THE DIFFERENTIAL ACTION OF DYES 24I 



ential colouring in another way. In so far as it enters the cytoplasm 

 and muscle and is taken up by them, it helps to exclude the aniline 

 blue, and thus appears to favour the background dye. All these 

 processes, however, have to be carefully controlled. If the aniline 

 blue or similar dye is allowed to act for too long, it will spread to 

 the cytoplasm and muscle and eventually replace the background 

 dye. 



It is particularly to be noticed that the phosphomolybdic acid 

 opposes the action of the aniline blue. One sometimes sees state- 

 ments to the effect that it 'mordants' the tissues for the aniline 

 blue. Not only is it impossible for such a substance to mordant for 

 an acid dye, but in fact the aniline blue colours everyAvhere more 

 powerfully if the treatment with phosphomolybdic acid be 

 omitted. To prove this it is only necessary to take two sections 

 from the same block, put one of them in a solution of phospho- 

 molybdic acid, and then colour both sections with aniline blue for 

 the same period. 



An object that is difficult to penetrate will resist the escape of a 

 dye that has succeeded in entering it. One may take a dye that does 

 not diffuse readily, heat it until the ion-aggregates have dispersed, 

 allow it to enter the tissues in this form, and then cool the dyed 

 object: the dye is unable to escape. This process is much more 

 applicable to textiles than to microscopical preparations, for the 

 former are nearly always dyed at high temperatures. Polar yellow 

 R, for instance, will not enter wool at all below 40° C, because the 

 ion-aggregates are too large. ^^^ Similarly one can scarcely colour 

 mitochondria strongly with cold acid fuchsine solution, but the 

 dye enters them readily when the temperature is raised to near 

 boiling point. If subsequently the section be treated at room- 

 temperature wdth another acid dye, even a readily- diffusing one, 

 the acid fuchsine will be replaced in the cytoplasm before it leaves 

 the mitochondria. This is probably the basis of Metzner's ^*^ and 

 several other methods for mitochondria, in which acid fuchsine is 

 used hot and another dye (or dyes) at room-temperature sub- 

 sequently. In Altmann's original method,^ hot acid fuchsine was 

 followed by warm picric acid. He himself admitted that the 

 differentiation was difficult. It is far easier to use cold picric acid 

 solution, as Metzner ^*^ did. (See also Meves ^^^ for details of 

 Metzner's method.) 



That this is the correct explanation of the usual mitochondrial 

 Q 



