( 150 ) 



With a view to isomorphism chromium might be placed in the 

 eventh, the sixth, the third, and on account of chrom-ammonium 

 in the eig;lith group. For telhirium you have choice between three 

 groups. If the principal weight is accorded to the analogy between 

 H3 Te and Hj S, it must be placed in the sixth group; if you 

 follow Mr. Ektgers, in putting the isomorphism of K3 Te Clo with 

 K3 Os Cle and Kj Ii" Cle upon the foreground, a place must be 

 found for it in the eighth group; if the attention is fixed on the 

 somorphism of Csj Te Cls with Csg Su Clg and on the analogy 

 of potassium -tellurium -oxalate with potassium -zirconium -oxalate, 

 it may be put into the fourth group. Meanwhile it may not 

 be overlooked, that Te owes its place in the sixth group solely 

 to the analogy of H2 Te and Hj S ; that, to fit into this group, 

 its atomic weight ought to range below 126 (Sb = 120, Te = 123(?), 

 J := 12G.5) and that the atomic weight of Te stands likewise 

 in the way of a migration to the fourth and to the eighth group. 

 In the fourth group no place is open next Sn for an element 

 having an atomic weight about 120 (Sn = 117.4, Ce = 141), and 

 if Te should be placed in the eighth group, between Ru and Os, 

 search would have to be made for seventeen new elements in this 

 group. The difference between the atomic weights of two acknow- 

 ledged membres of this group, taken in a vertical column, amounts 

 to 45 — 48; between Ru and Te it would be half as great (Ru= 104, 

 Te := 126 — 128), thus: three unknown elements, filling the gap 

 between the first and second file, eight between the second and 

 third (Ru — 104, Os = 195) and six' below the third (Pt = 195, 

 U = 240). Not impossible, but certainly not probable. Probably, 

 if IMr. Retgeks had been acquainted with the isomorphism of 

 Cs2 Mu 01(5 and Cso Sn Clo, he would have stopped at the statement, 

 that isomorphism between tcllurates and sulfates does not exist, 

 and would not have proceeded to seek a place for tellurium in 

 the eighth group with a difference of 22 between two consecutive 

 atomic weights instead af 45. 



The weight of isomorphism in chemical speculations must be 

 further reduced than has been doue by Mr. Rktukks. The hypo- 

 thesis, that isomorpliism among a group of compounds does point 

 to isomorphism of the elements which these compounds have in 

 common, is abandoned by most chemists, but isomorpliism between 

 sulfates and chromates, between phosphates, arsenates and vanadates, 

 stated without restriction, is even objectionable. In alum trivalent 

 chromium can take the place of aluminium, while sulfuric acid 

 cannot be exchanged for eliromic acid, and yet this exchange can 



