94 SCIENCE REMAKING THE WORLD 



ual atoms. The key to the atom is its nuclear composi- 

 tion. Its weight depends upon the number of protons 

 in its nucleus. Its normal possibilities of combination 

 into molecular form, that is, its chemical properties, 

 depend upon the excess of protons in the nucleus. That 



From "Letters of a Radio-Engineer to His Son," by John 

 Mills. Published by Harcourt, Brace and Company, IDC. 



The Thermionic Vacuum Tube. Electrons emitted by a heated fila- 

 ment, F, are drawn across a highly evacuated space to a plate, P. The 

 stream is very sensitive to changes in the electrical potential of the 

 grid, G. The device is widely used in the Bell System as an amplifier of 

 telephone currents 



determines what is its normal complement of planetary 

 electrons and their configuration. 



In terms of excess protons in the nucleus, that is, the 

 so-called atomic number, there are ninety-two classes, 

 of which eighty-seven are known chemical elements. 

 In each class, however, there may be required a further 

 subdivision on the basis of total number of protons in 

 the nucleus. In addition, although the point has not 

 previously been developed, there may be a difference 

 in nuclear history which predisposes a nucleus to one 



