CHAPTER IV. 



FIXING AND HARDENING AGENTS MINERAL ACIDS AND THEIR 



SALTS. 



35. Osmic Acid. --The tetroxide of osmium (Os0 4 ) 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 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 be 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. I therefore keep the bulk of my osmium 

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

 aqueous chromic acid solution. This solution serves 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 

 (N enrol. 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 cent, 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 must be free from all traces of chrome and platinum 

 salts, etc. 



