STAINING 137 



iron-ha?niatoxylin method, serves both as preHiniiiary mordant 

 and as differentiating agent. This double action is not easy to 

 explain and confirms what was said above as to the complexity 

 of the process, 



245. Metachromasy. There are a few dyes, mostly of the 

 basic anilin series, which stain certain elements in the colour of 

 the ordinary solution 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 

 being 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 Miciiaelis, 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 black to red by water shows 

 that the change is not one involving great alternations 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. 

 Holmes {Stain Technology, vol. i, 1926) has shown that in such a 

 case an addition product may result in which the pentavalent 

 nitrogen becomes trivalent, with corresponding change in colour. 

 In the case of thionin, the acid is supposed to change its connection 

 to the nitrogen which unites the two benzene rings. What 

 conditions regulate the change from one form to the other 

 are 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 arc 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. 



246. 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. 



The most distinctive processes, involving the use of mordants 

 and regressive differentiation, can only be undertaken on sections. 

 Staining in bulk is useful when the general anatomy is the object 

 of study. For cytological work it is of little value. 



