Inorganic Substances 31 



Another reaction for potassium has been described by 

 Carere-Comes/ based on the dipicrylamine test of Poluek- 

 toff.^ 



Formalin fixation is recommended by the author. No pre- 

 cautions are taken in the course of further handhng of the 

 tissue. Under such conditions, only a minute fraction of 

 potassium will remain at the original sites, and the technique 

 would have little value even if the reaction were a satis- 

 factory one, which it is not. If a drop of a dilute solution of 

 KCl is mixed with a drop of the reagent on a slide, one 

 can observe under the microscope that precipitation of the 

 orange-red K-dipicrylamine crystals starts only after about 

 90 seconds, and complete precipitation requires several min- 

 utes. The crystals formed are very coarse. The criticism of 

 Claesson^ that this method is unsuitable for the identification 

 and locahzation of potassium appears to be fully justified. 

 The explanation of "positive reactions" is this: The reagent 

 is a good plasma stain, antiquated now but much used during 

 the pioneer period of histology under the name of "Aurantia." 

 It will stain all acidophilic structures, some of which (muscle, 

 red cells ) happen to be rich in potassium. 



Calcium— Soluble calcium could possibly be demonstrated 

 by treating frozen-dried sections with a solution of ammo- 

 nium oxalate, although there is no record of this ever having 

 been attempted. RabP proposed the use of a fixative com- 

 posed of formalin and oxalate; however, localization under 

 such conditions would be far from accurate. The octahedric 

 crystals of calcium oxalate are easily recognizable. 



For insoluble deposits of Ca salts it is important to use 

 neutral fixatives because both Ca phosphate and carbonate 

 are acid-soluble. Neutral formalin or alcohol or their mixtures 

 are suitable fixatives. 



4. Carere-Comes, O.: Ztsclir. f. wissensch. Mikr., 55:1, 1938. 



5. Poluektoff, N. S.: Mikrochemie, 14:265, 1933. 



6. Claesson, L.: Acta anat., 3:1, 1947. 



7. Rabl, C. R. H.: Klin. V^chnschr., 2:1644, 1923. 



