METACHROMASY 253 



imino-base is in fact present in a particular solution of a meta- 

 chromatic dye, it can be extracted with chloroform without 

 affecting the capacity of the solution to show its metachromatic 

 effect. Even more conclusive than these arguments is the fact that 

 although the metachromatic colour and that of the imino-base are 

 similar, yet they are distinct spectroscopically.^^^ 



It was pointed out long ago ^^^ that concentrated solutions of 

 metachromatic dyes show to some extent the metachromatic 

 colour. It is, in fact, quite clear that although there may be no 

 allochromasy whatever, yet the dye may appear in two forms — 

 orthochromatic and metachromatic — in aqueous solution. When, 

 however, the dye is dissolved in ethanol, only the orthochromatic 

 colour is seen, however concentrated the solution may be. This is 

 shown in fig. 27. It has already been remarked (p. 250) that when 

 ethanol is the solvent, the wave-length of minimum transmission 

 (or maximum absorption) by toluidine blue is 630 m/x, while in 

 water it is about 590-600 m^ (at the dye-concentrations used in 

 the observations). 



The factors that promote and antagonize the presence of the 

 metachromatic form of the dye in solution have been specially 

 studied by Lison.^^^ As an aqueous solution is made more and 

 more concentrated, there is a progressive approximation towards 

 the metachromatic colour, while at extreme dilutions the colour is 

 purely orthochromatic. 



The following experiments would demonstrate the spectro- 

 scopic difference between an orthochromatic and a metachromatic 

 dye. In practice one would carry them out rather differently, but 

 the methods used would not differ in principle. 



Take two exactly similar glass tanks, flat-sided and rectangular, 

 and place them in a spectrophotometer in such a way that the light 

 must pass through both. Fill both with water. Now add a weighed 

 quantity of an orthochromatic dye to the water in the tanks, allow 

 it to dissolve fully, and draw a curv^e to show the absorption at 

 different wave-lengths. Methyl green would be a particularly suit- 

 able dye, because it shows no trace of metachromasy. A series of 

 such experiments will show that if the same weight of the dye is 

 used each time, it makes no difference to the curve whether all the 

 dye is put in one of the tanks, or some in each, in any proportion. 

 If, however, a metachromatic dye be used, this is not so. The 

 absorption maximum will be at one wave-length when the dye is 



