CHAPTER XI 

 NEUTRAL STAINS 



THERE are two ways in which dyes may be compounded. In 

 the first place it is possible to mix mechanically any two dyes, 

 and if they are of different colors with different selective powers, 

 double staining effects may be procured. In the second place, it is 

 often possible to form a chemical union between two dyes and thus 

 to obtain an entirely new compound which may have quite strik- 

 ing staining properties. It is such compounds as these, rather than 

 simple mechanical mixtures, that are ordinarily referred to as 

 compound dyes. 



The simple anilin dyes, it will be recalled (see Chapter II), owe 

 their properties as dyes or as biological stains to the basic or acidic 

 character of the dye molecule. Those parts of the protoplasm which 

 are acid in nature (e. g., chromatin) tend to react with the basic 

 dyes and to be colored by them; while those which are basic 

 (e. g., cytoplasm) react similarly with the acid dyes. (This, to be 

 sure, is not the whole theory of staining, as the process is quite 

 complex and involves physical and mechanical factors as well; but 

 it serves to illustrate the difference between the two kinds of stains.) 

 Now, as already explained, the dyes are not used as free acids or 

 free bases; but rather as sodium or potassium salts of the acid 

 dyes, and as chlorides (or salts of some other colorless acid) of the 

 basic dyes. 



It is well known that when two salts, such as sodium chloride 

 and ammonium nitrate, are mixed in solution there is an inter- 

 change of ions (metathesis), and the resulting solution, when it 

 reaches equilibrium, contains not only the original salts but also 

 the four free ions and the two alternate compounds as well, in this 

 case sodium nitrate and ammonium chloride. Now if one of these 

 two new compounds happens to be insoluble, as silver chloride for 

 example, which would have been formed if silver nitrate had been 

 substituted for ammonium nitrate, it is thrown out of solution, 

 and equilibrium is not reached until the solution is free (or at least 

 practically free) from the two ions which are insoluble in combina- 

 tion. In the same way, when a sodium salt of an acid dye and a 

 chloride of a basic dye are mixed in solution, there is a similar 

 tendency for the ions to interchange. Ordinarily the dyes are 

 weaker acids and bases than the chlorine and sodium ions respec- 

 tively; and if the compound dye formed were soluble in water 

 there would be little chance for much of it to be produced. As a 

 matter of fact, however, it is generally insoluble and is therefore 

 thrown out of solution; hence the compound dye can be formed 

 in considerable quantity. 



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