304 DYEING 



The crystals of the pure substance are colourless ^*^' ^^^ or at most 

 just tinged with yellow.^-^ There is not much information about 

 the colour of pure proteins in which iodine has been incorporated 

 by reaction with the phenyl rings of the tyrosine residues, but 

 casein that has been modified in this way and then washed free of 

 loose iodine is white. ^'^ It is probable, therefore, that when pro- 

 teins are browned by iodine/iodide solutions, the tri-iodide ion (or 

 iodine in some other non-molecular form) is loosely adsorbed; 

 but this rather evasive verb only veils our ignorance of the actual 

 process involved. 



In microtechnique we very frequently cause the local formation 

 of a coloured substance that is not a dye. We soak the section in 

 a colourless solution, and certain tissue-constituents become 

 coloured (or black) ; or alternatively we soak it in a coloured solu- 

 tion, and certain tissue-constituents take on a different colour. 

 Very many histochemical tests belong to this category of colouring 

 processes. 



It is an essential part of this method that the coloured (or black) 

 reaction-product should not diffuse away from its site of produc- 

 tion; otherwise vague or misleading results would be given. The 

 substance produced must be either insoluble or else capable of 

 adsorption to the object in or on which it originated. 



There seems to be no limit to the variety of different processes 

 that are used in microtechnique to achieve the local formation of 

 coloured substances (other than dyes) in particular tissue- 

 constituents. The reactions vary in complexity from the simple 

 reduction of a metal from its salt to the trapping of the reaction- 

 product of a cellular enzyme with a provided substrate, and its 

 subsequent visualization. 



One of the most familiar and striking examples of this kind of 

 colouring is the blueing of starch by the same potassium tri-iodide 

 solution that colours protein brown; but unfortunately, despite a 

 great deal of study, the reactions concerned in this colour-change 

 are not yet established with certainty. ^^^ 



Colouring of this sort is particularly satisfactory when we can 

 actually write the equation for the local reaction. Perls's test ^^^ for 

 ferric iron is a good example. A section is soaked in a yellow solu- 

 tion of potassium ferrocyanide, and sites of ferric iron stand out 

 sharply by the deposition in them of insoluble Prussian blue : — 

 4Fe^-+- + 3Fe(CN)e= ► Fe4[Fe(CN)e]3i 



