132 STAINING 



tion, much as charcoal adsorbs colour from solutions on which 

 it is employed as a decoloriser. The physical theories of stain- 

 ing are well presented by Holmes in the paper cited above, and 

 are very generally held by dye chemists at present. Chemical or 

 electrical theories, on the other hand, are favoured by some bio- 

 chemists for the reasons pointed out by the Stearns. 



240. Removal of Dyes. One strong argument in favour of 

 chemical or electrical fixation of dyes is the difficulty with which 

 they are ordinarily removed. On the electrical theory above 

 outlined one can explain this by assuming that the dye can only 

 be set free by reversing the sign of the charge on the surface. 

 This cannot be done by pure water alone. It can be done, how- 

 ever, by acid or alkali in the appropriate case. For example, if 

 an acidic dye has been fixed on a negative surface by the aid of 

 cations, which convert the charge to a positive one, OH' ions, 

 provided by alkali, are powerful enough to change the sign of the 

 charge back again to negative and thus free the dye, whereas H- 

 ions from an acid only increase the positive charge and fix the 

 dye more firmly. Hence the statement that acidic dyes are fast 

 to acids. A basic dye, adsorbed by a negative surface, is removed 

 by acids and intensified by alkalies. A corresponding explanation 

 holds. Thus, H* ions from acids make the surface more positive, 

 hence the dye is released. OH' ions make it more negative, 

 hence the dye is held faster. In all cases, if the acid or alkali is 

 strong enough, any dye-salt adsorbed is decoinposed, sometimes 

 with change of colour. 



The process of " differentiation " by alcohol or other agent, to 

 be referred to below, is an application of these facts. Alcohol 

 removes a " basic " dye because it reduces the negative charge 

 of the tissue elements and thus releases part of the positively 

 charged constituent of the dye adsorbed. 



241. "Specific" Stains. Certain tissue elements and cell- 

 constituents have the property of staining deeply with particular 

 dyes. That of nervous structures with methylen blue and of 

 mitochondria with dyes containing di-ethyl-safranin, such as 

 Janus green, may be given as examples. The property may be 

 shown either by their taking up the stain from a dilute solution 

 more rapidly than other structures present do (" progressive " 

 staining), or by their holding on to it more tightly when excess 

 of general stain is washed away by appropriate treatment. This 

 latter process is sometimes known as " differentiation " or as 

 " regressive " staining. 



It is natural to interpret this behaviour as due to a chemical 

 combination of a special kind, as did Ehrlich in his well-known 

 theory of " chemo-receptors," according to which certain " side- 

 chains " of protoplasmic molecules have special affinities for 

 particular groups in the dye molecules. While this may be the 



