22 FIXING AGENTS. 



with haematoxylin (the preparations are not favorable for 

 staining with safranin or other coal-tar colours) . 



32. Chromo-formic Acid (RABL, Morph. Jahrb., x, 1884, pp. 

 215, 216). Four or five drops of concentrated formic acid are 

 added to 200 c.c. of 0'33 per cent, chromic acid solution. The 

 mixture must be freshly prepared at the instant of using. 

 Fix for twelve to twenty-four hours, wash out with water, 

 harden in alcohol, stain with haematoxylin or safranin. For 

 the study of karyokinesis. This is acknowledged to be one of 

 the very best reagents for the purpose. 



33. Chromic Acid and Spirit. A mixture of 2 parts of 

 per cent, chromic acid solution with 1 part of methylated 

 spirit was much used by Klein in his investigations into the 

 structure of cells and nuclei, and found to give better results 

 than the ordinary reagents (including even osmic acid). 

 Haematoxylin was used for staining. 



The addition of alcohol to augment the penetrating power 

 of chromic acid seems to be a step in the right direction, and 

 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. 



34. Chromo-osmic Acid (MAX FLESCH, Arch.f. mik. Anat., xvi, 1878, 

 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. Flemming 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 haematoxylin, picro-carmine, or gentian violet. He recommends 

 the following formula, which may be considered to have superseded M;ix 

 Flesch's. 



35. Chromo-aceto-osmic Acid (FLEMMING, FIRST or WEAK 

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

 Chromic acid . . 0*25 per cent. "I 

 Osmic acid . .O'l per cent. Mn water. 

 Glacial acetic acid . O'l per cent. J 



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



