100 FIXATION 



give (HgCl)20 or HgClOH, with hydronium and chloride ions. 

 [HgClJ^, [HgCl]^, and the mercuric [Hg]^^ ion are present, in 

 addition to undissociated HgClg.^^' [HgClJ^ is particularly 

 readily formed in the presence of other chlorides, and mercuric 

 chloride is more soluble in solutions of sodium chloride than in 

 distilled water (about io% in 0-75% sodium chloride). 



The hydronium ion produced by hydrolysis results in moderate 

 acidity. The figures given for the pH of saturated aqueous solu- 

 tions vary from 2-8 to ^.6.i^6'285, 397, 459 j^ careful reading of the 

 pH of a 5% solution gives 3-25.^^^ 



Solutions of mercuric chloride become more acid during fixa- 

 tion. ^^^ All other acidic fixatives maintain their pH or become less 

 acid during fixation. 



Oxidation-potential. The mercuric ion is capable of reduction to 

 the mercurous [Hga]^^ ion, and mercuric chloride can therefore 

 act as an oxidizer. A 5% aqueous solution shows an oxidation- 

 potential of 0-75 volt. This is a little less than the figure for 2-5% 

 potassium dichromate and a little more than that for 2% osmium 

 tetroxide. Thus mercuric chloride is a moderately strong oxidizer. 



Manufacture. Metallic mercury is dissolved in concentrated 

 sulphuric acid; the solution is heated and sodium chloride is 

 added, with a little manganese dioxide. Mercuric chloride is 

 formed. The temperature is increased and it sublimes. It is caught 

 on the interior surfaces of glass funnels. 



Alternatively mercuric chloride may be made by the direct 

 chlorination of mercury. The metal 'burns' in an atmosphere of 

 chlorine. The product sublimes and is deposited on the walls of 

 the chamber in which the combination occurs. 



Introduction as fixative. Mercuric chloride appears to have been 

 first used as a preservative for anatomical specimens by the French 

 surgeon and anatomist, Chaussier (1746-1828).^^ Goadby included 

 it in his preservative mixture, which was recommended by 

 Quekett; *^^ but the substance was so dilute (about 0-023%) "^^^^ 

 it cannot have acted as a fixative, and Goadby's fluid must have 

 relied upon its other constituents for its preservative qualities. In 

 1847 the distinguished French morphologist Blanchard '^^ added 

 a solution of mercuric chloride to sea- water to fix marine Turbel- 

 laria. Corti ^^^ used it as a fixative in his histological study of the 

 inner ear, and so did Remak *^^ in his work on the multinucleate 

 cells of the liver. 



Lang, the celebrated Swiss zoologist and microtechnician. 



