SECTION ONE 



permit chemical interaction to take place between them. Phloxine 

 is much more soluble (about 9%) in cellosolve than is tartrazine 

 (about 2-5%) which is used as the differentiating dye. Also the 

 strength of phloxine used in the aqueous staining solution is only 

 0-5%, as against the 2-5% of tartrazine (in cellosolve), but this 

 matter will be discussed later in this chapter. 



A differentiating dye can be regarded as a decolorizer of tissues, 

 not of dyes. A decolorizing dye can be regarded as a decolorizer 

 both of tissues and of other dyes. Both differentiating and de- 

 colorizing dyes if allowed to act long enough will not only cast out 

 the synthetic dye with which the tissues have been previously 

 stained, but will also take its place and colour the tissues. 



The synthetic basic dyes used without a mordant, and carboxyl- 

 ated dyes devoid of sulphonic groups, such as the eosin group, 

 which includes phloxine, are generally readily removed by im- 

 mersing tissues stained with them in alcohol. Therefore the 

 differentiating action of aurantia or picric acid must be due, to a 

 large extent, to the alcohol in which these two differentiators are 

 dissolved. Cellosolve may have the same action as alcohol in this 

 respect. Like alcohol, it is an aliphatic solvent. Most dyes are 

 more soluble in cellosolve than they are in alcohol. Alcohol, of 

 course, has the power to remove picric acid itself from tissues, as 

 is well known. It may be that dyes such as aurantia, picric acid 

 and tartrazine reinforce the action of alcohol or cellosolve in 

 detaching certain basic and acid dyes from tissues. Possibly the 

 action of the molecules of these dyes is to bombard the other 

 dyes out of the tissue elements. 



Differentiation by dyes might, then, be due to: 



(a) Physical phenomena, e.g. molecular bombardment, as 

 mentioned above, by which the dye to be differentiated is dis- 

 lodged from the tissues and is then carried away by the solvent of 

 the differentiating dye. 



(b) Chemical affinities. The tissue elements may have a greater 

 affinity for the differentiating dye than for the dye which is cast 

 off by them in favour of the differentiating dye. This appears to 

 be the situation in the case of Orange G in the Falgog method, 

 sun yellow in the standard Faviol method, and also probably in the 

 case of the tartrazine method (page 368). However, it is hardly likely 

 to apply to unmordanted basic synthetic dyes differentiated by 



II 



