THE DIFFERENTIAL ACTION OF DYES 235 



the acid dyes according to the way in which they can best be used to 

 colour textile fibres. ^^^''^'^^^'^^^ It is desirable to mention this 

 classification, partly because one can only follow the textile litera- 

 ture if it is understood, partly because it has direct significance for 

 microtechnique. The classification is not based on chemical re- 

 lationship: dyes in a single group may possess quite different 

 chromophores. The limits of some of the groups are not sharp, and 

 the textile authorities do not all use exactly the same classification. 

 Three groups will be defined here. 



The 'levelling' dyes are acid dyes that are used to colour wool 

 and other protein fibres from a strongly acid bath. They have not 

 a very high affinity for such fibres and will not dye them at neutral- 

 ity, and they have no affinity for cellulose fibres. The word 'level- 

 ling' (or 'equalizing') means that they dye very evenly. They are 

 often called simply 'acid dyes', ^^^'^^^ in reference to the pH of the 

 bath in which they are dissolved, but this confusing name will not 

 be used here. 



The 'milling' dyes colour protein fibres very strongly at low pH, 

 but their action is so uneven that they are not used in this way in 

 practice. They are dissolved, on the contrary, in neutral or weakly 

 acid solution and are therefore sometimes called 'neutral- dyeing' 

 dyes. Their special character is that they are not decolorized nor 

 extracted by 'milling', which is a felting process applied to wet 

 wool after dyeing, often in the presence of soap. It is characteristic 

 of milling dyes that they are of greater molecular weight than the 

 levelling ones, and form colloidal solutions. ^^* 



The 'direct cotton' dyes colour cellulose fibres without the use of 

 any mordant. 



The relevance to microtechnique of this classification of acid 

 dyes will appear shortly. 



Acid dyes are very often used in pairs. Sometimes the dyes are 

 mixed together, sometimes one is used after the other. If the 

 affinities of the two were exactly the same, no advantage would be 

 secured, for the appearance given would be exactly the same as 

 though only one dye had been used (apart from the colour being 

 mixed). In fact, however, the dyes are selected in such a way that 

 certain objects are coloured by one of them, others by the other, 

 and some (usually) by both. Sometimes three acid dyes are used. 



One component of the pair (or trio) colours collagen fibres, 

 another the ground cytoplasm. Other tissue-constituents are 

 coloured by the one or the other or by both, but for our present 



