio- 

 activity is different; but the atomic weight is reduced 

 by four units. The new element is an isotope of the 

 old, chemically identical, but with a lower atomic 

 weight. 



Such is a brief outline of the present ideas as to the 

 matter of which our universe is composed. It is gran- 

 ular in structure and electrical in nature, being com- 

 posed of definite specks. Upon the number and 

 arrangement of these specks, protons and electrons, 

 depend all the physical and chemical characteristics of 

 matter. 



Instead of classifying matter inexactly by the average 

 behaviour of a number of atoms it is now possible to 

 extend a rigorous and numerical classification to individ- 



