xvm THE CLASSIFICATION OF THE ELEMENTS 4 79 



dominant property of the periodic system, and has been 

 made the basis for the descriptive portion of many text- 

 books. The undue emphasis of this property, which is 

 mainly an expression of valency-relationships, has created 

 an artificial atmosphere, in which the close similarity 

 between manganese and iron or between the compounds 

 of copper and mercury, is overlooked in favour of a strained 

 analogy between manganese and chlorine, or between 

 cuprous chloride and common salt. In the same way the 

 two combustible elements, sulphur and phosphorus, are 

 only rarely mentioned together, since they have the mis- 

 fortune to differ in valency. 



For this fault Mendeleeff cannot be blamed, since he 

 insisted quite as much on the analogy between adjacent 

 elements in the same horizontal series as on that between 

 alternate elements in the same vertical group and used both 

 of these "atomic analogies" in predicting the properties of 

 missing elements. He was also careful to point out the 

 relationships which are observed between certain elements 

 lying on the same diagonal line (see above, p. 475.)- 



Some of these diagonal analogies are strong enough to 

 overcome the limitations imposed by unequal valencies, as 

 in the case of boron and silicon, their fluorides and double 

 fluorides, BF 3 , SiF 4 , KBF 4 , K 2 SiF 6 , and the glass-forming 

 borates and silicates. In other cases valencies are developed 

 in defiance of the simpler rules of periodicity, as when 

 boron becomes quadrivalent in the hydrides B 2 H 6 and B 4 H 10 , 

 as if in protest against its separation from the non-metals 

 carbon and silicon. Many similar cases might be quoted 

 of unexpected resemblance between elements lying 

 diagonally on lines sloping down from left to right in the 

 table. 



The importance of the horizontal relationships is seen 

 most clearly in Mendele'efFs group of transition-elements. 

 Metallurgists will recognise in the series 

 V, Cr, Mn, Fe, Co, Ni, 



