10 W. H. E M I G 



solution of Haematoxylin, a blue-black precipitate appears. This 

 pigment, Hematein, is the oxidized substance seen in tissues stained 

 with Haematoxylin. In staining, the section is placed first in a 

 solution of ferric alum, whereupon certain types of proteins absorb 

 the iron salt. A rinse in distilled water removes the ferric alum 

 from the surface of the tissue. Now, those parts that have re- 

 tained the mordant are colored in Haematoxylin ; other parts from 

 which the mordant is absent remain colorless. 



Reversible actions. In the production of color precipitates, 

 the action remains more or less incomplete because the reverse ac- 

 tion also takes place under the same conditions. Since the extent 

 to which the precipitate is affected depends on the concentration of 

 the original salts, it is possible to adjust the conditions in staining 

 and thereby regulate the process in one direction. 



With the formation of ferric f errocyanide, some of the precipi- 

 tate is dissolved in the presence of an excess of potassium ferro- 

 cyanide giving either a suspension of the precipitate or only a 

 colored solution. On the other hand, with a slight excess of ferric 

 ammonium sulfate, the pigment is not dissolved to an appreciable 

 extent. 



A tissue left in tannic acid for 15 minutes, rinsed with distilled 

 water, and treated with ferric alum becomes blue-black. By re- 

 versing the order of application, using ferric alum first and then 

 tannic acid, some of the iron tannate dissolves in the tannic acid so 

 that within 15 minutes only a part of the original precipitate re- 

 mains. This Hematein-like pigment is very soluble in tannic acid 

 but less so in ferric alum. 



A precipitate forms immediately after a drop of Haematoxylin 

 is added to 1 per cent ferric ammonium sulfate, but the Hematein 

 soon dissolves in ferric alum. In Heidenhain's method of stain- 

 ing, ferric ammonium sulfate is needed first as a mordant. After 

 a rinse in distilled water the tissue is overstained in Haematoxylin ; 

 the excess pigment is removed with ferric alum. During the pro- 

 cess of destaining the material must be observed under a micro- 

 scope. As soon as the correct degree of color has been attained, 

 the section is taken out of the ferric alum, but the reversible action 



