30 FIXING AND HARDENING AGENTS 



Professor Partington informs us, the amount of water is not 

 definite. 



Estimation of Osmium Tetroxide. Tschugaeff (C. R. Acad. 

 Sci., 167, 1918) discovered a test for concentration of osmic acid, 

 which has been discussed and usefully amplified by Richard 

 Palmer {J.R.M.S., 1930). A solution of OSO4, heated with 

 thiourea in excess, with a few drops of HCl diluted to y in. 

 strength, gives a clear red colour, varying in depth with the 

 concentration of osmic acid used. 



42. Osmic Mixtures. Nicolas (Intern. Monatsschr., 1891, p. 3) adds 

 \ per cent, of osmic acid to nitric acid of 3 per cent. Lee has employed 

 a similar mixture and not had good results, though he found the mixture 

 kept perfectly. 



BuscH (Neurol. Centralb., xvii, 1898, No. 10, p. 476 ; Zeit. wiss. Mik., 

 XV, p. 373) finds that the penetration of osmic acid is enhanced by 

 combining it with iodate of sodiimn, which by hindering its too rapid 

 decomposition in the tissues ensures a more energetic action in the 

 deeper layers. He adds 3 per cent, of sodium iodate to a 1 per cent, 

 solution of osmic acid. 



Unna (Monatsschr. prakt. Derm., xxvi, 1898, p. 602) adds 1 per cent, 

 of alum to a 1 per cent, solution. For some mixtures of Kolossow, 

 see 5th ed., or Zeit. wiss. Mikr., v, 1888, p. 51, and ix, 1892, p. 39. 



43. Chromic Acid. Chromic anhydride, CrOg, is found in 

 commerce in the form of red crystals that dissolve readily in 

 water, forming chromic acid, H2Cr04. These crystals are very 

 deliquescent, and it is therefore well to keep the acid in stock in 

 the shape of a 1 per cent, solution. Care must be taken not to 

 allow the crystals to be contaminated by organic matter, in the 

 presence of which the anhydride is readily reduced into sesquioxide. 



Chromic acid is generally employed in aqueous solution. Some 

 observers (Klein ; Urban Pritchard ; Perenyi) have recom- 

 mended alcoholic solutions ; but this is evidently irrational. For 

 in the presence of alcohol chromic acid has a great tendency to 

 become reduced to chromous oxide or sesquioxide, neither of which 

 appears to have any fixing power. 



The most useful strengths in which it is employed in aqueous 

 solution are from 0-1 to 1-0 per cent, for a period of immersion of 

 a few hours (structure of cells and ova). For nerve tissues weaker 

 solutions are taken, 5^0 to ^ per cent, for a few hours. Stronger 

 solutions, such as 5 per cent., should only be allowed to act for a 

 few seconds. 



Washing out. The general practice is to wash out very 

 thoroughly with water (by preference running water, for many 

 hours) before bringing into alcohol or any staining liquid. For 

 if the objects are put direct into alcohol it is found that after a 

 short time a fine precipitate is thrown down on the surface of the 

 preparations, thus forming an obstacle to the further penetration 

 of the alcohol. Previous washing by water does not prevent the 



