July 14, 1916] 



SCIENCE 



41 



new physical chemistry, so that the accurate 

 determination of atomic weights really be- 

 longs as much in that field as in the field of 

 analytical chemistry. 



At Harvard during the last thirty years 

 the values of the atomic weights of thirty 

 of the most frequently occurring among the 

 eighty or more chemical elements have been 

 redetermined. From data secured here and 

 elsewhere is compiled an international table 

 of atomic weights, revised from year to year 

 by an authoritative committee composed of 

 representatives of various nations. The 

 values thus recorded are in daily use in 

 every chemical laboratory throughout the 

 world, serving as the basis for the compu- 

 tation of countless 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 pro- 

 tracted labor of revising these fundamental 

 quantities was the hope of finding some clue 

 as to the reasons for their several magni- 

 tudes, and for the manifest but incompre- 

 hensible relationships of the elements to one 

 another. 



The unsolved cosmic riddle of the mean- 

 ing 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 discovery of 

 the mechanism of gravitation. The mutual 

 attraction of the earth and sun, for exam- 

 ple, 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 infin- 

 itesimal gravitational force attracting other 

 atoms. If we could discover the reasons 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 num- 

 bers 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 sky, in meteor- 

 ites having their birth far beyond the ter- 

 restrial 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 know only one kind of 

 copper and iron and silver, evidence has 

 recently been discovered which points to- 

 wards 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 of its properties, 

 including its spectrum, it seems to be iden- 

 tical. This recent conclusion, reached only 

 two years ago at Harvard, has been eon- 

 firmed in other laboratories, and it now 

 seems to be beyond question. Whatever 

 may be the ultimate interpretation of the 

 anomaly, the solution of this cosmic conun- 

 drum must surely give us a new idea of the 

 essential nature of matter. Indeed, the 

 fascinating subject of radioactivity bids 



