Metachromasy of nucleic acids

If nucleic acids and a metachromatic dye are mixed in amounts giving a low nucleic acid: dye ratio a 'positive ' metachromatic effect occurs similar to that observed with truly chromotropic substances: for instance, thionine or toluidine blue become red or reddish-violet. This may be observed in test-tubes with pure solutions of the sub-stances. In microscopical preparations this occurs when the staining, is strong. If the amount of N.A. is such that the ratio N.A.: dye is high, a 'negative ' meta-chromasy occurs, that is, shades of colour are produced that are greener than those obtained in pure water solutions. In microscopical preparations this occurs when the staining of nuclei is very light. Both 'positive ' and 'negative ' metachromasy have the characteristics of truly meta-chromatic phenomena—specific absorption curves and suppression by heat. EVERY histologist who has had some experience in the use of meta-chromatic dyes is sure to have noticed how differently the nuclei may appear in the preparations. If they are stained, for example, with toluidine blue, they appear in some cases blue-green or blue, while in other cases they are distinctly metachromatic and stain purple or violet. This metachromati