508 



Mr. F. W. Aston 



[Feb. 11, 



The following is a list of elements and isotopes determined to 

 date : — 



Table of Elements and Isotopes. 



(Numbers in brackets are provisional only.) 



By far the most important result of these measurements is that, 

 with the exception of hydrogen, the weights of the atoms of all the 

 elements measured, and therefore almost certainly of all elements, 

 are whole numbers to the accuracy of experiment, namely, about one 

 part in a thousand. Of course, the error expressed in fractions of 

 a unit increases with the weight measured, but with the lighter 

 elements the divergence from the whole-number rule is extremely 

 small. 



This enables the most sweeping simplifications to be made in our 

 ideas of mass. The original hypothesis of Prout, put forward in 

 1815, that all atoms were themselves built of atoms of Protyle, a 

 hypothetical element which he tried to identify with hydrogen, is 

 now re-established, with the modification that the primordial atoms 

 are of two kinds : atoms of positive aud negative electricity. 



Although the latter unit has long been known to us as an 

 " Electron," its mate, which appears to be the real unit of mass, has 

 only recently been given the name of " Proton." 



The Rutherford atom, whether we take Bohr's or Langmuir's 

 development of it, consists essentially of a positively charged central 



