CHAPTER 17 



A Comparison between Dyeing and 

 other Processes oj Colouring 



The words 'stain' and 'staining' have been avoided in this book, 

 lest they should convey the impression of a particular, definable 

 process. These words are often used in biology with such latitude 

 that they no longer have recognizable meanings. It is said that 

 basic fuchsine 'stains' chromatin, silver compounds 'stain' 

 nerve-fibres, Sudan black 'stains' lipid. The three processes are 

 fundamentally diflFerent from the chemical and physical points of 

 view. It is a curious fact that the layman is more careful in his use 

 of the word. He would not say that he 'stains' a fluid by dissolving 

 a soluble coloured substance in it, yet that is what we do when we 

 colour lipids with Sudan black. He would not say he 'stains' a 

 piece of wood when he applies an insoluble pigment to it, yet the 

 expression 'vital staining' is often used in biology in reference to 

 the uptake of insoluble pigments by cells. 



We need a phrase to cover all materials that we use to confer 

 colour (or blackness) on the parts of an organism. The simplest and 

 most self-explanatory general expression seems to be 'colouring 

 agent' or 'colorant', and the corresponding verb is to 'colour'. 

 These words suggest nothing but the demonstrable facts. 



Anyone may define words as he chooses, yet his usage will not 

 be logical unless the objects or processes called by his names possess 

 genuine similarities that are not shared by objects or processes to 

 which the names are not applied. Now the words 'dye' and 'dyeing' 

 do have meanings, even if the limits of what is meant may be rather 

 difficult to define. We shall here pass in review a series of processes 

 departing in varying degrees from the process of dyeing as de- 

 scribed in the preceding chapters. We shall start with colorations 

 that obviously do not involve dyeing, and pass on step by step 

 through various intermediates until we reach once again the sub- 

 stances and reactions with which we have familiarized ourselves. 



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