84 SCIENCE REMAKING THE WORLD 



polite dancing, form a molecule of nitrogen. Of such mol- 

 ecules you inhale with each breath a few score billion, 

 to say nothing about one fourth as many oxygen mole- 

 cules each formed of two atoms. In the air around 

 you all these molecules are moving at about one thou- 

 sand miles an hour, in a haphazard way, for on the aver- 

 age each can travel less than one thousandth of an inch 

 before it must dodge another. That means a change 

 of front about fifty million times a second, and estab- 

 lishes a world's record in expediency. 



Now imagine a cube of this air about three eighths of 

 an inch on a side, a cubic centimeter. Let the cube and 

 its contents grow until its edge would reach from New 

 York to Cleveland. Then about every foot along the 

 way there would be a molecule; but you could not ex- 

 pect to see one because it would be only a few tiny 

 specks, each speck about one hundred thousandth of an 

 inch. 



Nitrogen has an excess of seven protons in each 

 atomic nucleus. Helium has two. So far as concerns 

 an excess of protons in the nucleus there are just ninety- 

 two possible conditions, extending from a nucleus of one 

 proton and no electrons to the condition of ninety-two 

 more protons in the nucleus. Whether or not there 

 ever were any nuclei with an excess of more than ninety- 

 two no one knows. If there were in the early geologic 

 ages, they have now disappeared, for nuclei with more 

 than eighty-two excess protons spontaneously disinte- 

 grate. This is the secret of the radioactive elements of 

 which radium is the widest known, but uranium and 

 thorium are the unrelated parents. 



