METACHROMASY 255 



and at any concentration of salt or dehydrating agent, there is an 

 equiUbrium between the orthochromatic and metachromatic 

 forms of the dye, and this equihbrium will be affected by a change 

 in any of the factors mentioned. Every alteration in the equilibrium 

 between the two forms of the dye is reversible. It is stressed by 

 Lison ^^^ that chromotropes are substances that shift the equili- 

 brium to an extreme extent in favour of the metachromatic form, 

 without producing any irreversible effect. 



The factors that influence the degree of metachromasy exhibited 

 by solutions of a dye also influence the appearance of micro- 

 scopical preparations coloured by that dye. The metachromatic 

 effect is most strongly shown when the section is still in the dye 

 solution, but is also well seen when this is replaced by distilled 

 water. Unfortunately this is not a good mounting medium, because 

 of the wide difference in refractive index from that of the fixed 

 proteins. Hansen ^^^ recommended a saturated aqueous solution 

 of potassium acetate, which is indeed usable, though there is some 

 loss of metachromatic colour. If the dyed preparation be heated or 

 acidified, or if the salts listed above or a dehydrating agent be 

 added, the metachromatic effect will be reduced or abolished. 

 Ordinary glycerine-jelly is acidic and contains a high proportion of 

 a dehydrating agent: it therefore abolishes all except rather strong 

 metachromatic reactions. Passage through absolute alcohol has an 

 even more extreme effect, and preparations mounted in Canada 

 balsam therefore show metachromasy only in the granules of 

 Mastzellen and other particularly chromotropic objects, and even 

 these would presumably not show it if dehydration had really been 

 complete. 



It has been suggested ^^^ that one should not speak of meta- 

 chromasy unless the colour-shift is shown in preparations mounted 

 in glycerine-jelly, balsam, or other commonly-used media. It 

 must be observed, however, that this would be a very arbitrary 

 decision. These mounting media have been chosen, not because 

 they are adapted to studies of metachromasy, but solely because 

 they give good optical results and preserve ordinary microscopical 

 specimens permanently. It would be a strange chance if they 

 happened to be the ideal media for quite another purpose. It 

 would seem more rational to use various media that slightly oppose 

 the metachromatic effect to varying degrees. One might bring 

 dyed sections into i% and 2% sodium chloride, for instance, or 

 25% and 45% ethanol, or into solutions at pH 4 and 3. In this 



