250 DYEING 



When a metachromatic dye is dissolved in absolute alcohol or 

 other organic solvent, the absorption curve shoves a single main 

 peak, called the a band. With toluidine blue the peak is at 630 mjit 

 or a little less, in the reddish orange. The transmission curve 

 naturally shows a trough, which w^ill here be called the a trough. 

 A glance at the transmission curves (fig. 27, a) suggests rightly 



100 



80 - 



3? 



60 



in 



to 

 Z40 



20 - 



400 



450 



500 



550 



600 



650 



WAVELENGTH, m|i 



FIG. 27. Graph showing the transmission of light of various wave- 

 lengths through a layer of toluidine blue solution i cm thick. ^^ 



A, the dye was dissolved at 0-0002% in 80% alcohol; B, at 0'00i% in distillsd 

 water; c, the same as B, with the addition of 2 drops of heparin solution to 3*4 ml 

 of solution. The heparin solution used contained 5000 international units per ml. 

 The a, 13, and y troughs in the curves are marked by arrows. 



that the colour of the solution is blue. When the dye is dissolved 

 in aqueous solution a second or p trough appears, corresponding 

 to a p band or hump in the absorption curve. It is claimed that this 

 can just be detected even in non-aqueous solutions. ^^^ It is not 

 ordinarily detectable in very dilute aqueous solutions, but becomes 

 more and more marked as the concentration rises, while the a 

 trough becomes shallower. The ^ trough is always situated on the 

 side of the a trough towards the shorter wave-lengths. In fig. 27, 

 c, the p trough is deeper than the a, and the two are almost 

 smoothed out into one. The wave-length of the p trough of 

 toluidine blue is about 590-600 m/x (yellowish orange). 



