OSM10 ACID. 17 



CHAPTER IV. 



FIXING AGENTS. MINERAL ACIDS AND THEIR SALTS. 



26. Osmic Acid. The tetroxyde of osmium (Os0 4 ) is the 

 substance commonly known as osmic acid, though it does not 

 possess acid properties. It is a substance that is exceedingly 

 difficult to keep in use for any length of time. It is extremely 

 volatile, and in the form of an aqueous solution becomes par- 

 tially 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, but 

 this is not the case ; they may be exposed to the light with 

 impunity if dust be absolutely denied access to them.) 

 After having carefully tried several of the plans that have 

 been recommended for keeping the working solutions free 

 from dust, I have come to the conclusion that the following is 

 the most practical plan : 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. 



1 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. 

 A small quantity of osmic acid may also be made up in 1 per 

 cent, solution in distilled water, and kept carefully protected 

 from dust. Those who have to do a great deal of fixing by 

 means of the vapours may also keep a supply of the solid 

 oxide for this purpose. 



Great stress is laid by authors on the fact that the vapour of osmium is 

 very irritating to mucous tissues. It is said that the slightest exposure to 

 it is sufficient to give rise to serious catarrh, irritation of the bronchial 

 tubes, laryngeal catarrh, conjunctivitis, &c. I think these statements greatly 

 exaggerated. 



