METACHROMASY 259 



toluidine blue itself, however dilute, is never green, nor is the 

 alcoholic solution. 



Lison and Mutsaars ^^^ took a very dilute solution of toluidine 

 blue (about 0-0015%) and added varying amounts of DNA or 

 RNA to it. When the nucleic acid was very dilute, a small hypso- 

 chrome change occurred, the colour going from blue towards red, 

 and red itself was reached when the concentration of the nucleic 

 acid was 0-0025%. When the concentration rose higher than this, 

 the colour became violet-blue, and finally blue-green. 



A study with the spectrophotometer gave results in conformity 

 with what could be seen with the eye. As the concentration of the 

 nucleic acid increased, the peak of the original absorption curve 

 sank while a new one (^ band) arose on the side of it towards the 

 shorter wavelengths ; with further increase in the concentration of 

 nucleic acid the curve moved bodily in the opposite direction, its 

 peak passing the wave-length of the original peak and going beyond 

 it towards longer wave-lengths. At no concentration was the 

 actual shape of the original curve restored, and it w^ould therefore 

 be wrong to refer to a reversion to the orthochromatic form of the 

 dye, with subsequent movement beyond it. 



When the blue-green colour had been obtained, heating 

 abolished it and restored the truly orthochromatic form of the dye. 



The concentration of DNA is particularly high in the heads of 

 spermatozoa, and it is interesting to notice that these are rather 

 easily dyed blue-green by toluidine blue. 



No satisfactory explanation has been given of bathochrome or 

 'negative' metachromasy. 



We now leave the metachromasy of basic dyes and turn to a 

 subject that has received much less attention. A few^ acid dyes are 

 metachromatic in peculiar ways. The rules that govern the meta- 

 chromasy of basic dyes do not apply here. Thus azo dyes can be 

 metachromatic, there is no correlation between metachromasy and 

 the capacity to form imino-bases, and the effect is usually batho- 

 chrome. 



Indigocarmine (indigo-dye) and orange G (mono-azo) are meta- 

 chromatic towards solutions of clupeine, which is a protamine in 

 combination with nucleic acid, extracted from the heads of the 

 spermatozoa of certain fishes; it is strongly basic (acidophil). The 

 same dyes show metachromasy in the presence of certain large 

 crystalloid cations, especially those of quinine and strychnine.*^ 



