THE INDIRECT ATTACHMENT OF DYES TO TISSUES 213 



veniently situated to donate electrons to an iron atom that has re- 

 placed the hydrogen of the -OH group. Thus a dative covalency is 

 formed, as is shown in the formula by an arrow, and the iron atom 



HO— C I HO— C 



II ' 

 O OH 



Salicylic acid Salicylic acid combined zvith iron 



is now gripped by a pair of pincers. It wdll be noticed that the iron 

 atom now forms part of a six-membered ring, the atoms being in 

 this order: — iron, oxygen, carbon, carbon, carbon, oxygen. 

 Actually the iron atom is gripped by three pairs of pincers (though 

 this fact is not shown in the formula), for the iron atom can 

 replace hydrogen in three molecules of salicylic acid, each of 

 which can form a second bond with the iron through an oxygen 

 of its carboxyl group. 



To achieve this result, one must have a bi- or multivalent metal 

 that is capable of forming covalent bonds, and an organic com- 

 pound that possesses both a hydrogen atom replaceable by the 

 metal and also a donor atom (here oxygen), capable of donating 

 electrons to the metal to form a second link. These are exactly the 

 conditions that are fulfilled when a lake is made. This fact was 

 first realized nearly half a century ago by the celebrated Swiss 

 chemist, Werner, in 1908,^^^ though naturally he did not express 

 himself in terms of the electronic theory of valency. He realized 

 that the mordants were salts of metals that could form innere 

 Komplexsalze, and he quoted [Cr(OH2)6]X3 (X standing for the 

 anion). The fact that chromium is capable of receiving dative 

 bonds is implied by the formula he wrote, for, as we have seen, six 

 molecules of water are co-ordinated to the metal through their 

 oxygen atoms. Werner expressed himself thus: — 'From the re- 

 ported results of the experiments it can no longer be doubtful 

 that the property of chemical compounds to connect wdth mordants 

 depends upon the property of the latter to form internal metal- 

 complex salts.' ^^^ Of the other partner to the transaction he wTOte, 

 'Dyes that connect with mordants are hereafter constitutionally 

 characterized by the fact that a salt-forming group and a group 

 capable of producing a co-ordinate bond with the metal atom are 

 so arranged that an internal metal-complex salt can originate'. ^^^ 



