CHAPTER X 



THE THEORY OF STAINING 



THRUOL T the preceding pages of this book an effort has been 

 made so far as possible to avoid theoretical discussions. Altho 

 thev contain some statements the truth of which cannot be 

 regarded as fully established, as in the case of the chemical com- 

 position of some of the dyes, the discussion in general has been con- 

 fined to observations and to chemical information for which there 

 is good authority, without any attempt to introduce explanations 

 of a theoretical nature. The present chapter, therefore, was not 

 part of the original plan of the book, and the decision has finally 

 been made to introduce it merelv because it is felt that a brief 

 statement of some of the most probable theories to explain staining 

 may be of value in assisting the histologist in the intelligent use of 

 stains for his purposes. 



A long theoretical discussion of this subject might be included 

 here, basing it upon the lengthy arguments supporting the various 

 theories that have appeared in the literature. Such a detailed dis- 

 cussion, however, would probably be of little value. Hence this 

 chapter is confined to a bare outline of the important points of the 

 different theories. 



Theoretically the dyeing of textile fabrics and the staining of 

 microscopic structures are the same. In one case only the gross 

 effects are observed, in the other the microscopic details. Any 

 theory, therefore, that will explain the details of microscopic stain- 

 ing will be fully adequate to account for dyeing in bulk. 



Theories to account for dyeing or staining have in general been 

 based exclusively upon either physical or chemical phenomena. It 

 would seem at first thought that the dves combine so firmlv with 

 the tissues stained by them that the phenomenon must be a 

 chemical one; but the exponents of physical theories have taken 

 pains to show that all the observed facts can be explained on a 

 physical basis, and that some observations are hard to explain if a 

 chemical union between tissue and dye actually takes place. In a 

 chemical union a new substance is formed which does not neces- 

 sarily have the properties of either substance entering into its 

 formation, and it is ordinarily impossible to recover the original 

 substances by means of simple solvents, ^^hen tissue is stained 

 there is no evidence of any new substance having been formed, the 

 colored tissue merely taking on one of the characteristics of the 

 dye (color) in addition to the properties which it originally possess- 

 ed; it is, moreover, ordinarily possible to extract all or nearly all of 

 the color by sufficiently long immersion in water, or by the fairly 

 brief action of alcohol. Another observation which points against 



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