36 Microscopic Histochemistry 



and Hg give a precipitate of a shade very similar to that 

 obtained with Ba. 



Magnesium.— Quinsilizsinn, titan yellow, and a number of 

 azo dyes have been suggested for the demonstration of this 

 element. ^^ In model experiments some of them appear to be 

 sufficiently specific; however, in sections of animal tissues 

 no staining can be obtained with any of them. Presumably 

 their sensitivity is relatively low. 



Iron— This metal occurs in two forms in animal tissue. In 

 one form, represented by hemosiderin, it behaves like any 

 poorly soluble inorganic ferric compound, such as ferric 

 oxide, readily demonstrable by the common reagents of ana- 

 lytical chemistry. In the other form (occult or masked iron), 

 exemplified by hemoglobin, iron is a part of complex organic 

 molecules and demonstrable only after destruction of the 

 organic part. Iron appears to be present in the tissues ex- 

 clusively in the ferric state, although there are a few reports 

 on the finding of ferrous iron. 



For the demonstration of iron, tissues should be fixed in a 

 neutral fixative, such as neutral formalin, alcohol, or a mix- 

 ture of the two. Although hemosiderin is far less acid-soluble 

 than Ca phosphate, it is attacked to a noticeable degree by 

 acid-containing fixatives, such as Bouin's fluid. The result 

 is not only blurring of the picture but also a false localization. 

 Certain morphological structures, especially nuclei^^ and, to a 

 lesser extent, coarse connective-tissue fibers, have an amazing 

 affinity for the ferric ion and will adsorb it even from ex- 

 tremely dilute solutions (Gilson,^^ Wiener^^) and hold it 

 tenaciously. 



The oldest technique for the demonstration of iron is 



• 19. Broda, B.: Wiadomosci Farm., 63:6 and 15, 1936; and Okamoto, K., 

 Seno, M., and Shibata, D.: Taishitzu Gaku Zasshi, 13:97, 1944. 



20. Macallum, A. B.: Quart. J. Micr. Sc, 38:175, 1896, and Proc. Roy. 

 Soc. London, 50:277, 1891. 



21. Gilson, G.: Rep. British A. Adv. Sc, p. 778, 1892. 



22. W^iener, A.: Biochem. Ztschr., 77:27, 1916. 



