FIXING AND HARDENING AGENTS. 43 



You may make up and keep separately 



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



Distilled water . . . 4 ,, 



Glacial acetic acid ... 1 part, 

 and (B) a 2 per cent, solution of osmic acid 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. 



According to Flemming, it is better not to make up very 

 large quantities 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 may be insufficiently 

 controlled. 



Merk (Denksch. d. Math. Nittunr. C!l. d. K. Acad.d. Wiss. Wien,lSS7 ; 

 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 part, 



and (B), some 1 per cent, osmic acid 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, i, 2, 1885, p. 211) has employed a mixture one third stronger in 

 osmic acid 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 ,, 



PODWYSSOZKI recommends (for glands especially) the following modifi- 

 cation : 



1 per cent. Cr0 3 dissolved in O'o per cent, solution of 

 corrosive sublimate ...... 15 c.c. 



2 per cent, osmic acid solution ..... 4 c.c. 

 Glacial acetic acid 6 to 8 drops. 



The sublimate is said to augment the penetration of the osmium, but is 

 unfavourable to staining. The proportion of acetic acid is reduced in order 

 to avoid swelling of the tissue elements (ZIEGLEK'S Beitrdge z. path. Anat., 

 i, 1886 ; cf. Zeit.f. wiss. Mile., iii, 3, 1886, p. 405). 



The characters of the fixation are, in the main, those of 

 the weak formula, but the action is more energetic, the 



