FIXING AND HARDENING AGENTS. 41 



Chromic acid . . 0*25 per cent.^j 



Osmic acid . .0*1 per cent, j- In water. 



Glacial acetic acid . 0*1 per cent.J 

 This liquid may, without inconvenience, be allowed to act 

 for many hours or days, or according to some workers even 

 weeks or months. Wash out very thoroughly in water (see 

 also 40). Stain with haematoxylin if you wish to stain in 

 toto (staining in this way with other reagents is possible, but 

 very difficult, and not to be recommended). Stain sections 

 with safranin or other coal-tar colour, or with hasmatoxylin 

 or Kernschwarz. 



To make up this mixture with the usual stock solutions, 

 you take 



Chromic acid of 1 per cent. . . 25 volumes. 



Osmic acid of 1 per cent. . .10 ,, 



Acetic acid of 1 per cent. . .10 



Water . . . . 55 ,, 



If you keep your osmium in 2 per cent, solution in 

 chromic acid of 1 per cent., as I have recommended, you 

 will have to take only 20 vols. of chromic acid, 5 of your 

 osmium solution, and 65 of water. See also the remarks on 

 the deterioration of these solutions by keeping, in the next 

 section. 



It is not necessary in all cases to observe the exact proportions of the in- 

 gredients in this mixture. FOL (Lehrb. d. vergl. mik. Anat., 1884, p. 100) 

 recommends the following : 



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. 



A mixture still weaker than this is osmium, viz. with 1 vol. osmic acid 

 solution instead of 2, has been recommended by GOBI (Zeit.f. wiss. Mik. 

 vi, 1, 1890, p. 441). 



This mixture, though less in vogue now than the follow- 

 ing or strong mixture, is one of the most celebrated of fixa- 

 tives. I think justly so, for, with the possible exception of 

 HERMANN'S mixture, no known fixative seems to me to afford 

 such fine images of cellular structures as Flemming's two 

 fluids. That is to say, that the fixation is pre-eminent both 

 as regards the preservation of the structures and as regards 

 their optical differentiation. But this is meant with the re- 



