THE ELECTRON 93 



hydrogen, with only one electron, and the unstable 

 uranium, with its 92, lie all the possible elements. If 

 one considers that an atom must have a whole number 

 of electrons it will appear that the other 84 known ele- 

 ments may fit in between hydrogen and uranium and 

 still leave six yet to be discovered. The names of the 

 elements with their numbers of electrons are given in 

 Table I. The spaces indicate possible elements which 

 have not yet been discovered. These may some day 

 be discovered in the composition of some distant star 

 or perhaps hi some rare earth. 



What is the character of the nucleus and why if 

 electrons repel each other, are they not all dispelled 

 and all atoms disrupted? The answer is that the 

 character of the nucleus is such as to attract towards 

 itself and in general to hold within the atomic radius 

 the various electrons of the atom. We may say, then, 

 that the nucleus is itself electricity, but of a different 

 kind from that of the electron. For convenience and 

 for reasons connected with the history of the science, 

 we call the electricity of the electron "negative" and 

 that of the nucleus "positive." 



The names were introduced by Benjamin Franklin 

 about 1756, long before the electron was known, for 

 our knowledge of the latter has all developed since 

 Rontgen's discovery in 1895 of X-rays. 1 In Franklin's 

 time it was known that if a glass rod is rubbed with 

 silk it is electrified," that is, acquires the ability to at- 

 tract light particles, as bits of paper or pith balls. Two 



1 The reader probably knows most of X-rays as a means of taking 

 pictures of bones or teeth for surgical diagnosis. The phenomenon 

 is discussed further in Chapter XIV. 



