158 SCIENCE -PROGRESS 
certain of the elements involved the manifestation of more com- 
bining units, ze. 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, 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 KCl. [The argument is not invalidated by the fact 
that it has now been proved that the double salt is FeCl. 
2KC1.H,.O, and not, as Werner had supposed, FeCl;.3KCl1; 
the place of one molecule of KCl is taken by one of H,0.} 
He says: “It is assumed that in the double salt FeCl,.3KCl 
all the six Cl atoms are linked with the Fe atom, that it contains 
the radicle FeCl, 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 
