312 DYEING 



of a dye. A dye may be defined, for the purposes of microtech- 

 nique, as an aromatic, salt-Hke compound having these char- 

 acters : — ■ 



(i) it ionizes in the presence of water; 



(2) either the cations or the anions are coloured (sometimes 

 both) ; 



(3) the coloured ions are able to make chemical linkages with 

 the proteins (and generally also with other constituents) 

 of the fixed tissues of organisms (and in some cases with 

 constituents of living cells as well) ; 



(4) when the coloured ions make their linkages with the 

 tissues, they do not lose colour, and generally they do not 

 change it. 



It is not necessary to mention mordants in the definition of 

 dyes, since all dyes that work with mordants are able to colour the 

 fixed tissues of organisms in the absence of any mordant, though 

 the results are then very different. 



The definition includes no substance that is not obviously a dye, 

 and excludes scarcely any that deserve the name. The number of 

 substances left in doubt is very small. Methylene violet (p. 268) is 

 an example. This certainly acts as a dye, in the presence of related 

 substances that are dyes, but when isolated is a non-ionic com- 

 pound. It might fall into place under the definition if we knew its 

 chemical structure in the presence of other dyes. 



