DYEING AND OTHER PROCESSES OF COLOURING 3OI 



and retained in these against solvents. ^^° It would appear that 

 Sudan black is able to permeate phospholipids on account of its 

 lipid-solubility, and to combine with phosphoric radicles on 

 account of its possession of -N- groups. Thus it has some of the 



H 



characters of both lysochromes and dyes. Acidic substances other 

 than lipids may sometimes be coloured by untreated Sudan black. 

 Tests with the acetylated colorant should always be carried out if 

 the colour is not readily removed by solvents. It must be remem- 

 bered, though, that resistance to solution does not necessarily 

 indicate that the substance coloured is not a lipid. 



Nile red is a lysochrome of altogether special interest, on account 

 of the complications that surround it in ordinary use. It is an 

 oxazone; that is to say, a non-ionic substance formed by the 



H5C/ 



Nile red 



oxidation of an oxazine dye (see p. 180). It can be dissolved in 70% 

 ethanol and used in exactly the same way as Sudan III or IV. It 

 has the advantage of possessing no potential auxochrome. 



Nile red appears by spontaneous oxidation in aqueous solutions 

 of Nile blue A. The anion of this dye is sulphate, because the 

 chloride is rather insoluble. (The dye is often called Nile blue 

 sulphate, but there is no more reason for doing this than for speak- 

 ing of basic fuchsine chloride or sodium eosin.) Although Nile red 

 is insoluble in distilled water, it is soluble in aqueous solutions of 



+ ^ + 



Hr/ ' I I I III 



Nile blue A 



Nile blue. Solutions of this dye have therefore very interesting 

 properties, to which attention was called nearly half a century ago 

 by the English pathologist. Smith. "^^^'^^^ There has been much 

 controversy about the action on tissues of solutions of Nile blue, 

 but the main facts have been exposed by the careful studies of 

 Lison ^^^ and Cain.^^ 



