92 THE REALITIES OF MODERN SCIENCE 



If then, as there seems reason to believe, the hydrogen 

 atom consists of a nucleus and one electron revolving 

 about it, the size of both may well be very small as 

 compared to what we should measure as the size of 

 the atom or of its inclosing sphere. Although the re- 

 lations of size in the atom are much like those of our 

 earth and its orbit, the nucleus is very much smaller in 

 proportion than is our sun. 



In the case of atoms there are two ways, which we 

 shall discuss more fully later, in which electrons may be 

 obtained. In the first case we may jar electrons loose 

 from the atom. In the second case the electron may 

 be thrown off by some disturbance which has its 

 origin in the atom itself. In this second case we call 

 the substance "radioactive." Of such substances ra- 

 dium, discovered in 1897 by Monsieur and Madame 

 Curie, is the best example. Other radioactive sub- 

 stances are uranium, thorium, actinium, and polonium. 

 The atoms of such substances appear to be disrupting ; 

 not that all of them do so at once, but of a bit of such a 

 substance some of the atoms are always breaking down 

 in this way. In such a breakdown electrons are shot 

 out or other changes take place and the result is new 

 elements. 1 



Uranium, which is the heaviest known atom, has at 

 least 92 electrons, 2 that is, it has 92 electrons exclusive 

 of any which may be contained in the nucleus, the con- 

 struction of which we are about to consider. Between 



1 Radium is believed to be a product due to the disintegration of 

 uranium, and this may later be found true of the other radioactive 

 elements. 



2 The determination of "atomic numbers" is discussed in Chap- 

 ter XXII. 



