CHROMO-ACETO-OSMIC ACID. 25 



it is matter for surprise that such mixtures are not more used. 

 The alcohol should be added to the acid in aqueous solution, 

 as if strong alcohol be added to crystals of chromic anhydride,, 

 a very violent reaction is set up. The mixture should be kept 

 in the dark. 



34. Chromo-osmic Acid (MAX FLESCH, Arch.f. mik. Anat., xvi, 1879, 

 p. 300). This mixture (osmium O'lO, chromic acid 0.25, water lOO'O), origi- 

 nally introduced for the preparation of the auditory organ of vertebrates, is- 

 of general application. It does not require to be kept in the dark. Objects 

 may remain in it for twenty-four or thirty-six hours without risk of the- 

 osmic acid over-blackening them. Memming found it to preserve nuclear 

 figures well, but the preparations are pale, and difficult to stain well. He- 

 finds that the action of the mixture is improved (for nuclear figures) by the 

 addition of acetic, formic, or other acid. This addition brings out the- 

 figures more sharply, and has the further advantage of allowing of a sharper 

 stain with hsematoxylin, picro-carmine, or gentian violet. He recommends, 

 the following formula, which may be considered to have superseded Max 

 Flesch's. 



35. Chromic-aceto-osmic Acid (FLEMMING, FIKST or WEAK 

 formula, Zellsubstanz, Kern imd Zelltheilung, 1882, p. 381). 

 Chromic acid . . 0'25 per cent.] 

 Osmic acid . . O'l per cent. >In water. 

 Glacial acetic acid . 0*1 per cent.J 



The best results (as regards faithfulness of fixation) are ob- 

 tained with this mixture when it is allowed to act for only a 

 short time (about half an hour). 



But it may, without inconvenience, be allowed to act for 

 many hours or even days. Wash out, very thoroughly, in 

 water. 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 anilin, or with haematoxylin or Kern- 

 schwarz. 



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, 



