300 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Uranium, the highest of all the atomic weights, is one of the least 

 satisfactorily determined. Recent investigation upon this subject at 

 Harvard seems to show that all the published data are complicated by 

 constant errors, which, however, sometimes partly eliminate one another ; 

 accordingly the round number 240, instead of Clarke's 239.59, is printed 

 in the table. The discussion is better deferred until our work has taken 

 more definite shajDe. 



Besides these seven elements, there are seventeen more whose atomic 

 weights, as given in the following table, differ from the results of Profes- 

 sor Clarke's computations by one part in a thousand or more. As none 

 of the seventeen values is known certainly to within one part in five 

 hundred, and some of them are probably at least as much as one per cent 

 in error, differences in these cases will excite no especial remark. The 

 elements are: cerium, columbium, erbium, gadolinium, gallium, glu- 

 cinum, indium, lanthanum, neodymium, osmium, palladium, praseody- 

 mium, samarium, scandium, thorium, ytterbium, and zirconium. This is 

 by no means a complete list of the uncertain elements, but only of 

 those for which the data admit of slightly different interpretations. 



It is rather unfortunate that most classified tables of the elements omit the 

 fractional parts of the atomic weights, for these fractions will undoubtedly 

 play an essential part in the ultimate solution of the de-Chancourtois- 

 Newlands-Mendeleev-Meyer mystery. In order to present a modern 

 table which will supply these omissions, as well as afford at a glance 

 an approximate idea of the probable error in each case, I have repeated 

 the values in the natural order. From the classified table have been 

 omitted several elements whose atomic weights and properties are 

 uncertain ; their presence in the system serves rather to obscure well 

 known relations than to elucidate new ones. That this procedure is 

 an admission of incompleteness is evident ; the last part of the table is 

 at best a lame affair. The form proposed by Thomsen * is perhaps the 

 most generally satisfactory of the current modes of arrangement, but 

 his last row is unwieldy both for printing and for thinking. In the 

 accompanying table a compromise has been adopted which cannot fail 

 to be comprehensible ; it is not my purpose here to enter into the 

 arguments regarding the adequacy of the various forms. 



This table will be reprinted, with any changes which may be ne- 

 cessary, every year. Since the commencement of the work I have 

 been glad to hear of the appointment of a committee by the German 



* Zeit. Anorg. Cliem., IX. 190. 



