248 DYEING 



Certain inorganic substances with complex anions are markedly 

 chromotropic.*^'*^^ Among these anions are ferricyanide, thio- 

 cyanate, and especially phosphotungstate. The subject has not 

 been fully investigated, presumably because it is not of much 

 interest to biologists, who are the chief people concerned with 

 metachromasy. 



More than two dozen basic dyes are known to be definitely 

 metachromatic, most of them being triarylmethanes and azines. 

 No azo-dye is metachromatic except Janus green, which owes this 

 character to the fact that it is also an azine. Among the lake-dyes 

 certain oxazines are remarkable for giving strongly metachromatic 

 effects. ^^*' *^'^' ^^^ The most useful metachromatic dyes are prob- 

 ably these : — 



methyl violet (triarylmethane) 



brilliant cresyl blue (oxazine) 



cupric, ferric, and aluminium lakes of coelestine blue 



(oxazine) 

 thionine (thiazine) 

 azure A (thiazine) 

 azure B (thiazine) 

 toluidine blue (thiazine) 



It will be noticed that the thiazines are pre-eminent in providing 

 us with valuable metachromatic dyes. 



Certain dyes show their metachromatic effect when used vitally. 

 These will be considered in one of the chapters devoted to vital 

 dyeing (p. 281). 



It will be convenient to consider first an ideal dye that has a 

 nearly symmetrical absorption curve with a peak in the middle of 

 the visible spectrum. Basic fuchsine w^ould serve as a fairly good 

 example, though the curve is not very symmetrical. Such a dye 

 will necessarily transmit light from the two ends of the spectrum 

 (compare fig. 17, p. 161), and if the transmission curve is regular 

 on the two sides, the colour will be purple. (Basic fuchsine trans- 

 mits a high proportion of red and the colour is magenta.) 



If now we somehow influence our dye in such a way that the 

 peak of its absorption curve (or the trough of its transmission 

 curve) is slowly shifted towards the right (that is, towards the 

 longer wave-lengths), the transmitted colour will change gradually 

 from purple through violet to blue and then to green. The 



