32 FIXING AND HARDENING AGENTS. 



In general, osmic acid, especially when used in the form of vapour, 

 fixes protoplasm very faithfully, nuclei badly. It is pre-eminently 

 a fixative of the hyaloplasm or enchylema of cells. The penetrating 

 power of the solution is very low, so that if any but very small pieces 

 of tissue be taken the outer layers become over-fixed before the 

 reagent has penetrated to the deeper layers. Over-fixed cells have 

 a certain homogeneous, glassy, or colloid look, and are unfit for 

 study, and attention should be confined to cells four or five layers 

 deeper down, which will generally be found to present the required 

 intensity of fixation. In these the fixation is admirable, with no 

 shrinkage and next to no swelling of anything. 



36. The Osmium Tetroxide Reaction. MANN believed that during 

 the osmic reaction on fatty substances the Os0 4 was reduced to 

 osmium tetra-hydroxide Os(OH) 4 . Other observers have assumed 

 the reaction to be the reduction of the Os0 4 to some lower oxide. 

 The matter has recently been reviewed by Professor J. K. 

 PARTINGTON and Mr. D. B. HUNTINGFORD, who find that the 

 reduced substance is a hydrated form of Os0 2 , possibly Os0 2 , 5H 2 0, 

 or Os0 2) 6H 2 0. In all probability, Professor PARTINGTON informs 

 me, the amount of water is not definite. (See also 768 on " Fat.") 



37. Osmic Mixtures. NICOLAS (Intern. Monatsschr., 1891, p. 3) adds 

 | per cent, of osmic acid to nitric acid of 3 per cent. I have employed 

 a similar mixture and not had good results, though I find the mixture 

 keeps perfectly. 



BTJSCH (Neurol. Centralb., xvii, 1898, No. 10, p. 476 ; Zeit. iciss. Mik., 

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

 combining it with iodate of sodium, 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 (Monatschr. 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. See 

 also under " Cytology," 677. 



38. Chromic Acid. Chromic anhydride, Cr0 3 , is found in com- 

 merce in the form of red crystals that dissolve readily in water, 

 forming chromic acid, H 2 Cr0 4 . 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 



