THE ACTION OF MORDANTS 121 



generally used. The ferric lake is black or blue-black. It attaches 

 itself more securely to the tissues than the aluminium lake does, 

 and is less easily removed by acids. It does not act simply as a 

 basic dye, but attaches itself also to certain objects that are not 

 basiphil: in particular, to mitochondria, centrioles, and certain 

 kinds of lipid droplets. It is probable that a covalent link is estab- 

 lished between the iron-haematein complex and the tissue. ^^^ The 

 difference between this link and that formed by the aluminium 

 haematein complex has not been satisfactorily explained. 



When ferric salts and haematein are dissolved together, an in- 

 soluble lake tends to be deposited. For this reason the two-bath 

 method is generally used. The section is first soaked in iron alum 

 or some other ferric salt and then transferred to a simple solution 

 of 'ripened' (partly oxidized) haematoxylin. The latter is usually 

 dissolved in ethanol, in which it is very soluble, and the solution 

 subsequently diluted with water. The regressive method is used. 

 The excess of the dye is removed by soaking the section a second 

 time in the mordant. 



Heidenhain's method^'^- ^^- ^^ is the best-known example of this 

 way of using iron haematein. Experienced workers can get pre- 

 cisely the result they require, but beginners find the differentiation 

 rather difficult, because the extraction by the mordant seems to go 

 faster and faster towards the end, just when one would like it to 

 go slowly. Heidenhain's is one of the most important of all 

 methods of dyeing in microtechnique. Almost everything in the 

 cell can be revealed by careful differentiation, if the tissue was 

 appropriately fixed. The black or blue-black colour gives excellent 

 images in the microscope and is very convenient for photomicro- 

 graphy. Preparations are permanent in hydrophobe mounting 

 media (p. 125), even in those (such as Canada balsam) that tend 

 to bleach certain dyes. It is generally best not to use an acid dye 

 in addition, because the contrast between black objects and their 

 surroundings is necessarily reduced by any background colour. 



It will be remembered that carminic acid, like purpurine, is an 

 anthraquinonoid dye. It possesses an =0 and a phenolic -OH 

 placed in the same relation to each other as in purpurine. It 



