28 SECTIONAL ADDRESSES. 



remarkable change in the ionisation of the molecule. Thus, platinic 

 chloride PtCl 4 combines with six molecules of ammonia forming a com- 

 pound Pt(NH 3 ) 6 Cl 4 , in which all four chlorine atoms are ionised. As 

 the ammonia molecules are removed one by one, the chlorine atoms 

 appear to take their places in the non-ionised part of the molecule, until 

 we reach Pt(NH 3 ) 2 Cl 4 , which is not ionised at all, and is not a salt : 

 every replacement diminishes the positive charge on the platinum -com- 

 plex by one. If more ammonia molecules are replaced by chlorine atoms, 

 the ionisation occurs again, but now the complex has acquired a negative 

 charge, so that we finally reach the well-known ' double salt ' K 2 PtCl 6 . 



To explain these phenomena, Werner proposed a theory of molecular 

 structure founded on entirely new principles : that it was determined by 

 the tendency of atoms, irrespective of the periodic groups to which they 

 belonged, to attach to themselves a definite number (usually six, sometimes 

 four, and less often other numbers) of other atoms or groups, which 

 might either be univalent radicals or whole molecules capable of independent 

 existence. These groups, together with the central atom, formed the 

 ' co-ordination complex,' and the groups were said to occupy the ' first 

 sphere ' of combination of the central atom ; the molecule might also 

 contain other atoms or groups occupying a ' second sphere,' which were 

 less firmly attached, and did not count as part of the co-ordination com- 

 plex. For example, in the hexammine of platinic chloride [Pt(NH,) 6 ]Cl 4 , 

 the ammonia molecules were regarded as occupying the first sphere of 

 the platinum and satisfying its co-ordination number 6, while the chlorine 

 atoms occupied the second sphere. Experimentally , the groups in the 

 second sphere were distinguished by the fact that they were ionised in 

 water, while those forming part of the co-ordination complex were not. 

 Werner produced a great mass of evidence in support of these views ; 

 the chemical public in general did not, however, pay much attention to 

 them until in 1911 Werner was able to show that certain compounds of 

 chromium and other elements which, on his theory, should have asymmetric 

 molecules could actually be resolved into their optically active forms. It 

 then became evident that the theory must at least contain a large element 

 of truth. 



Thus, some fifteen years ago, Werner had been able to demonstrate 

 that his theory accounted for the structure of a large number of (mainly 

 inorganic) compounds, with which the ordinary structural theory was not 

 able to deal. He himself applied the theory to organic compounds as 

 well : he regarded it as a general theory of molecular constitution, and 

 sought to show that the structural theory failed even in dealing with 

 organic compounds. But this must be admitted to be the weakest 

 part of his argument : he was not really able to prove that the structural 

 theory was inadequate in the sphere of its greatest triumphs, that of 

 organic chemistry. 



An impartial critic writing at this time (say, in 1913) might have 

 summed up the position thus : The theory of structural chemistry gives 

 a satisfactory account of the molecular constitution of nearly all organic 

 and a certain number of inorganic compounds, but it is unable to deal 

 with a large number of substances of the latter class. The theory of 

 co-ordination, which proceeds on wholly different lines, is able to explain 



