258 DYEING 



charges happened to be so spaced as to conform with this require- 

 ment. If there were a regular alternation of water-molecules with 

 dye-ions, the latter would be about 0-4 mjit apart. 



As Sylven points out, each dye-ion consists of a large hydro- 

 phobe and a small hydrophil (auxochrome) part, and this in itself 

 will facilitate the orderly arrangement of the ions. It is certain that 

 some objects can arrange the ions of dyes in an orderly way, not 

 necessarily with the production of metachromasy. This was shown 

 in the last century by Ambronn,^ in the course of his studies with 

 polarized light. If a needle-shaped crystal of methylene blue be 

 placed above a Nicol, with its long axis parallel to the longer 

 diameter of the lozenge-shaped top of the Nicol, it appears dark 

 violet; if it be now rotated through a right angle, it nearly loses its 

 colour and becomes a pale greenish blue. Ambronn showed that 

 if cell-membranes were dyed with methylene blue and then rotated 

 above a Nicol, the dichroic effect was again exhibited, for in one 

 position they appeared strongly dyed, while on rotation through 

 a right angle the intensity of the colour was reduced. We need a 

 full investigation along these lines of the behaviour of dichroic 

 metachromatic dyes taken up by chromotropic substrates. 



Sylven *^^ has shown that when carboxyl groups are introduced 

 into cellulose fibres, metachromasy starts as soon as every second 

 glucose unit of the chain has become carboxylated. The average 

 distance between the charged groups is now about i m/x. Much 

 stronger metachromasy is exhibited by polysaccharides containing 

 one sulphate and one uronic group per disaccharide unit: the 

 distance between the charged groups now alternates along the 

 chain between o-6 m/x and 0-4 m/x. 



Attention must now be directed to a most curious departure 

 from the ordinary rules of metachromasy. 



It has been remarked above that the nucleic acids, especially 

 RNA, are rather feebly metachromatic, but chromatin does often 

 give a violet reaction with toluidine blue in microscopical prepara- 

 tions (and a distinct purple with the azures; see p. 269). Occasion- 

 ally, however, a most strange reaction is noted : chromatin is dyed 

 blue-green by this blue dye, and the colour-change is therefore 

 bathochrome. 



When microscopical preparations are strongly dyed with 

 toluidine blue, chromatin is violet; when feebly, it appears blue, 

 blue-green, or even green, as though dyed by methyl green. Now 



