CHAPTER IV 



FIXING AND HARDENING AGENTS MINERAL ACIDS AND 



THEIR SALTS 



40. Osmic Acid, The tctroxide of osmium (OSO4) is the 

 substance commonly known as osmic acid, though it does not 

 possess acid properties. It is extremely volatile, and in the form 

 of an aqueous solution becomes partially reduced with great 

 readiness in the presence of the slightest contaminating particle of 

 organic matter. It is generally believed that the aqueous solutions 

 are reduced by light alone, but this is not the case : they may be 

 exposed to the light with impunity if dust he absolutely denied 

 access to them. 



The solution of osmic acid in chromic acid solution is not, like the 

 solution in pure water, easily reducible, but may be kept without any 

 special precautions. Bolles Lke used to keep the bulk of osmium 

 in the shape of a 2 per cent, solution of osmic acid in 1 per cent, aqueous 

 chromic acid solution. This solution served for fixation by osmium 

 vapours, and for making up solution of Flemming, which is the form 

 in which osmium is most generally employed. A small quantity of 

 osmic acid may also be made up in 1 per cent, solution in distilled water, 

 and kept in a drop-bottle with grooved stopper, from which quantities 

 can be obtained when required without removing the stopper. 



CoRi {Zeit. wiss. Mik., vi, 1890, p. 442) finds that solutions in distilled 

 water keep perfectly if there be added to them enough permanganate 

 of potassium to give a very slight rosy tint to the liquid. From time 

 to time, as the solution becomes colourless, further small quantities of 

 the salt should be added, so as to keep up the rosy tint. 



BuscH finds that the addition of sodium iodate hinders reduction 

 (Neurol. Centralb., xvii, 1898, p. 476). 



PiNTNER finds that a slight addition of corrosive sublimate has the 

 same effect, e.g. 10 drops of 5 per cen'i;. solution of sublimate added to 

 100 c.c. of 1 per cent, solution of osmic acid. 



For the Kopsch, Mann-Kopsch and Sjovall methods the osmic 

 acid solution should be free from all traces of chrome nd platinum 

 salts, etc. We recommend that osmic acid be bought in ^ grm. 

 tubes and a fresh supply be made up at frequent intervals. 



For the so-called " regeneration " of reduced solutions, see 

 previous editions. 



Osmic acid is met with in commerce in the solid form in sealed tubes. 

 The assigned weights should be checked, as they may vary greatly. 



Fixation by the Vapours. This is indicated in most of the 

 cases in which it is possible to expose the tissues directly to the 



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