DOCTOR ASTON 'S EXPERIMENTS ON THE MASS 

 SPECTRA OF THE CHEMICAL ELEMENTS. 



With introduction by C. G. Abbot. 



[With 1 plate.] 



In his paper, " The problem of radioactive lead," 1 Dr. T. W. Rich- 

 ards showed that there exist well-marked differences between the 

 atomic weights of lead from different sources. To every chemical 

 and optical test other than the atomic weights and closely related 

 properties the samples are indistinguishable, and, when mixed, in- 

 separable. The recent beautiful experiments of Dr. F. W. Aston, 

 of the University of Cambridge, throw a clear light on this matter 

 and show that many others of the chemical elements contain com- 

 ponents of different atomic weights. These are identical in all 

 chemical and optical properties, so that the several varieties are 

 indistinguishable and inseparable by ordinary means. 



In order to clearly grasp the profound significance of these results 

 and to appreciate the ingenuity which has brought them to light it 

 will be well to review a variety of phenomena before giving Doctor 

 Aston's observations in his own words. 



It is but a few years since a chemical element was defined as a sub- 

 stance which can not be decomposed into unlike components, and an 

 atom as the smallest particle o-f such a substance. While the atoms 

 could neither be seen nor isolated, their relative weights in different 

 elements were known by noting the proportions in which the elements 

 combine. Thus Morley, in his classical investigation, 2 determined 

 the relative atomic weights of oxygen and hydrogen at 16.000 and 

 1.0076. 



So many of the atomic weights are exact whole numbers that a 

 suspicion was entertained by many chemists of the nineteenth century 

 that all atomic weights would be integers if correct determinations of 

 them could be obtained. Doctor Richard's classical work made this 



1 Smithsonian Report for 1918, pp. 205-219 (separate publication No. 2557). 



2 Smithsonian Contributions to Knowledge, Vol. XXIX. 



223 



