92 SCIENCE REMAKING THE WORLD 



them with a definite muscular force. As to knocking 

 electrons off from atoms, and thus ionizing, there are 

 many ways but they are not our present concern. 



The important point is that because of the electrical 

 nature of the tiny particles which compose atoms it 

 becomes possible not only to determine the masses of 

 the atoms but also to separate atoms by weight. 



The chemist has always separated atoms on the cri- 

 terion of chemical behaviour, that is, unconsciously un- 

 til recently, on the basis of atomic number. For atoms 

 so separated there have long been available very accurate 

 determinations of the relative weights of the atoms of 

 different chemical elements. The results of the elec- 

 tronic physicist agree with these in those cases where 

 the chemist had found for the atomic weight a whole 

 number, when compared to the weight of the oxygen 

 atom assumed as sixteen. The weights should be re- 

 lated as integers since the nucleus should contain whole 

 numbers of protons. 



The chemical method determined the average atomic 

 weight of a large number of atoms of a chemically pure 

 element. The physical method could be applied to a 

 mixture of elements and would separate the atoms 

 according to their weight. The chemical method 

 applied to chlorine obtained an atomic weight of 35.45 

 on a scale where the oxygen atom was 16.00. The 

 physical method applied to chlorine indicated a mixture 

 of two distinct types of atoms, one with a weight of 

 35 and the other with a weight of 37. There were ap- 

 parently in any sample of chemically pure chlorine 

 about three times as many atoms with weights of 35 



