CHAPTER IV. 



FIXING AND HARDENING AGENTS MINERAL ACIDS AND 



THEIR SALTS. 



33. 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 a substance that it 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 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 abso- 

 lutely denied access to them. It would even seem that the 

 solutions are improved for some purposes by exposure to 

 sunlight (vide infra, 336, the remarks on solutions of metallic 

 salts). (Some observations communicated to me by Dr. 

 Lindsay Johnson go to prove that, if dust be avoided, solu- 

 tions 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, etc. 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. 



34. Eow to keep the Solutions. After having carefully tried 

 several of the plans that have been recommended for keep- 

 ing the working solutions free from dust, I have come to 

 the conclusion 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 solu- 

 tion in pure water, easily reducible, but may be kept with- 

 out any special precautions. I therefore keep the bulk of 



