130 STAINING 



possible by converting one of the trivalent N atoms into the 

 pentavalent form to introduce another methyl group, and methyl 

 green results ; i.e. the colour has been changed to that of deeper 

 hue, namely, green. 



Tliis principle applies in practically any group of dyes, and 

 may often be taken into account to advantage in the search for 

 some new stain to give a particular desired effect. 



239. The Nature of the Staining Process. From what has been 

 said in the preceding paragraphs it may be realised that a solution 

 of a dye is a complex system from a physico-chemical standpoint. 

 Moreover, the structures to be stained are present as separate 

 phases, solid or liquid, of a heterogeneous system. Much dis- 

 cussion has taken place with respect to the process of dyeing, and 

 various theories of its nature as being essentially chemical or 

 essentially physical, in the sense of adsorption, mechanical or 

 electrical, or in the sense of solid solution, involving partition 

 between the solution and the tissue elements according to relative 

 solubility of the dye therein, have been advocated. It is probable 

 that all these factors play their part in varying proportion and 

 that no one theory alone can explain all the facts. 



We shall be in a better position to appreciate the complexity 

 of the conditions present if we examine, to begin with, the case 

 of a pure substance, cellulose, in relation to pure solutions of an 

 acidic and a basic dye respectively. 



We take, then, a piece of the purest analytical filter paper, 

 wash it with distilled water to remove possible traces of acid, 

 and place it in a dilute solution of the acid dye Congo red, freed 

 from foreign salts. It is scarcely stained at all. Add next a very 

 small amount of a neutral salt, say, sodium chloride. The paper 

 is deeply stained. How are these facts to be explained ? In 

 view of the chemical inertness of cellulose, it seems unlikely that a 

 chemical combination occurs between the dye and the paper 

 under the influence of a neutral salt at ordinary temperatures. 

 Moreover, the same behaviour is shown by such different sub- 

 stances as charcoal, silk, alumina, silica, and so on. The process 

 must be one of adsorption or deposition of the dye on the surface 

 by some means. In other words, it must be associated with the 

 decrease of surface energy of some kind. In the absence of foreign 

 electrolytes, adsorption may be due to decrease of surface energy 

 of the ordinary kind, known as surface tension. This is confirmed 

 by the fact that the dye, in the absence of electrolytes, can be 

 washed out again by water. But since the degree of staining is 

 very small, there must be some influence at work restricting the 

 mechanical adsorption. There is, indeed, another property of the 

 boundary surfaces between phases which demands attention here. 

 This is the electrical charge, nearly always present. If we test 

 paper in water, we find that it has a negative charge. Similarly, 



