i6o POPULAR SCIENCE MONTHLY. 



are quite a number of elements before the next group of seven is 

 reached, and the same is true in each succeeding column. The next 

 year, 1870, Meyer published a table in which he brought these outside 

 elements into something of order by pointing out the existence of a 

 double periodicity after the first two octaves have been passed, and 

 showing that the alternate periods resemble each other closely. This 

 was brought out with greater clearness in the revised table which Men- 

 deleetf published in 1871. The skill of the author of this table is 

 apparent when we consider that it is, with few additions, the generally 

 .accepted table in use at the present day. This table, which is given on 

 the following page, when compared with that of two years before, 

 shows how great had been the development. 



One well-recognized test of the truth of any theory is its use in 

 prediction. In this table Mendeleeff did not hesitate to make certain 

 changes in the generally received atomic weights, in order to bring 

 facts into conformity with his table. His was not the position of the 

 ancient philosopher who would have all phenomena bend to his precon- 

 ceived theory, and if the facts failed to yield, so much the worse for 

 the facts. Mendeleefi: had confidence that this Periodic Law was the 

 expression of a gTeat truth of nature, and so firm was his confidence 

 that he could not but believe that when the phenomena did not agree, 

 it was from imperfect observation and interpretation of the facts. A 

 good instance of this is seen in the case of the metals of the platinum 

 group. As far as observation had gone, osmium had the largest atomic 

 weight of these metals, followed by iridium and platinum, of equal 

 weight, and all these metals were lighter than gold. According to the 

 Periodic Law the reverse should be the case. Mendeleeff affirmed that 

 the discrepancy in this case was probably due to the fact that the 

 atomic weights of these metals had not been determined with an accu- 

 racy commensurate with the work of the table. It is an interesting 

 confirmation that some years later, Seubert took up this atomic weight 

 problem, and found that the views of Mendeleeff were correct. Gold 

 has the highest atomic weight of these elements, platinimi the next 

 highest, iridium follows and osmium comes lowest of all, its previously 

 determined weight having been seven or eight units too high. 



Along the line of predictions a still more remarkable use of the 

 table appeared in connection with the vacant spaces. There were many 

 places in the table where elements might be expected, which were, 

 however, then unknown. Could the table stand the test of actually 

 predicting the existence of an unknown element? Mendeleeff did not 

 think this too great a strain to put upon his work, and he ventured 

 not merely to predict that elements might be expected with atomic 

 weights of 44, 68 and 72, but he was even bold enough to describe 

 these elements under the names of eka-boron, eka-aluminum and eka- 



