CHAPTER XL 133 



differentiating agent. This double action is not easy to explain 

 and confirms what was said above as to the complexity of the 



process. 



210. Metachromasy. There are a few dyes, mostly of the basic 

 aniline series, which stain certain elements in the colour of the 

 ordinary solutions of the dye, other elements in that of the free 

 colour-base. Safranin stains nuclei red ; mucin and the ground 

 substance of cartilage, orange. Methyl violet stains ' amyloid," 

 and mucin red. 



Although in a few cases this behaviour may be due to the dye 

 beino; really a mixture of two dyes, as in the case of iodine green, 

 there is no doubt that this is not the explanation of genuine cases. 

 According to MICHAELIS, the appearance of the colour of the base is 

 not due to the alkalinity of the elements in question. The fact 

 that the red stain given to mucin by thionin can be changed into blue 

 by alcohol and back to red by water shows that the change is not 

 one involving great alterations of chemical structure, and a 

 tautomeric one is naturally suggested. It would appear that the 

 change is one by which an amino-group becomes freed from its 

 combination with the mineral acid of the salt. In the case of 

 thionin, the acid is supposed to change its connection to the nitrogen 

 which unites the two benzene rings. What are the conditions which 

 regulate the change from one form to the other is unknown. A 

 similar difficulty is met with in the case of iodine, which is brown in 

 solution in alcohol, violet in chloroform. 



We must, however, not overlook the changes in colour shown by 

 substances in the colloidal state merely in consequence of a decrease 

 in their degree of dispersion or increase in size of particles. Gold is 

 a notable case. It may be red, violet, blue or green. All of these 

 tints are met with in its use as a histological reagent. Whether 

 similar phenomena may occur in the adsorption of dyes is uncertain, 

 but cannot be dismissed without further evidence. 



The Use of Stains in Practice. Stains for special purposes are 

 described in other pages of this book. It will be obvious from the 

 contents of the present chapter that caution must be exercised in 

 making deductions as to chemical composition from behaviour to 

 dyes. 



In general, the dyes of the anilin series, although of great value 

 for particular purposes, are apt to be less permanent than the old 

 logwood and carmine stains. The Canada balsam used must be as 

 pure as possible if the stain is to last for any length of time. 



