336 TRANSACTIONS OF SECTION B. 
emanating from the central atom, but also to the mutual attractions exerted on 
one another by the co-ordinating or associating radicals or groups. It is pre- 
cisely those radicals or groups possessing considerable residual affinity which 
function most frequently in the formation of co-ordinated complexes. The 
general tendency to form hydrated and ammoniated metallic salts is to be 
attributed ‘in the main to the capacity for association exhibited by water and 
ammonia molecules respectively. 
The phenomenon of co-ordination may be compared with the formation of fog 
in moist air containing minute dust-particles, and it is even more closely akin to 
the formation of the large ions of the atmosphere by the association of small 
ions with uncharged invisible water-drops. 
The size of the atomic volume of the central element has a threefold influ- 
ence on the stability of co-ordination compounds. First, if the atomic volume 
is small, the residual affinity of the atom is exerted ‘in a more concentrated 
form. Secondly, the co-ordinating molecules or radicals can approach nearer 
to the centre of the central atom when its volume is small and therefore nearer 
to one another so that their mutual attractions become more effective. Thirdly, 
as the dimensions of the co-ordinating molecules or radicals are of molecular or 
atomic magnitude, these segregating units will fill more completely the available 
space round an atom of small volume than that round an atom occupying a 
larger sphere. This filling up of the available space also conduces to stability, 
as is manifested by many stereoisomeric compounds. 
The number and arrangement of the associating units have also an important 
bearing on the stability of the co-ordination complex. It is obvious that the 
most stable system will be that in which there is a symmetrical distribution of 
the forces interacting between the associating units, a condition which is 
attained by taking such a number of units that they can be arranged symmetri- 
cally over the surface of a sphere. This problem has but few solutions, inas- 
much as there are only five regular solids, the tetrahedron, octahedron, cube, 
icosahedron, and dodecahedron, with four, six, eight, twelve, and twenty vertices 
respectively. These integers will be the co-ordination numbers corresponding 
with the theoretically possible most stable systems. Molecular aggregations exist 
corresponding with four of these arrangements—that is, with all possible cases 
except that of the dodecahedron. There are also examples of less symmetric 
arrangements which become stable in certain circumstances. 
In addition to the centric co-ordination complexes with associating units 
arranged round a central atom, it is highly probable that co-ordination some- 
times leads to cyclic arrangements, as, for example, in the following instances :— 
The basic glucinum acetate, butyrate, &c., the dichloride and dibromide of 
molybdenum, and the reduction products of the pentahalides of columbium and 
tantalum. 
As an example of the application of the co-ordination theory to compounds 
of technical importance may be cited the case of the lakes of acidic colouring- 
matters developed in mordant dyeing. The simplest of these are the iron, 
chromium, and cobalt lakes of the ortho-quinoneoxime dyes, which are 
undoubtedly internally co-ordinated compounds, 
2. A Device for the Representation of the Natural Classification of the 
Elements. By Professor Ormu Masson, I’.R.S. 
U'UESDAY, AUGUST 18. 
Jot Discussion with Section A on the Structure of Atoms and 
Molecules.—See p. 293. 
2 SS 
