Miscellaneous Dyes 237 



This like the other tetrazolium salts is colorless itself, but oxidizes 

 into a colored formazan, known in this case as blue forniazan, be- 

 cause of its color. This deep blue pigment is a strong indicator 

 of certain types of enzymatic activity, but is much more toxic 

 than ordinary tetrazolium chloride. See Smith (1951). 



6. MINERAL PIGMENTS 



' There are a considerable number of pigments of non-organic 

 nature, which are much better known to the manufacturer of 

 paints than to the biologist. They are in no sense dyes; but a few 

 of them are occasionally used in microscopic work or for some other 

 biological purpose. They were not listed in the first editions of this 

 book because they were regarded as lying outside the field covered. 

 The following three, however, are occasionally mentioned by the 

 biologist; and they seem worth mentioning, if for no other reason, 

 to keep them from being confused with the dyes. Of these three, 

 the first one is generally known for what it is; but the other two 

 are more apt to be ordered as though they were dyes instead 

 of mineral salts. 



PRUSSIAN BLUE C. I. NO. 12.88 



Synonyms: Berlin blue. Chinese blue. Paris blue. Milori blue. 



Steel blue. 



The well-known colored salt, ferric ferrocyanide, Fe4(FeC6N6)3, 

 finds its greatest use in the manufacture of paints, but it has oc- 

 casional biologic employment, chiefly for coloring anatomical 

 specimens, for demonstration. of the circulatory system by means of 

 injection. It is mentioned in connection with the Gomori (1936) 

 technic in which K4Fe(CN)6 is used as a microchemical reagent for 

 iron in tissues; by this procedure Prussian blue is formed in situ in 

 the tissues. 



SKY BLUE C. I. NO. 12.86 



Synonyms: Coelestin blue. Coeline. Coeruleum. 



This pigment is essentially cobaltous stannate, CoOSn02. It is 

 called for primarily as a constituent of artists' oil or water colors, 

 but has been given occasional mention in biological literature. 



RUTHENIUM RED 



An ammoniated ruthenium oxychloride prepared by making an 

 ammoniacal solution of ruthenic chloride, RuCla. It has been 

 employed, with or without the addition of acetic acid, in micro- 

 scopic work, primarily as a test for pectin. It is thought by some 

 to be specific for this purpose. 



