CHAPTER 8 



Introduction to the Chemical 

 Composition oj Dyes 



The constituent parts of cells and of intercellular material are 

 usually transparent and colourless, and therefore not distinguish- 

 able from one another unless there are appreciable differences of 

 refractive index. It was Leeuwenhoek (17 19) ^^^ who first used a 

 colouring agent in an attempt to overcome this difficulty. He was 

 studying mammaUan muscle, and tried the effect of an alcoholic 

 solution of saffron. ^^^ His experiment was not very successful, and 

 others wxre slow to follow his example. Iodine was used from time 

 to time for the same purpose, but during a very long period cells 

 and tissues were commonly examined in their natural, transparent 

 state. During the decade 1 848-1 858 the use of colouring agents in 

 microtechnique was repeatedly rediscovered,-^ and from that time 

 to the present day enormous use has been made of artificially 

 coloured preparations. 



The colouring agents used in microtechnique are of diverse 

 kinds, but the majority of them are dyes. It will be best to consider 

 carefully what dyes are, before discussing the other kinds of 

 colouring agents that can be used (p. 296). 



If a porous body, such as a sponge, be placed in a vessel con- 

 taining a solution of a coloured substance, some of the coloured 

 substance will be imbibed by the porous body, and if the latter be 

 lifted out of the solution, will be rem^oved with it. There are now 

 two possibilities about the solution that remains in the vessel. 

 Either the coloured substance is at the same concentration as it was 

 before the porous body was put in it, or else the concentration has 

 been reduced (that is to say, the solution has been to some extent 

 'exhausted'). If exhaustion has occurred, the porous body has 

 clearly shown a special affinity for the coloured substance in 

 preference to the solvent. It is characteristic of the process of dye- 

 ing that this special affinity exists, and indeed in the commercial 



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