116 CYTOLOGICAL TECHNIQUE 



remain in solution, but there is often a tendency towards general 

 precipitation of an insoluble lake, in which the mordant metal has 

 combined with the maximum number of dye ions. 



Although electrostatic attraction presumably draws the alu- 

 minium-purpurine ions towards sites of negative charge in the 

 tissues, yet a bond other than the ionic must eventually be estab- 

 lished, for otherwise the mordant/dye complex would be no more 

 tightly held to the tissues than basic dyes are, and like the latter it 

 would be washed away readily enough by neutral ethanol. It is 

 probable that covalent linkages are established between the 

 aluminium atom and acid groups in the tissues, such as the phos- 

 phoric group of the nucleic acids. If so, the tissue /mordant/dye 

 complex would have the structure shown here. It will be noticed 



OH 



OH 



o o 



HoO Al OHo 



L H^O 



o- 



OH2 



o- 



+ + 



0=F 



0=F 



sugar 



O 



O O 



\/ 



sugar 



O 



sugar 



sugar 



base base base base base base 



The mordant 'dye complex attracted to The tissue /mordant /dye 

 a site of negative charges (a nucleic acid) complex 



in the tissue 



Structural formulae showing hypothetically how purpurine 

 may be linked to nucleic acid through aluminium 



that two water molecules are freed from association with alu- 

 minium, and that the double positive charge on the mordant/dye 

 complex is cancelled by two negative charges on the acid. 



