252 DYEING 



a hypsochrome effect, for the dye is magenta, the base reddish 

 orange. PararosaniHne is metachromatic, and it exempHfies the 

 general rule that the transformation of a metachromatic dye to its 

 imino-base involves a heightening of colour : that is to say, a change 



NH, NH, 



NH 



Imino-base of pararosa?tiline 



in the same direction as the metachromatic shift. Because the 

 colour changes when the imino-base is produced, and because a 

 certain degree of alkalinity is necessary for its production, it 

 follows that metachromatic dyes can be used as indicators 

 of pH. 



It is most tempting to assume, as Hansen ^^^ did nearly half a 

 century ago, that chromotropes take up imino-bases from solutions 

 of metachromatic dyes. He considered that these bases were 

 present in ordinary solutions of the dyes and that they were some- 

 how specifically stored up by the chromotropic tissue-constituents. 

 He remarked that if one shakes up benzene, xylene, or chloroform 

 with an aqueous solution of thionine, the little oily droplets give 

 the impression under the microscope that they are composed of 

 'mucin', because they have the metachromatic colour. 



Although there may be some roundabout connexion between 

 the capacity of a dye to form an imino-base with heightened 

 colour on one hand and its capacity to give a metachromatic effect 

 on the other, yet Hansen was in fact mistaken, for these two kinds 

 of colour change are not due to the same cause. As Lison ^^^ 

 pointed out, metachromatic dyes give their colour-shift at a pH 

 far below that at which an imino-base could exist. Brilliant cresyl 

 blue, for instance, is pure blue at pH 10 and only becomes orange- 

 red at higher pH than this, but the dye can act metachromatically 

 down to pH 3 or even lower. There is no question of the chromo- 

 tropes being local alkalinizing agents, since, as we have seen, they 

 are acidic. At a low pH, at which the dye will still act metachro- 

 matically, no imino-base can be extracted by chloroform. If any 



