474 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP 



determined there is often some doubt as to its valency and 

 therefore as to its atomic weight. If the metal can be 

 separated, its specific heat will give the information required. 

 If not, the isomorphism of its compounds is the most trust- 

 worthy guide ; but there are so many exceptions to the law of 

 isomorphism (e.g. Ag 2 S and PbS are isomorphous) that 

 further confirmation is often needed. It is just here that 

 the periodic classification finds its most important practical 

 application. The following examples show also the advan- 

 tages of the periodic system over the earlier system of 

 classification into families. 



i. Beryllium. Equivalent 4'55- Atomic weight 

 4-55x2 = 9*1. The compounds of beryllium resemble 

 those of aluminium, e.g. the hydroxide is soluble in caustic 

 alkalis as well as in acids, the carbonate decomposes spon- 

 taneously at ordinary temperatures, the metal is prepared by 

 electrolysing the fusible double fluoride and dissolves in 

 alkalis with liberation of hydrogen. Beryllium was therefore 

 assumed, like aluminium, to be tervalent, with atomic weight 

 3 x 4-55 = 13*65. * But there is no vacant place here in the 

 periodic classification, and Newlands in his first complete 

 table of the elements (p. 457) was obliged to place beryllium 

 (with valency 2 and atomic weight 9) above magnesium, in 

 a position which he had assigned a few months before (from 

 a consideration of the individual family relationships) to 

 lithium (p. 455). Mendeleeff (Ann. Chem. Pharm., 1872, 

 8, SuppL, p. 1 66) was able to justify the position thus 

 assigned to beryllium by showing that 



(a) there was a steady gradation in the series Li, Be, B ; 



(b) the differences between beryllium and magnesium 

 were of the same character as in the corresponding 

 members of the two adjacent families, 



Li 



Na 



Be 



Mg 



B 



Al 



1 Equivalent 6'8, if O = 8. 



