METACHROMASY 261 



potassium chloride, which turns Congo rubin from red to violet 

 and finally flocculates it as a blue substance. This change of colour 

 accords with the general rule (disobeyed by the basic meta- 

 chromatic dyes) that increase in particle-size has a bathochrome 

 effect. Gold hydrosols, for instance, are light red, but if coagulated 

 by electrolytes change their colour through violet to blue and 

 finally form a blue precipitate. If a red hydrosol is injected into 

 a mouse, the gold is seen in the phagocytic cells as blue-black 

 particles. Thus the gold hydrosol is metachromatic.**^ This 

 strongly suggests that the same process accounts for the meta- 

 chromasy of Congo rubin and similar dyes. 



