THE DIRECT ATTACHMENT OF DYES TO TISSUES I93 



ents was remarked by Ehrlich ^^'^ in 1879. ^^ ^^^ same year he 

 noted ^^^ the affinity of acid dyes in general for the granules of 

 what he had named 'eosinophil' leucocytes. In the following year 

 he gave a short general statement of the differences between basic 

 and acid dyes in their reactions with cell-constituents, illustrating 

 his remarks by reference to leucocytes. ^^* He noted that the gran- 

 ules of eosinophils have an affinity for acid dyes, of Mastzellen for 

 basic dyes, and of polymorphs for both constituent parts of 

 'neutral' dyes (p. 262). These papers, published when Ehrlich 

 was about 25 years old, mark a turning-point in the history of 

 scientific microtechnique. 



In preliminary studies of the reactions of dyes with electrically- 

 charged objects it is convenient to use simple, homogeneous 

 models to represent the tissue-constituents. The acidic and ampho- 

 teric constituents are the most worthy of attention, because every 

 cell contains them. Seki *^^ chose collodion as a model for the 

 acidic components, gelatine gel for the amphoteric. These sub- 

 stances are very convenient, because they can so easily be cut in 

 slices of uniform thickness, but collodion is more acidic than 

 most tissue-constituents. The depth of colour may be recorded on 

 an arbitrary scale, or determined by photometry. Practical instruc- 

 tions for some experiments resembling Seki's are given in the 

 Appendix (p. 323). 



The results with collodion (fig. 25) are simpler than those with 

 gelatine because the former, when in water or aqueous solutions, 

 maintains its negative charge throughout the relevant range of pH. 

 It is strongly dyed by crystal violet or any other typical basic dye 

 throughout the range. Typical acid dyes, however, scarcely tinge 

 collodion, except in strongly acid solutions, in which the charge on 

 the collodion is somewhat lessened. 



An amphoteric dye behaves as one would expect on theoretical 

 grounds. On the acid side of its iso-electric point it is positively 

 charged and therefore dyes collodion strongly, but on the less acid 

 side it becomes negatively charged and its tendency to colour 

 collodion disappears about neutrality. These facts are well exempli- 

 fied in the practical use of carminic acid, the iso-electric point 

 of which is about pH 4-2. It is commonly used for dyeing chro- 

 matin in the form of aceto-carmine.*'*^ In this strongly acid solu- 

 tion it acts as a basic dye.*^*' *^^ In alkaline solution, however, in 

 the form of borax-carmine, ^^^ it acts as an acid dye, but is usually 

 converted subsequently into a basic one by treatment with acid.*^* 



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