Inorganic Substances 45 



zone; see method under "Zinc"). Hand and associates'^ re- 

 duce mercurous mercury with 10 per cent thioglycoHc acid, 

 and both the mercurous and mercuric form with stannous 

 chloride. The metalHc mercury which appears is in the form 

 of dark droplets. All the techniques mentioned require 

 thorough testing for specificity. 



Bismuth, silver, and gold.— Alter being introduced into the 

 animal organism, these metals are converted in a short time 

 into dark-brownish or blackish, granular, rather unreactive 

 compounds the chemical nature of which is not well under- 

 stood (proteinate, sulfide, or reduced metal?). 



The identification of heavy metals in the tissues is a prob- 

 lem of analytical chemistry rather than of histochemistry. 

 In histochemical studies the identity of the metal to be dem- 

 onstrated is usually known in advance (having been intro- 

 duced artificially ) . Therefore, the task is not so much to dif- 

 ferentiate between two or more metals as to estabhsh the 

 metalhc nature of the pigment granules and to distinguish 

 them from other colored granular substances such as soot, 

 melanin, formalin pigment, pigment of wear and tear, etc. 

 In most cases purely morphological criteria (type of cells 

 in which the granules are seen) will do more than histo- 

 chemical tests to decide this question. 



The following relatively simple tests are recommended: 



1. Apply 30 per cent hydrogen peroxide to the section for 

 5 minutes. Silver and gold remain unchanged (the latter will 

 actually darken; method of Elftman);'^ bismuth'* and most 

 other pigments will be bleached. 



For the identification of bismuth, rinse the bleached shde 

 thoroughly and proceed according to one of the following 

 two methods: 



52. Hand, W. C, Edwards, B. B., and Caley, E. R.: J. Lab. & Clin. Med., 

 28:1835, 1943. 



53. Elftman, H., and Elftman, A. G.: Stain Technol., 20:59, 1945; Elft- 

 man, H., Elftman, A. G., and Zwemer, R. L.: Anat. Rec, 96:341, 1946. 



54. Wachstein, M., and Zak, F. G.: Am. J. Path., 22:603, 1946. 



