CHAPTBE IV. 



FIXING AND PIARDJ3NING AGENTS — ^MINIilllAL ACIUS AND 

 THEIR SALTS. 



35. Osmic Acid.— The tetroxide of osmium (OsO^) is the .sub- 

 stance commonly known as osniic a.cid, though it does not 

 possess acid properties. It is e.xtremely volatile, and in the 

 form of an aqueous solution become 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 he ahsolutelij denied access to Ihem. 



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. 



Goiu (Zeit. wiss. Mih., 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. 



Bdsch finds that the addition of sodium iodate hinders 

 reduction {Neurol. Genlralli., xvii, 1898, p. 476). 



PiNTNBE finds that a slight addition of corrosive subliuja.te 



