92 



Prof. H. E. Armstrong. 



[Mar. 5, 

















156 Gd 



157 

 158 

 159 

 160 

 161 

 162 



163 Tb 



164 

 165 



166 Er 



167 

 loo 

 169 



170 Tu 





185 



186 



187 



188 

 189 

 190 



191 Os 

 192 



193 Ir 



194 Pt 



195 



196 



197 Au 



198 

 199 



200 Hg- 

 201 





212 



213 



214 



215 

 216 

 217 

 218 

 219 

 220 



221 



222 



223 



224 

 225 

 226 

 227 

 228 





In the fourth period, a similar step down in the fourth column from 

 iron to cobalt and nickel is necessary to bring copper into an appro- 

 priate position ; and for a like reason zinc is included in a group with 

 copper. There are similar drops in the eleventh and twelfth columns. 

 Selenium appears with 77 instead of 79 as its atomic weight, in order 

 to preserve it in the oxygen-sulphur series : bearing in mind that 

 the atomic weight of sulphur differs from that of chlorine by fully 

 three units, whereas that of selenium is ordinarily supposed to differ 



