OSMIO ACID. 19 



CHAPTER IV. 



FIXING AGENTS. MINERAL ACIDS AND THEIR SALTS. 



25. Osmic Acid. The tetroxyde of osmium (OsOJ 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 alone, 

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

 with impunity if dust be absolutely denied access to them. 

 It would even seem that the solutions are improved for some 

 purposes by exposure to sunlight, vide infra, the remarks on 

 gold chloride solutions. Some observations communicated to 

 me by Dr. Lindsay-Johnson go to prove that, if dust be 

 avoided, solutions keep better in the light, with occasional 

 " sunning/' than in the dark.) 



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 have never myself suffered in this 

 way, but there is no doubt that many persons do, and such susceptible sub- 

 jects should be very careful in handling osmium in any form. 



26. How to keep the Solutions. 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 con- 

 clusion that the following is the most practical plan for 

 preventing them from " going bad " : 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 



