CHAPTER 7 



Introduction to the Chemical 

 Composition of Dyes 



It is best to give the general name of 'colouring agents' or 

 'colorants' to substances used in microtechnique to colour or 

 blacken the parts of organisms. 



Colouring agents are used in very diverse ways. It is a strange 

 fact that the housewife is more careful in her terminology of 

 colouring agents than many microscopists are. She distinguishes 

 clearly between staining and painting the floor, while they often 

 use the word 'staining' without regard for the diversity of the 

 processes grouped by them under this single name. The 'staining' 

 of specimens by electron-microscopists has no connexion with 

 dyeing. The word was formerly used as a synonym for 'dyeing', 

 but has come to be treated so loosely in microtechnique that it is 

 avoided in this book. 



• There are five principal ways of using colouring agents to 

 distinguish the microscopical parts of organisms. In this book we 

 are primarily concerned with only one of these, namely, dyeing \ 

 but this one is best understood by contrast with the others. The 

 five will be briefly described here. 



Injection of suspended coloured particles into closed spaces. We 

 may suspend minute, insoluble, coloured particles in a fluid and 

 inject this into the blood-vessels or other internal spaces of an 

 organism. The final distribution of the coloured particles will be 

 determined by their incapacity to penetrate the walls of the vessels 

 into which the fluid has been forced. The chemical nature of the 

 colouring agent is irrelevant, provided that it is insoluble. 



Uptake of suspended coloured particles by phagocytic cells. We 



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