THE INDIRECT ATTACHMENT OF DYES TO TISSUES 221 



minic acid to i g of alum crystals. Now Mayer's carmalum only 

 contains o-i g of carminic acid to i g of alum crystals. 



If the amount of potassium alum in Mayer's carmalum is re- 

 duced by one-half, precipitation eventually occurs. It is evident 

 that a dye-lake only remains in permanent solution if the mordant 

 is present in great excess. 



When a piece of tissue is mordanted with iron alum and then 

 treated with haematein, elastic fibres are not coloured. When, 

 however, the dyed tissue is put once more into iron alum, these 

 fibres take up the colour.^^^ This shows that during differentiation 

 by the mordant a soluble lake is formed, which is capable in certain 

 circumstances of dyeing, and also that the dye-lake penetrates the 

 elastic fibres more easily than its two components separately. 



The differentiation of mordant-dyes by acids is more compli- 

 cated than that by mordants. Both the links in the tissue/mordant/ 

 dye complex may be attacked. When a dye has a different colour 

 from its lake, it is easy to see that acidity undoes the bond between 

 mordant and dye. Aluminium haematein, for instance, is blue; but 

 when the lake is undone by the addition of a little acetic acid (as in 

 Ehrlich's haematein, ^^^ for instance), the colour of the dye itself is 

 seen. It must be supposed that the hydroxyl group of the dye, 

 which lost a hydrogen ion to form one-half of the chela that holds 

 the metal in its pincer-grip, is reconstituted in strongly acid 

 solution. The anion of the differentiating acid also plays a part, for 

 it must be one that does not make an insoluble salt by reaction 

 with the released cationic complex. Acetic and hydrochloric acids 

 are generally used. Neither of these forms an insoluble salt with 

 iron or aluminium. 



The hydrogen ions of the differentiating acid presumably tend 

 to reconstitute, in unionized form, those acidic groups in the tissue 

 (phosphoric, carboxyl, hydroxyl, etc.) that were ionized when they 

 made their first contact with the cationic metal complex. The re- 

 actions, however, must be more complex than these few words 

 suggest. The bonds of the metal with the dye and with the tissue 

 are not, in their final forms, electrovalent. Further, it must be re- 

 membered that the mordant solutions themselves, in the absence 

 of added acid, have already quite a low pH (p. 212), and dye-lakes 

 are nevertheless taken up by the tissues from such solutions. The 

 addition of weak acids to mordant solutions does not have a big 

 effect on pH. For instance, 5% potassium alum shows a pH of 3-2 ; 

 if 3% of glacial acetic acid is added, the figure only falls to 2*45; 



