CHAPTER V 



FIXING AND HARDENING AGENTS -CHLORIDES, ORGANIC 



ACIDS, AND OTHERS 



CHLORIDES 



68. Bichloride of Mercury (Corrosive Sublimate). Corrosive 

 sublimate 1 part is soluble in about 16 parts of cold and 3 of 

 boiling distilled water. It is more soluble in alcohol (1 : 3) or in 

 ether (1:4) than in water. Its solubility in all these menstrua is 

 augmented by the addition of hydrochloric acid, ammonium 

 chloride, or camphor. With sodium chloride it forms a more 

 easily soluble double salt ; hence sea-water may dissolve over 

 15 per cent. 



The simple aqueous solutions should always be made with 

 distilled — not spring — water. The HgClg in them has been assumed 

 partly to split up by hydrolysis into CI, H, and (HgCl)2, or 

 HgClOH (see Chem. Centralb., 1904, i, p. 571). But J. Baker 

 {Cytological Technique, 1933) points out that mercuric chloride 

 partly hydrolyses into hydrogen and chlorine ions and (HgCl)20 

 or HgClOH, according to Luther [Chem. Centralbl., 8, i, 1904). 

 These solutions should give an acid reaction with litmus paper, 

 whilst those made with strong sodium chloride solution are 

 neutral. 



Carleton (Q. J. M. S., vol. Ixvi, 1922) has investigated the com- 

 parative effects of isotonic saline and distilled water when used as 

 solvents for mercuric chloride and formol in histological fixation. 

 From this work it appears to be of no histological importance whether 

 saturated (6 per cent.) solutions of mercuric chloride be dissolved in 

 normal saline or in distilled water. No differences could be detected 

 in specimens of liver, small intestine, and kidney fixed in either way, 

 nor would there be any reason to expect such differences on a priori 

 grounds. For the relatively high molecular concentration of the HgClj 

 is only very slightly altered by dissolving it in either isotonic saline or 

 in hypertonic saline of double the normal concentration. In fact, the 

 only effect of making up a concentrated solution of mercuric chloride 

 in normal saline is slightly to increase the tonicity of the mixture. 

 There is still (1937) a question of the exact molecular condition of 

 fixing fluids with NaCl which has been raised again by J. Z. Young 

 (§ 31), and the question will have to be reviewed in the future. 



In the case of a 5 per cent, solution of formol the evidence is 

 that this reagent fixes tissues more faithfully when made up in 

 normal saline than in distilled water. When dissolved in the 



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