126 STAINING. 



by electrical forces. The reason is, in all probability, 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, however, 

 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 decomposed, 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. 



204. "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 parti- 

 cular groups in the dye molecules. While this may be the case in 

 isolated instances, there are many facts which show that it cannot 

 be accepted as a general law. It is difficult to see what purely 

 chemical relationship can exist between complex, substituted, 

 diazo-sulphonates, as a large number of these specific dyes are, and 

 the chemical components of cells. Moreover, although methylen 



