158 SCIENCE PROGRESS 



certain of the elements involved the manifestation of more com- 

 bining units, i.e. of a higher valency than that deduced from 

 their simpler combinations, which are the more numerous and 

 the more stable. Now what Werner does is to conceive of two 

 kinds of valency manifestations, the chief or primary valencies 

 (Hauptvalenzen), which are effective in the compounds from 

 whose composition the ordinary valency values are deduced, 

 and the subsidiary or secondary valencies (Nebenvalenzen), 

 which are different in nature from the first, as they bind entirely 

 different radicles. The number of "subsidiary" valencies, like 

 that of the "chief" valencies, is limited. He assumes that in 

 compounds of a higher order the atom considered to form the 

 nucleus can bind a definite number of other atoms, independent 

 of the nature of these atoms, monovalent chlorine ranking alike 

 with divalent oxygen. This number, which arises from the 

 joint effect of "chief" and "subsidiary" valencies, is called the 

 co-ordination number, and is defined as the limiting number 

 which indicates how many atoms, independent of their valencies, 

 are directly linked with one other special atom. The co-ordina- 

 tion number of an elementary atom can be deduced from the 

 composition of its structurally known compounds, that is, from 

 the maximum number of atoms, radicles, or groups linked with 

 the elementary atom. In the majority of cases this number is 

 found to be six, though four is of not infrequent occurrence. 

 Thus when, in a simple salt, according to its ordinary valency 

 value, the binding capacity of the metallic atom seems exhausted, 

 it may still possess the power of certain atomic linkings whereby 

 it participates in the building up of more complex molecules. 

 Taking FeCl 3 as example, Werner points out how this salt, 

 which by itself appears saturated, yet possesses the power of 

 further combination with three apparently equally saturated 

 molecules of KC1. [The argument is not invalidated by the fact 

 that it has now been proved that the double salt is FeCl 3 . 

 2KC1.H 2 .0, and not, as Werner had supposed, FeCl 3 .3KCl; 

 the place of one molecule of KC1 is taken by one of H 2 0.] 

 He says: "It is assumed that in the double salt FeCl 3 .3KCl 

 all the six CI atoms are linked with the Fe atom, that it contains 

 the radicle FeCl 6 , whose existence finds its explanation in the 

 capacity of the iron to directly link six atoms— that is, in the 

 co-ordination number six. Hence the co-ordination number is 

 intended to represent that property of the atoms which makes 



