CHAPTER HI 

 THE THEORY OF STAINING 



THROUGHOUT the other chapters of this book an effort has 

 been made so far as possible to avoid theoretical discussions. 

 Although they contain some statements the truth of which 

 cannot be regarded as fully established, as in the case of the chemi- 

 cal composition of some of the dyes, the discussion in general is 

 confined to observations and to chemical information for which 

 there is good authority, without any attempt to introduce ex- 

 planations of a theoretical nature. The present chapter, dealing 

 whollv with theorv, is included because it is felt that a brief state- 

 ment of some of the most probable theories to explain stainmg 

 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. 



The majority of such theories have depended upon either physi- 

 cal or chemical phenomena rather than on both simultaneously. 

 It would seem at first thought that the dyes combine so firmly 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. When 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 

 possessed; it is, moreover, ordinarily possible to extract all or near- 

 ly all of the color by sufficiently long immersion in water, or by the 

 fairly brief action of alcohol. Another observation which points 

 against chemical action is that the tissue never removes the dye 

 completely from solution, even though very dilute; whereas or- 



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