CHROMO-ACETO-OSMIC ACID. 27 



If this mixture be kept in stock in large quantities, it may 

 go bad, probably on account of the large proportion of 

 organic acid contained in it. I therefore recommend that it 

 be made up from time to time from stock solutions, in which 

 the osmium is kept separate from the acetic acid. The pro- 

 portions being as follows : 



Cr0 3 . . . . . . 0-15 



Os 0-08 



Acid. acet. ..... 1-00 



Aq. .... 19-00 



You may make up and keep separately 



(A) 1 per cent, chromic acid . .11 parts. 

 Distilled water . . . 4 



Glacial acetic acid . . 1 



and (B) a 2 per cent, solution of osmium in 1 per cent, 

 chromic acid solution, and when required, mix four parts of 

 A with one of B ; or, of course, if you prefer it, you may 

 keep the osmic and chromic acid ready mixed in the propor- 

 tions given, and add 5 per cent, of acetic acid at the moment 

 of using. 



In any case, it is better not to make up very large quan- 

 tities of the mixture at once, as osmium being very volatile 

 it will be found that solutions that have been long in use 

 no longer contain the proper proportion of that ingredient, 

 and the hardening action being thus weakened the swelling 

 action of the acetic acid will be insufficiently controlled. 



Merk (Denksch. d. Math. Naturw. Cl. d. K. Acad. d. Wiss. Wien, 1887 ; 

 cf . Zeit. f. wiss. Mik., v, 2, 1888, p. 237) proposes to make up separately (A) 



2 per cent, chromic acid 7'5 parts. 



Water 3'5 



Acetic acid 1 



and (B), some 1 per cent, osmium solution, and to mix for use 12 parts of 

 A with 8 of B. But this plan leaves you in the old difficulty of keeping 

 your osmium in aqueous solution. 



It does not appear necessary to observe the exact proportions of the ingre- 

 dients of these mixtures, a certain latitude is allowable. Thus CAENOY (La 

 Cellule, 1, 2, 1885, p. 211) has employed a mixture one third stronger in 

 osmium and twice as strong in chromic acid, viz. 



Chromic acid of 2 per cent, (or even stronger) . 45 parts. 



Osmic acid of 2 per cent 16 



Glacial acetic acid 3 



In the Traite des Meth. Techniques, 1887, I treated this 



