218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1016. 



less analyses performed by the analytical chemist, whether for tech- 

 nical or for scientific purposes. 



This practical utility of atomic weights, although not forgotten, 

 was not the prime incentive in the work under discussion. The 

 real inspiration leading to the protracted labor of revising these 

 fundamental quantities was the hope of finding some clue as to 

 the reasons for their several magnitudes and for the manifest but 

 incomprehensible relationships of the elements to one another. 



The unsolved cosmic riddle of the meaning of the atomic weights 

 may have far-reaching significance in another direction, because the 

 atomic weights may be supposed to hold one of the keys to the dis- 

 covery of the mechanism of gi-avitation. The mutual attraction of 

 the earth and sun, for example, must be due to the countless myriads 

 of atoms which compose them, each atom possessing, because of its 

 own appointed relative atomic weight, a definite if infinitesimal gi'avi- 

 tational force attracting other atoms. If we could discover the rea- 

 sons for the individual atomic weights we should probably gain a 

 far better understanding of the all-embracing force built up of the 

 infinitesimal effects represented by their individual magnitudes. 



Among the striking facts to be considered is the constancy of 

 gravity (and therefore of the sum total of the weights of all the atoms 

 concerned) as shown in many ways. Moreover, not only is the sum 

 total of the weights of the atoms remarkably constant, but also in 

 many cases the values for the individual elements are found to be 

 numbers of amazing constancy. Silver from all parts of the world 

 and from many different ores yields always the same value ; copper 

 from Europe has the same atomic weight as the native metal mined 

 under the bottom of Lake Superior; and yet more wonderful, the 

 iron which falls from the slcy in meteorites having their birth far 

 beyond the terrestrial orbit has precisely the same atomic weight 

 as that smelted in Norway. Many atomic weights therefore must 

 be supposed to be constant, whatever the source of the elements. 



Although thus we Imow only one kind of copper and iron and 

 silver, evidence has recently been discovered which points toward 

 the existence of at least two kinds of metallic lead. Every sample 

 of ordinary lead always has exactly the same atomic weight as every 

 other sample: but lead from radioactive minerals— lead which seems 

 to have come from the decomposition of radium — has neither the 

 same atomic weight nor the same density as ordinary lead, although 

 in many properties, including their spectra, they seem to be iden- 

 tical. This recent conclusion, reached only two years ago at Har- 

 vard, has been confirmed in other laboratories, and it now seems 

 to be beyond question. Whatever irnxj be the ultimate interpretation 

 of the anomaly, the solution of this cosmic conundrum must surely 

 give us a new idea of the essential nature of matter. Indeed, the 



