24 FIXING AGENTS. 



1 per cent, chromic acid . . . . .25 vols. 



1 per cent, osmic acid 2 



2 per cent, acetic acid . . . . 5 ,, 

 Water 68 ,. 



That is to say, a mixture much weaker in osmium than Flemming's. In the 

 Traite des Methodes Techniques, &c., Lee et Henneguy,1887, 1 recommended 

 this mixture, as giving better results in general, but am now inclined to 

 think that, at all events as regards fidelity of fixation, it is a step in the 

 wrong direction, and that, on the contrary, the stronger mixture of Flemming 

 (next ) is a step in the right direction. Fol's formula has the advantage of 

 allowing better staining with carmine, that is all. 



36. Chromo-aceto-osmic Acid (FLEMMING, SECOND or STEONG 

 formula, Zeit.f. wiss. Milt., 1, 1884, p. 349). 



1 per cent, chromic acid . .15 parts. 



2 per cent, osmic acid . 4 

 Glacial acetic acid . . . 1 



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 proportions being 

 as follows : 



Cr0 3 0-15 



Os 0-08 



Acid, acet I'OO 



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. 



Merk (DenJcsch. d. Math. Naturic. Cl d. K. Acad. d. Wiss. tTien., 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 CARNOY (La 



